Negatively charged fast moving particle Atom is no longer unbreakable
Idea of atom introduced No experimental evidence Atom is the smallest unit of matter and is not breakable With experimental evidence from the study of gas Invention of cathode ray tube Behaviour of cathode ray in magnetic and electric field Invention of the word "electron". Raisin pudding model of atom Gold foil scattering experiment The idea of presence of tiny nucleus
Democritus
Dalton Goldstein
Thomson
Rutherford Chadwick
Study of radioactivity
Discovery of neutron The idea of nucleus is breakable
Vaporization chamber Ionization chamber Accelerating plates Construction
Mass of an atom
Radioactivity
Atomic Structure
Development of atomic model
Mass spectrometer
Relative isotopic mass
Relative atomic mass
Relative molecular mass
Interpretation of mass spectrum
Working principle - response to m/e ratio
Vacuum Pump
Recorder
Detector
Deflecting magnetic field
To determine the ionization energy of an atom Determination of isotopic mass Determination of atomic mass Determination of molecular mass Determination of molecular structure The relative mass of a particular isotope in C-12 scale where C-12 isotope is defined to have exactly 12 units of mass.
The weighted average of the relative isotopic masses for a particular element in C-12 scale according to their natural abundance The relative mass of a molecule where that the value is the same as the sum of relative atomic mass of all atoms present in the molecule.
1. Atomic Structure.mmap - 2005/5/23 -
History of discovery
Nature of radioactivity
Discovery of radioactivity
,
and
First order reaction
Independent of temp.
Nuclear bomb
Co-60 / Cs-137 (tumor)
I-131 (thyroid cancer)
P-32 (metabolism of plant)
I-131 (thyroid disease)
Geiger-Muller counter
Constant Half-life
Meaning of Half-life
re-orientation of the spinning proton
Nature state of atoms after the big bang / nucelus formation
increasing of p:n ration
instablility of neutron when isolated
lowering of p:n ratio
binding force between proton and neutron
replusion among proton
100% speed of light
high energy EM wave
up to 99% speed of light
very fast moving electron
5% of speed of light
fast moving helium nuclei
Study the nature of radiation
Extraction of radium and polonium
Becquerel
Curie
Rutherford
particle
particle
radiation
particle
particle
radiation
Natural decay
Origin of radioactivity
Nuclear Reaction
Bio-tracer
Leak Detection
Artificial transmutation
Use of Radioactive isotope
Radiotherapy
Carbon-14 dating
Nuclear power
Molecular mass is numerically the same as molar mass
Measurement of T
Measurement of m
Measurement of V
Measurement of P
Conditions a real gas behaves virtually ideal
Assuming the vapor behaves ideally
Volatile liquid vaporizes easily by heating No interaction among particles Individual particle occupies no volume
interaction among particles is negligible High temperature
Low pressure Barometer in the laboratory
Increase in volume of the gas syringe
Difference in mass of the hypodermic syringe before and after injection
Thermometer (final reading)
Definition of partial pressure - the pressure of a component in the gaseous mixture if that component occupies the same volume by itself alone.
Principle
Setup of apparatus
Operation
T
1/V
Charles' Law V N
Boyle's Law P Avogadro's Law V
Ideal gas Law / Equation PV = nRT
Dalton's Law of partial pressure
Determination of molecular mass of a volatile liquid
Total pressure of a gaseous mixture is the same of the sum of the partial pressure of all of the component present
Take final reading when all readings are steady
Take all initial readings
Allow the system to reach a thermo-equilibrium
Precaution
Precaution
Precaution
particles are far apart
particles are moving fast
Insert the needle fully into the gas syringe
Hold the end of the syringe only
Hold the hypodermic syringe horizontal
Take reading only when the reading is steady
space occupied by individual particle is negligible
Some liquid may have not vaporized The vapor may be still expanding The liquid may leak out. Volatile liquid usually has low surface tension. Body warmth may heat up the liquid and expel it from the syringe Inject the liquid into the deeper part, usually hotter, of the gas syringe to ensure better vaporization.
Take reading only when the reading is steady
mole fraction (percentage by number)
The system may have not reach thermo-equilibrium yet.
2. Mole Concept.mmap - 21/5/2005 -
Behaviour of Ideal gas
How much is one mole ?
Mole Concept
How many ?
How heavy ?
Definition
1st Law
2nd Law
m
1/n
Q
1 F = 96500 C
Faraday constant
m
Avogadro's law
s.t.p.
r.t.p.
Carbon-12 Scale
Avogadro constant (Experimental measurement required)
Molar mass
Molar volume
Molar volume of a gas
Faraday's Laws of electrolysis
Relationship between Current I, Amount of charge Q and time t i.e Q=It
How large ?
Charges
Charges carried by 1 mole of electron
A solution with accurately known concentration Process of determination of concentration of a non-standard solution
criteria
A substance which can be used to prepare a standard solution directly known composition stable composition readily soluble preferably with higher molecular mass
strong acid vs strong base strong acid vs weak base weak acid vs strong base
Choice of indicator
pH meter (titration curve) thermometric titration
Standard solution
Standardization
Primary standard
Equivalence point
Titration
Acid-base titration
Back titration
Redox titration
End point
Without indicator
Example
KMnO4 vs H2C2O4.2H2O
KMnO4 vs FeSO4
Iodometric titration
conductometric titration
Using indicator
The point when the end of titration is detected
The point at which the 2 reagents just react with each other completely.
methyl orange or phenolphthalein methyl orange phenolphthalein
methods for titration of weak acid vs weak alkali iodine / potassium iodide starch solution sodium thiosulphate Determination of concentration of chlorine bleach Constant heating is required
Determination of percentage by mass of calcium carbonate in chalk Analysis of aspirin
Used if the reaction of direct titration is not instantaneous / if no suitable indicator for direct titration
3. Chemical Equations and Stoichiometry.mmap - 8/1/2005 -
Volumetric Analysis
Formulae
Chemical Equations and Stoichiometry
Formulae of Compounds
Determination of Formulae
Chemical Equations
Empirical formula
Molecular formula
Structural formula
Empirical formula
Molecular formula
Word Equation
Chemical Equation
Simplest whole number ratio
Actual no. of atoms
Connectivity of atoms
By Combustion Analysis
From Composition by mass
From empirical formula and molecular mass
Water of crystallization
Stoichiometry
Volumes of gases
Reacting masses
Stoichiometric coefficients
Calculation based on equations
Electromagnetic Spectrum Continuous Spectrum Line Spectrum
Different models of atom
The Electronic Structure of Atoms
Background knowledge
4. The Electronic Structure of Atoms.mmap - 19/1/2005 -
Bohr's model
Setup of the apparatus
gas discharge tube slit
prism or diffraction grating
Line spectrum
Continuum
Ionization of atom
Flame test
Red shift
Planck's equation
ground state
excited state
principal quantum number
Identification of element at remote star
Discovery of helium
Atomic emission spectroscopy
Unevenly spaced energy levels
Multiple transitions take place at the same time
Presence of numerous energy levels (shells)
Process of excitation and relaxation
Introduction of concept of quantum
Convergence limits at the higher frequency end of each series
Unevenly spaced lines in each series
Different series
Numerous lines
photographic film
Characteristic of the hydrogen spectrum
Interpretation of the hydrogen spectrum
Numerous lines
Unevenly spaced lines
Convergence limit and continuum
As a mean of identification
Neon light with different colours
Evidence of subshell
Evidence of shell
Uniqueness of atomic emission spectra
Successive ionization energies
Definition
Atomic Emission spectrum of hydrogen
Ionization energies
1st ionization energies across periods
Electrons are orbiting around the nucleus in orbits with with different energy
The allowed energy of the electron in an atom is quantumized
Able to explain the emission spectrum of hydrogen spectrum but fail to explain the emission spectrum of those multi-electron system.
Quantum numbers
Subsidiary (l)
Principal (n)
energy in multi-electron system
shape of orbital / subshell
l = 0,1,2...(n-1)
shell no.
size
principal energy
n = 1,2,3 ...
By mathematical modeling
Heisenberg's uncertainty principle
Wave-particle duality of electron
Features of Bohr's model
Wave-mechanical model / Orbital model
m = -l,...,0,...+l
spatial orientation of orbital spin of electron
s = +½ or -½
Magnetic (m) Spin (s)
Nodal plane
Nodal surface
Radial probability distribution
Pauli Exclusion principle Orbital / Electron cloud representation
repulsion with the inner eusually stronger repulsion with the e- in the same shell usually weaker
Gets more endothermic when increase in attraction with e> increase in repulsion among electrons, and vice versa Value reflects the net attraction between the outermost electron and the nucleus
Attraction with the nucleus Relative position of the outermost e(Electronic configuration of the atom) Repulsion among electrons
Energy required to remove 1 mole of electrons from 1 mole of gaseous atom
Depending on
General pattern
Definition
Variation
Periodic Table
Atomic radius
Aufbau Principle
Electrons occupy the orbital with the lowest energy first
more evenly distributed electron cloud
full-filled 3d and half-filled 4s orbital
less repulsion
less repulsion
less repulsion
more evenly distributed electron cloud
no electron pair-up in 3d and 4s
half-filled 3d and 4s orbital
3d electrons shield 4s electron from the nucleus and make 4s electrons more energetic
Electrons will occupy degenerate orbitals singly with parallel spin before pairing up occurs.
Each orbital can accommodate 2 electrons with opposite spin
All electrons behave different in an atom i.e. no 2 electrons in an atom have the same set of quantum no.
Order of energy of different orbitals
Pauli exclusion principle
Copper
Completely filled quantum shell e.g. K, L, M
Noble gas cores e.g. [He], [Ne] etc.
Chromium
The difference gets even smaller when 3d orbitals are being filled up
Energy of 4s orbital is only a little bit lower than that of 3d orbital
Hund's Rule (Rule of Maximum Multiplicity)
Irregularity Exception
by notation by electron in boxes use of abbreviation
the p to e- ratio is increasing
the repulsion/shielding among electron is getting less
more difficult to remove electron from a positive particle
usually weaker
repulsion with the e- in the same shell
usually stronger
repulsion with the inner e-
the electronic configuration of an ion is the same as the electron configuration of the preceding atom
the p to e- ratio is increasing
Proof of the presence of shells
the increases in successive ionization energies are too large to be represented by linear scale
shift to the right
shift upwards
appear in tiers
In log scale
Successive ionization energy is always much higher than its predecessor
Plot of Successive ionization energies of an atom
Plot of successive ionization energies of different atoms
including metallic radius
Nuclear charge
Primary shielding effect
Secondary shielding effect
the increase in primary shielding effect outweighs the increase in nuclear charge
the increase in nuclear charge outweighs the increase in secondary shielding effect.
Relative position of the outermost e(Electronic configuration of the atom)
Attraction with the nucleus
half of the distance between 2 chemically bonded nuclei of the same element
Depending on
Repulsion among electrons
contract across a period
expand down a group
half of the distance between 2 nuclei of the same element which are not chemically bonded together in a crystal lattice
General trend
van der Waals' radius
Covalent radius
Variation of Successive ionization energies
Representation of electronic configuration
Building up of electronic configuration on paper
Electronic configurations and the Periodic table
Variation of First Ionization energy across the first 20 elements
s-block, p-block, d-block and f-block elements
Showing 2-3-3 pattern across period 2 and 3
Main trend
Effective nuclear charge (nuclear charge - shielding effect) experienced by the outermost e-
Energy required to move the outermost electron to infinity and escape from the attraction of the nucleus Nuclear charge Primary shielding effect Secondary shielding effect
Getting less endothermic down a group
Getting more endothermic across a period
Position of the outermost e- (Size of atom) the increase in nuclear charge outweighs the increase in secondary shielding effect the increase in primary shielding effect outweighs the increase in nuclear charge Imply presence of subshell
Unexpected drops at O and S (or peaks at N and P)
Unexpected drops at B and Al (or peaks at Be and Mg)
Imply pairing up of electrons in an orbital Electron goes into a more energetic p-subshell (or full filled s-subsell in Be and Mg - more evenly distributed charges) Electrons starts pairing up in O and S - extra repulsion (or half filled p-subshell in N and P more evenly distributed charges)
5. Electronic configurations and the Periodic table.mmap - 6/1/2005 -
Different forms of the same element / compound in the same physical state e.g. graphite and diamond, ozone and diatomic oxygen
298K 1 atm
Standard conditions
pure, if it is a liquid or solid Standard state
Experimental determination
Examples and definitions
Need of standard state and condition
1M if it is an solution
most stable form at 298K, 1 atm
Presence of allotrope and polymorph
Heat of Combustion
Heat of Formation
Heat of Solution
Heat of Neutralization
Enthalpy change as Heat flow (q) of a system at constant pressure returning the system to the original temperature under a specific condition. Formation of 1 mole of water Neutralization of strong/weak acid / alkali A solution with specific concentration / an infinitely dilute solution is prepared. 1 mole of compound is formed from its constituent elements in their standard states Complete combustion of 1 mole of a specific substance in excess oxygen
Heat of Hydration
bomb calorimetry to determine the internal energy change
Formation of 1 mole of hydrated crystal from anhydrous solid Experimental setup conversion to enthalpy change by calculation
simple calorimetry to determine the enthaply change
Philips Harris calorimeter to determine heat of combustion
Common assumptions
Experimental setup
Experimental setup Polystyrene cup + cover Insulating layer thermometer stirrer Specific heat capacity of solution is the same as that of water The rate of heat loss is uniform / No heat loss to the surrounding. The heat capacity of some components in the system can be ignored. The specific heat capacities of some components are only approximation. Heat evolved will be the same as heat flow in the original definition of enthalpy change
6. Energetics.mmap - 17/1/2005 -
Standard enthalpy changes
Significance
Enthalpy and Internal energy
Definition : heat change of a system at constant volume
Definition : heat change of a system at constant pressure
The link between the microscopic interaction among particles and macroscopic observable change.
Enthalpy change
Internal energy change
Enthalpy change = Internal energy change + Work done on the atmosphere
Heat flows out of the system because of the increase in temperature of the system
Potential energy among the particles is converted to heat energy when there is an overall strengthening of attractions among the particles.
Bond formation > Bond breaking
Heat flows into the system as the temperature of the system decreases
Heat energy is converted to potential energy when there is an overall weakening of attractions among the particles.
Bond formation < Bond breaking
Energy cannot be created or destroyed
Some changes has no direct reaction at all
The direct reaction may be too vigorous and too dangerous to be carried out
The extent (% of conversion) of reaction cannot be controlled.
Formation of side products
Balanced equation according to definitions
Calculation involving heat capacity
Calculation of mole
Unit, usually in kJmol-1 must be included
+ and - sign must be included
3. Constituent atoms
2. Combustion products
1. Constituent elements
Patience and careful manipulation
Some common references in constructing energy cycles
Prerequisite
Need of Hess's Law
Born-Haber cycle as energy cycle used to determine the heat of formation
Energy cycle & energy level diagram
Another version of law of conservation of energy
The total enthalpy change of a chemical reaction is independent of the route by which the reaction takes place
Endothermic process
Exothermic process
Relationship between Enthalpy change and Internal energy change
Exothermic and Endothermic process
Energetics
Hess's Law
Calculation
unit and sign of the answer
Attraction among oppositely charged ions Repulsion among similarly charged ions
Some terms
Different arrangements of lattice points
Balance between non-directional electrostatic attraction and repulsion
face centered cubic (f.c.c.) structure simple cubic structure Actual arrangement is depending on
ionic radius
Lattice point
Unit Cell
Coordination number (C.N.) no. of nearest neighbour
e.g. NaCl
Potential well diagram
C.N. = 6 for both ions C.N. = 8 for both ions e.g. CsCl
Stoichiometry of the compound ionic radius ratio Determination of empirical formula 1. the smallest unit containing all the fundamentals of a crystal without repetition. Definition
Determination of no. of different ions in an unit cell
2. the smallest unit of a crystal from which the original crystal structure can be recreated by repetition in 3-dimensional space. Determination of empirical formula The position of a formula unit of the compound considered. Definition - Distance from the centre of an ion to the point of zero electron density between 2 adjacent ions.
Size of isoelectronic ion
Value depending on
determined by X-ray diffraction
p to e- ratio
no. of atoms present
Electron density map polyatomic ion is bigger than simple ion In general, negative ion is bigger than positive ion
As the no. of electrons and electronic configurations are the same, the size is only depending on the number of protons present.
7. Ionic bonding.mmap - 19/1/2005 -
Energetics of formation
Ionic Bonding
Ionic crystals
Born Haber Cycle
Stoichiometry of ionic compounds
Heat of formation
always positive
Removal of 1 mole of electrons from 1 mole of gaseous atoms / ions
always positive
formation of 1 mole of gaseous atoms
formation of 1 mole of compound
Atomization energy of element
Ionization enthalpy of atom
Addition of 1 mole of electrons to 1 mole of gaseous atoms / ions
negative : attraction between the incoming electron and the nucleus > the repulsion between the incoming electron and the existing electrons
depending on the packing efficiency i.e. crystal arrangement
depending on charge density of ions involved
positive : attraction between the incoming electron and the nucleus < the repulsion between the incoming electron and the existing electrons
always negative
Formation of 1 mole of ionic compound from its constituent gaseous ions
Electron Affinity of atom
Lattice energy
Construction of Born Haber Cycle for the hypothetical ionic compounds e.g. MgCl and MgCl3
Cations and anions are just in contact with each other
All ions are perfect spheres
The lattice arrangement of the hypothetical compound is known
Hypothetical lattice energy calculated from the ionic model
Construction of Born Harber cycle for the naturally occurring ionic compound e.g MgCl2
Indeed, the stability of an ionic compound is mainly arising from the formation of ionic bond among oppositely charged ions.
Positive metal ion + free electron are actually more energetic (less stable) than a gaseous metal atom
a measure of strength of ionic bond
Limitation of using stable electronic configuration to explain
Nature prefers the stoichiometry with the most exothermic heat of formation i.e. highest stability comparing with the constituent elements
Assumptions of ionic model
The bond is purely ionic
The value of heat of formation is mainly determined by the values of ionization energies and lattice energy
Strength of Covalent bond
Attraction between nucleus, bonding electron and another nucleus
Replusion among the electrons
directional attraction
e.g. BF3
Expansion of octet
Can be less than octet
Period 2 elements never excess octet
Formation of more bonds
More overall exothermic change
High overall stability
e.g. PCl5, SO42-
Availability of low lying energy orbital (e.g. 3d orbital comparing with 3p orbital)
an electron acceptor with a vacant p-orbital for dative bond formation
Energetically unfavorable
3s orbital has much higher energy than 2p orbital
# of p - # of e- associated with an atom assuming that all bonding e- are belonging to the more electronegative atom
# of p - # of e- associated with an atom assuming that bonding e- are equally shared
e.g. NH4+, H3O+ NH3BF3, Al2Cl6
Overlapping of a lone pair with a vacant orbital
a covalent bond in which the bonding electrons are supplied by one atom only
Promotion of paired electrons to low lying energy orbital (e.g. 2p comparing with 2s, 3d comparing with 3p) to yield more unpaired electrons for bond formation
Increase in electron density along the inter-nuclear axis
Constructive interference of atomic orbitals
Replusion between the nuclei
Potential well diagram
Cases not obeying octet rule
Cases obeying octet rule
Oxidation number
Formal charge
Dative covalent bond
Pairing up of unpaired electrons (bonding electrons)
Overlapping of atomic orbital
Balance between electrostatic attraction and repulsion
Formation of covalent bond
Drawing of Lewis structure
Covalent bonding
Giant Covalent Structure
Shape of molecules and polyatomic ions
Factors affecting bond strength
(Average) Bond enthalpy/energy E(X-Y)
Bond Dissociation Energy D(X-Y)
In the range of 150-1000 kJmol-1 Energy required to break a particular bond The average energy required to break a type of bond Estimation using enthalpy change of atomization
Bond length
Bond order
Using bond energy to estimate the enthalpy change of a reaction no. of bonding electrons / 2
Estimation of bond length Breakup of additivity of covalent radius
Atomic size (covalent radius)
(Primary) shielding effect affects the nuclear attraction experienced by the bonding electrons
The longer the bond length, the weaker will be the bond
Resonance
Polarity of bond covalent bond with certain ionic character is found to be stronger than purely covalent or purely ionic bond bond polarity
Description of shapes
Valence shell electron pair repulsion theory
repulsion among the lone pairs on adjacent atoms
Electrostatic repulsion is very sensitive to distance
Electrons centres tend to separate from each other as far as possible to minimize mutual repulsion
Electron centres : lone pair, single bond, double bond, triple bond
e.g. weak F-F, O-O and N-N bond
repulsion between lone pair-lone pair > lone pair-bond pair > bond pair-bond pair lone pair always occupies equatorial position in a five electron centres system linear, angular (V-shaped or bent), trigonal planar, tetrahedral, trigonal pyramidal, trigonal bipyramidal, seesaw, T-shaped, octehedral, square planar In the describing of the shape of molecule, the presence of lone pair is not included.
Diamond
Graphite
Quartz (SiO2)
tetrahedrally arranged strong C-C single bond only high hardness, high melting point, electrical insulator, excellent heat conductor arrange in hexagonal layers strong C-C bond, bond order 1 1/3, within layer weak van der Waals force between layers
high hardness, high melting point
low hardness, very high melting point, conducting along the layer, slippery between layers
8. Covalent bonding.mmap - 2005/5/21 -
e.g. Cl2
e.g. HCl
The bond is non-polar
Polar covalent bond
Pure covalent bond
Polarity of a molecule
(Permanent) Dipole moment
The bonding electrons are not shared equally
The bonding electrons are shared equally
atoms have different electronegativity
Definition - product of quantity of charge (q) and distance of charge separation (d) The dipole moment along a bond is pointing from the less electronegativity atom to the more electronegativity atom Lone pair
Depending on the vector sum of all dipole moments
Contribute a dipole moment pointing away from the atom Geometry of the molecule magnitude of individual dipole moment A molecule with non-polar bond only
Non-polar molecule
Polar molecule (Dipole)
Do not / weakly attracted by an electric field
A molecule with all dipole moments canceling out each other Non-polar molecules are also attracted by an electric field as a weak temporary dipole moment will be induced onto them by the electric field A molecule with non-zero net dipole moment May not be able to tell the actual direction of the net dipole moment Attracted by an electric field
9. Intermeidate type of bonding.mmap - 26/1/2005 -
Starting from pure covalent bond
Starting from pure ionic bond
Intermediate Type of Bonding
Assumptions of simple ionic model
The lattice arrangement of the compound is known, if it is hypothetical All ions are perfect spheres
Cations and anions are just in contact with each other and clearly separated from each other
Polarization of anion
Mutual repulsion of electron cloud in an ionic lattice
e.g. NaCl (almost purely ionic)
The ions are not perfectly spherical
Certain covalent character
Polarization of anion
p to e- ratio
secondary shielding effect
primary shielding effect
e.g. AgCl, AgBr, AgI, ZnS
Ions may not clearly separated from each other
ease of distortion of electron cloud
inversely proportional to the nuclear attraction on the electron cloud
electronic configuration
shielding effect
nuclear charge
directly proportional to the charge density of a cation
reflects the attraction on the incoming electron to be added to an atom
reflects the attraction on the outermost electron in an atom
Depending on the effective nuclear charge experienced by the bonding electron
Not applicable to noble gases
Increase across a period and up a group
Polarizing power of a cation
Polarizability of an anion
Polarization - distortion of electron cloud of anion under the influence of cation
A very different value implies failure of the ionic model with strong covalent character
A more agreed value implies a situation similar to the assumptions of ionic model pure ionic bond
The bond is purely ionic. i.e. no sharing of electron or incomplete transfer of electron
The lattice energy calculated based on the ionic model is never the same as the experimental value
Anion is always polarized by the cation
ionization energy
Tendency of an atom to attract the bonding electron in a stable molecule
For all real ionic compounds
Electronegativity Pauling's scale
electron affinity
Fluorine has the highest assigned value : 4.0
The percentage of ionic character of a covalent bond is depending on the difference in electronegativity of the 2 elements bonded together
Nature of metallic bond
Sea of electrons model
lattice of positive ions immersed in a sea of delocalized electrons
Non-directional in nature
agitation of delocalized electrons
delocalized electrons capable to reform the metallic bond even if the positive ions are shifted
delocalized electrons get excited and give out luster
Electrostatic attraction among positive ions and sea of delocalized electrons
high ductility and malleability
high conductivity
metallic radius = covalent radius
unique properties of metal
metallic luster
Charge density of the positive ion e.g. Na+ > K+ > Rb+
abab... pattern
C.N. = 12
In molten state, most metallic bond are not broken
high charge density of the positive ion limited the availability of the valence electrons e.g. Hg
no. of valence electrons e.g. Al > Mg > Na
In the range of 100-500 kJmol-1 Attraction on the delocalized electrons
Availability of delocalized electron Big difference between m.p. and b.p.
hexagonal close packing (h.c.p.)
C.N. = 12
abcabc... pattern
C.N. = 8
(face centred) cubic close packing (f.c.c. or c.c.p.) tetrahedral holes and octahedral holes body centered cubic (b.c.c.) Determination of no. of atoms in an unit cell
open-structure (68%)
close-packed structure (74%)
Strength of metallic bond
Metallic crystal (Giant metallic structure)
Metallic bonding
10. Metallic bonding.mmap - 22/2/2005 -
2 per H2O molecule, 1 per HF, NH3, MeOH molecule
H on F, O and N Attraction between a naked proton and a lone pair
An exceptionally strong directional dipole-dipole interaction hydrogen atom bonded onto a very electronegative atom / entity
overall strength depends on
formed between molecules (usually) gives higher b.p.
intermolecular
intramolecular dimerization of carboxylic acids in vapour state / non-aqueous solvent
unique properties of water
formation of secondary structure in protein and DNA
Examples
intermolecular and intramolecular hydrogen bond
Estimation of strength of hydrogen bond
Strength (1/10 of covalent bond) 5-50 kJmol-1
lone pair on a very electronegative atom
including H on CHCl3 lone pair on F, O and N only difference in electronegatvitiy average no. of hydrogen bond per molecule
Hydrogen bonded molecule has higher boiling point that ordinary polar molecule with comparable size.
Experimental
hydrogen bonded molecules also possesses van der Waals' forces
H-bond breaking
H-bond formation
e.g. HF, H2O, NH3, MeOH comparing with other molecules with comparable size
formation of new intermolecular forces > breaking of old intermolecular forces
Period 2 hydrides comparing with hydrides of period 3 to 5
Mixing ethanol and hexane
Mixing trichoromethane and ethyl ethanoate
a substance would be soluble (usually) gives higher solubility in water
formed within a molecule prevents the formation of intermolecular hydrogen bond
overall polarity of the molecule (actually more important than ability to form H-bond)
ability to form hydrogen bond with water e.g. trans-butenedioic acid e.g. alcohols (usually) gives a lower b.p. forming bond is better than not forming bond
Low density of ice
highest density at 4 °C
forming strong bond is better than forming weak bond
open structure of ice
water expands upon freezing diamond like structure
at 0 °C
from below 0 °C to 0 °C melting of ice
DNA
Protein
Variation of density with temperature (K.E.)
double helical structure
single helical structure
from 4 °C to 100 °C
from 0 °C to 4 °C
rapid collapsing of open structure
thermal expansion of solid structure
six membered rings of O linked by O-H▪▪O linkage
density drops
density increases sharply continuous collapsing of open structure
thermal expansion of liquid water
still possesses certain hexagonal network
density increases continuously
density drops
cytosine - guanine pair thymine - adenine pair
hydrogen bond between heterocyclic bases on 2 separate strands
hydrogen bond between N-H and C=O group among different amide groups along the polypeptide chain
3 H-bonds 2 H-bonds
11. Intermolecular Forces.mmap - 2005/5/23 -
Hydrogen bond
Intermolecular Forces
Type of dipole (polar molecule)
van der Waals forces
Molecular crystal
permanent dipole
permanent separation of centre of positive charge and centre of negative charge
no interaction (attraction and repulsion) among particles
individual particle occupies no volume
possessed by all kinds of molecules including polar molecules
the fluctuation of electron cloud creates an instantaneous dipole
possessed by all polar molecules (dipoles) instantaneous/temporary dipole
induced by a permanent dipole
with instantaneous dipole-induced dipole interaction only
b - volume of individual particle
the opposite ends attracted each other
dispersion force
London force
temporary dipole-temporary dipole interaction
induced dipole-induced dipole interaction
branched isomer of hydrocarbon has lower boiling point
lower b.p. and m.p.
higher b.p. and m.p.
the opposite end of different temporary dipoles attract each other
stronger overall strength for molecules with comparable size
weaker overall strength for molecules with comparable size
polarizability of constituent atoms e.g. telfon/poly(tetrafluoroethene)
increases with increasing molecular size / surface area e.g butane vs 2-methylpropane
with dipole-dipole interaction, dipole-induced dipole interaction and instantaneous dipole-induced dipole interaction
an instantaneous dipole induce a temporary dipole on another molecule
other names
a permanent dipole induce a temporary dipole onto another polar/non-polar molecule
also called permanent dipole induced dipole interaction
attractions among opposite ends of permanent dipole
also called permanent dipole-permanent dipole interaction
a - attraction among particles
assumption of ideal gas law
van der Waals' constants
Real gas does not obey ideal gas law perfectly
induced by instantaneous dipole
dipole-induced dipole interaction
dipole-dipole interaction
Condensation of solid and liquid at low temp.
Real gas equation
induced dipole
Evidence
Origin
Polar molecules
instantaneous dipole-induced dipole interaction
Strength (1/10 of covalent bond) 5-50 kJmol-1
Non-polar molecules
depending on the polarizability of the molecule
half of the internuclear distance between 2 atoms of the same element which are not chemically bonded together in a crystal lattice
face-centred cubic structure (f.c.c)
face-centred cubic structure (f.c.c)
van der Waals' radius
iodine
dry ice
Simple molecular
Macromolecular
non-conducting
Giant metallic structure
Giant ionic structure
Giant covalent structure
Molecular structure
Structure and properties of materials
Giant structure
12. Structures and properties of materials.mmap - 2005/6/5 -
usually solid
In general, like dissolves like
the rigid structure transmits the vibration of the C atoms efficiently
bonding electrons in most covalent bonds are not delocalized
intermolecular forces never stronger than covalent bond
solubility depending on the polarity of the solute and solvent
soft and weak
usually gases, liquids or low melting solids
fat and oil - marginal cases, with just over 100 atoms
e.g. H2O, NO2, S8, P4 under 100 atoms
weaker intermolecular forces
e.g. starch, protein, plastic over thousands of atoms stronger intermolecular forces no delocalized electrons
insoluble in any solvent
high melting point
high density
hard
e.g. diamond, quartz, graphite
strong covalent bond
usually non-conductor of electricity
Exception - diamond is an excellent conductor of heat
presence of delocalized electrons along the hexagonal layers
weak van der Waals' forces between layers
flat hexagonal layers
weak van der Waals' forces between layers
usually poor conductor of heat brittle slippery conducting
repulsion cleaves the crystal
weak van der Waals' forces between layers
strong C-C bond, bond order 1 1/3
large space between layers
high melting, even higher than diamond
lower density than diamond
slight dislocation brings ions of similar charge together
only conducts in molten or aqueous state
increase in disorderness
dipole - ion interaction
the ions are not free to move in solid state
brittle
high density
hard
high m.p. and b.p.
Speical case : Graphite
strong ionic bond
presence of ions
hard
high m.p. and b.p.
If soluble, only soluble in aqueous solvent
sometime, strong metallic bond
high conductivity of heat and electricity
malleable and ductile
high density Presence of sea of delocalized electrons Soluble in mercury
Concentration of reactants Pressure Temperature Surface Area Catalyst Light
Definition of Rate
Measuring of Reaction Rate
Rate of Chemical Reactions
Factors affecting Reaction Rates
13. Rate of Chemical Reactions.mmap - 2005/6/5 -
Change in quantity per unit time
Quantity is usually expressed in moldm-3 Time is usually expressed in s
Depending on the units of quantity and time Usually expressed in moldm-3s-1
Instantaneous rate
Average Rate
Unit of rate of reaction
Different rate Initial Rate (Instantaneous rate at t = 0)
Other possible units : mol, g For gases : Pa, mmHg, atm, cm3, dm3
v
by sudden cooling
by dilution with a large amount of water
by removing the catalyst
Need of a calibration curve
Transmittance and Absorbance
by removing one of the reactant
constant amount of Br2 produced / phenol consumed
starch indicator
constant amount of I2 produced / S2O32- consumed
phenolphthalein indicator
constant amount of methanal reacted / HSO3- consumed
constant amount of S ppt. produced to mask the cross
Measuring of mass of the reaction mixture
Measuring of colour intensity
Measuring of volume of gas
Quenching of reaction before titration is done
Methanal Clock experiment
Disproportionation of Na2S2O3 in acidic medium
By measuring a physical quantity related to the amount of a chemical
By titration
Differential form of rate equation / law
Constant time approach (fix the time interval and determine the amount present)
Constant amount approach (fix the amount to be reacted and determine the time required to complete the reaction) Iodine Clock experiment I- vs H2O2
Reaction between Br- and BrO3- in acidic medium
methyl red indicator
Differential form
Order with respect to individual reactant
Order of reaction
Rate constant k
Overall order of the reaction Unit of rate constant is depending on the overall order of the reaction A value depending on temperature only the rate is independent of the concentration of the reactant
Ordinal form
Zeroth order Different order with respect to a reactant First order
Second order
Usually occurring in catalyzed reaction where the availability of the catalyst surface is the limiting factor of the rate the rate is directly proportional to the concentration of the reactant the rate is directly proportional to the square of the concentration of the reactant
Zeroth order reaction
Rate Equation / Law
half life
Rate Equations and Order of Reactions
The concentration of C-14 in atmosphere is assumed to be constant Integrated form
First order reaction
The percentage of C-14 in a living organism is assumed to be constant. half life
carbon-14 dating
By comparing the percentage/beta emission of C-14 in an archaeological sample and the percentage/beta emission of C-14 in a new sample, the age of the archaeological sample can be estimated. The rate of decay is independent of temperature as it doesn't involve collision among particles
Second order reaction half life
Initial Rate method
From the initial rate with different initial concentrations
If there is more than one reactant involved, keep the concentrations of all reactants other than the one being studied virtually constant by keeping them in large excess By plotting concentration against time
Determination of rate equation / law
Horizontal line By plotting graph
By trial and error
By plotting rate against concentration
zeroth order
Straight inclined line
zeroth order
Straight inclined line parabola
first order
second order
By plotting rate against [A] or [A]2 etc to see which will give a straight inclined line passing through the origin By plotting [A] or ln[A] or 1/[A] against time Plotting log (rate) against log (concentration) 14. Rate Equations and Order of Reactions.mmap - 2005/1/31 -
The slope will be the order of reaction with respect to the reactant
The distribution is temperature dependent The total area under the curve is independent of temperature. i.e. the total no. of particles present
e.g iron in Harber process By breaking up a single step into a multi-steps reaction with lower activation energy negative catalyst
positive catalyst
Providing an alternative pathway with a different activation energy, usually lower.
e.g. H+ in decomposition of H2O2 The catalyst has the same phase as the reactants and product
e.g. acid-catalysed esterification The catalyst has a different phase as the reactants and product
Effect of catalyst
homogeneous catalysis
Simple Collision Theory (Effective Collision)
(A)Arrhenius Constant
Energy larger than activation energy Arrhenius Equation
(Z) Collision Frequency
Depending on the nature of the reaction
Vary only a little when the temperature change
(P) Orientation / geometrical / probability factor
The step with the highest activation energy The step with the lowest rate
can be isolated at low temperature
e.g. carbocation
e.g. a pentavalent transition state
Repeat a reaction at two different temperature In the reaction k can be replaced by rate if initial rate method is used k can be replaced by 1/t if constant amount approach is adopted
An energy barrier the reactants need to overcome for the reaction to proceed. By varying the temperature and measuring the rate of a reaction A plot of ln k versus 1/T A plot of ln rate versus 1/T if initial rate method is used A plot of ln 1/t versus 1/T if constant amount approach is adopted
rate-determining step (r.d.s.)
Reaction intermediate
can never be isolated
Determination of activation energy
Multi-stage Reaction
Single stage reaction
transition state (or activated complex)
Activation Energy
Energy profile
The Effect of Temperature Change and Catalyst on Reaction Rate
Maxwell-Boltzmann distribution of molecular speeds
Catalysis
Other examples
heterogeneous catalysis
Haber process
e.g. Catalytic decomposition of hydrogen peroxide by maganese(IV) oxide Fe(s)
Contact process hydrogenation of oil
Enzymes
Catalytic Converter
Gasification of naphtha in product of town gas
Ni(s), Pt(s) or Pd(s)
Pt(s) or V2O5(s)
chemsorption / adsorption on the surface of the catalysis
Ni(s) and NiO
Rh(s) and Pt(s)
zymase in the fermentation by yeast 15. The Effect of Temperature Change and Catalyst on Reaction Rate.mmap - 21/5/2005 -
including all species Excluding those species having no effect on the equilibrium position e.g. solvent(species in large excess), insoluble solid/liquid Examples
Equilibrium constants
At equilibrium and only at equilibrium, the order of reaction with respect to a species will be the same as the stochiometric coefficient of the species Generic equilibrium constant, K Equilibrium constant in terms of concentration, Kc Equilibrium constant in terms of partial pressure, Kp
Significance of equilibrium constant
Determination of Equilibrium constant
Unit of equilibrium constant
Esterification
The equilibrium concentrations of all species are determined by the initial concentrations of them and the equilibrium concentration of any one of them.
Construction of a calibration curve using colorimeter Formation of [Fe(NCS)]2+(aq) complex
Reaction between Fe2+(aq) and Ag+(aq)
Assuming that the rate of reaction is extremely slow and quenching is not required before any titration is done. Preparation of solutions with known [Fe(NCS)]2+(aq) concentration using excess Fe3+(aq) or NCS-(aq) Determination of the equilibrium concentration of [Fe(NCS)]2+(aq) using colorimeter and the calibration curve prepared Assuming that the rate of reaction is extremely slow and quenching is not required before any titration is done The titration is self-indicating if standard KCNS(aq) is used in the determination of the concentration of Ag+(aq). The solution will turn red at the end point with the formation of blood red [Fe(NCS)]2+(aq) complex.
The value is temperature dependent.
The value is depending on the relative stability of the products comparing with the reactants
The value shows the relative amounts of products and reactants when the system is at equilibrium i.e. extent of reaction at equilibrium.
16. Dynamic Equilibrium.mmap - 6/4/2005 -
Equilibrium Law
Equilibrium
Dynamic Equilibrium
Equilibrium position
A state where no macroscopic change is observed.
All chemical equilibria are dynamic in nature. i.e. forward rate = backward rate
The amounts of reactant and product are usually not equal in an equilibrium.
Examples
Thermal dissociation of calcium carbonate
Esterification
Action of acid and alkali on bromine water
Action of acid and alkaline on potassium dichromate
Action of acid and alkali on bismuth chloride
Definition : a set of reactant and product concentration
Using of equilibrium constant and reaction quotient to predict the direction to which the equilibrium position will be shifted.
Q = K : the system is at equilibrium
Q > K : the equilibrium position will be shifted to the left
Q < K : the equilibrium position will be shifted to the right
Effect of change in pressure
this has no effect on the equilibrium position
The rate of a reaction with a higher activation energy is more sensitive to the change in temperature
Catalyst does not change the relative stability of the products comparing with the reactants
Catalyst has no effect on the equilibrium position. It only shortens the time required for a system to reach a state of equilibrium.
Use of change in distribution of molecular speed to explain the effect of change in temperature on the equilibrium position.
by adding an inert gas
by adding one of the product/reactant
by decreasing/increasing the volume
Effect of change in concentration
The principle states that when a system in equilibrium is subjected to a change, the system will response in a way so that the effect of the change imposed will be minimized.
Reaction Quotient (Q)
Le Chatelier's principle Effect of change in temperature
Effect of catalyst
Ka1 is always much larger than Ka2
Successive ionization of polyprotic (polybasic) acid
A more negative pKa (pKb) value means a larger Ka (Kb) value, thus a stronger acid (base).
HA- is relatively more stable when comparing with H2A than A2- when comparing with HAA2- has a much higher charge density than HA- and has a very high potential energy i.e. more unstable It is more difficult to remove a positive proton from negative HA-(aq) than from electrically neutral H2A(aq) Beside the small value of Ka2, the second step of the ionization is also suppressed by the presence of H3O+ from the first step of dissociation which will tend to shift the equilibrium position of the second step to the left. This is known as common ion effect (of H3O+ in this case). Eventually, the amount of H3O+ from the second step of ionization will be very minimal. Determination of acidity constant Ka by half-way neutralization The value of Ka is only depending on temperature but the % of dissociation of a weak acid is depending on concentration as well.
17. Acid Base Equilibrium I.mmap - 6/4/2005 -
Acidity and Basicity constants
Different definitions
Arrhenius definition Bronsted Lowry definition Lewis defintion
acid : substance that produces H+ in water
Introduction of the concept of conjugate acid-base pair
base : substance that produces OH- ions in water
acid : proton donor
base : proton acceptor
acid : electron acceptor
base : electron donor
An Arrhenius acid (base) must be a Bronsted Lowry acid (base) and a Bronsted Lowry acid must be a Lewis acid (base) but not vice versa.
A neutral solution has a pH 7 only at 25°C.
Water is a very poor conductor as it always possesses very small amount of H3O+(aq) and OH-(aq) ions i.e. 1 pair of ions for every 556 million water molecules
Need of calibration using a buffer solution before use
using pH paper / universal indicator
Every unit increase / decrease in pH scale means a ten-fold decrease / increase in the concentration of hydroxonium (H3O+) ion
Originated from French "pouvoir hydrogene" (power of hydrogen).
Auto-ionization (dissociation) of water
Acid Base Equilibrium I : Basic Concepts
pH scale
Measurement of pH
Using pH meter
phenolphthalein or methyl orange methyl orange phenolphthalein no suitable indicator
strong acid vs strong alkali strong acid vs weak alkali weak acid vs strong alkali weak acid vs weak alkali
Examples
Add phenolphthalein to the mixture, then do the first part of the titration (V1). Thereafter, add methyl orange and do the second part of the titration (V2). A mixture of Na2CO3 (V1+V2) and NaHCO3 (V2) A mixture of NaOH (V1) and Na2CO3 (V1+V2) A mixture of NaOH (V1) and NaHCO3 (V2)
18. Acid-base Equilibrium II.mmap - 21/5/2005 -
Choosing of indicator and pH titration Curves
Double indicator titration
Definition : A solution that resists change in pH when a small amount of acid or base is added to it.
Composition : A mixture of weak acid and weak base i.e. usually a weak acid (base) and its salt i.e. the conjugate base (acid).
In the calculation of the pH of a buffer solution, it is assumed that the inter-conversion between the acid (base) and its salt upon mixing is negligible.
The buffering capacity of a buffer solution is depending on the concentrations of the acid/base and its salt.
Human eyes are only capable to detect a colour change when the concentration of one coloured substance is more than 10 times of another coloured substance.
Acid-base indicator is usually a weak acid (base) which colour is different from its conjugate base (acid).
Buffer solution
Acid-base Equilibrium II : Buffers and Indicators
Acid-base indicators
reducing agent is the reagent which is oxidized in a redox reaction oxidizing agent is the reagent which is reduced in a redox reaction
Remember the common strong reducing agents
Remember the common strong oxidizing agents
Oxidation half equation
Reduction half equation
The change in oxidation no. per atom is the same as the no. of electron being lost or gained by that particular atom Figure out the no. of atom that will be oxidized and the no. of atom that will be reduced by using the change in O.N. Add H2O to either side of the equation to get the number of O atom balanced if necessary. Add H+ to either side of the equation to get the number of H atom balanced if necessary. Check the presence of H+ or OH- on the both sides to ensure it agrees with the acidity/alkalinity of the medium used. For reactions in acidic medium, there should be no OH- ion on the both sides of the equation For reactions in alkaline medium, there should be no H+ ion on both sides of the equation
19. Redox Reactions.mmap - 2005/5/23 -
By Combining two half equations
By change in oxidation no.
Check the medium of reaction
Balancing Redox Equations
Definition of redox reaction
Redox Reactions
Oxidation no.
Oxidation
Reduction
Addition of oxygen atom
Removal of hydrogen atom
Losing of electron
Increase in oxidation no.
Removal of oxygen atom
Addition of hydrogen atom
Gaining of electron
A redox reaction in which an element undergoes oxidation and reduction at the same time.
Decrease in oxidation no.
Disproportionation reaction
the sum of the oxidation no. of all atoms in a species equals the overall charge carried by the species.
no. of proton - no. of electron associated with an atom assuming that all bonding electrons are belonging to the more electronegative atom
Rules of assigning oxidation number
The species on the left of the table are oxidizing agents. The species on the right of the table are reducing agents.
Electrochemical series is constructed by arranging the standard electrode potentials measured in an ascending order. i.e. the most negative value first. By convention, the reduction half equations are tabulated in the ECS. Therefore, standard electrode potential is also known as standard reduction potential.
Use of electrochemical series
Concentration and temperature dependence of electrode potential
Those oxidizing agents at the bottom of the list are strong oxidizing agents while those reducing agents at the top of the list are strong reducing agents.
Dependence on concentration
The values list in an ECS are all measured at standard conditions and all species involved are in their standard states. An increase in concentration of an oxidzing agent will shift the equilibrium position to the right and make the value more positive.
The value is only depending on concentration (including partial pressure in case of gas) and temperature, the size of the electrode has no effect on the value.
An increase in concentration of a reducing agent will shift the equilibrium position to the left and make the value more negative. Nernst Equation (Not required in HKAL syllabus) Q : reaction quotient
To predict the overall e.m.f. of an electrochemical cell
To predict the energetic feasibility of a redox reaction
Compare the strengths of oxidizing agents (or reducing agents) A redox reaction with a positive overall e.m.f. will be energetically feasible.
e.g. disproportionation of Cu+(aq) in water
The feasibility of a reaction is also depending on the kinetic stability (activation energy) of the system which is not related to the overall e.m.f. determined. To ensure the overall e.m.f. of the cell be positive, E(cell) = E(cathode) - E(anode) = E(more positive) - E(less positive)
Primary cell
IUPAC Cell diagram
Electrode potential (of a half-cell)
Half cell
Electrochemical cells
Electrochemical Cells
Electrochemical Series (ECS)
A set of symbols are used to represent the set up of an electrochemical cell.
Symbols used Depending on the emphasis and interpretation of the actual setup, it may be possible to construct more than one IUPAC cell diagram for a given chemical cell.
e.g. Zinc-carbon cell
e.g. Lead acid accumulator
Secondary cell (rechargeable cell)
e.g. Hydrogen-oxygen fuel cell Fuel cell
Some examples
For a given half-cell, no matter it is being used as a left-hand or right-hand electrode, the reduced form should be placed on the left if the reduced form and oxidized form are in the same phase. e.g. Pt(s) | Fe2+(aq) , Fe3+(aq) and Fe2+(aq) , Fe3+(aq) | Pt(s)
20. Electrochemical Cells.mmap - 9/5/2005 -
Definition : The difference between the charge on an electrode and the charge in the solution in which the electrode is immersed in.
Practically, it is not possible to determine the absolute potential of a half cell. Therefore, for the sake of comparison, the electrode potential of standard hydrogen electrode is defined as 0 V.
The value is depending on the activity of the electrons in the electrode
This value is also related to the nature, including concentration, of the electrolyte in which the electrode is immersed in.
By definition, overall e.m.f of a cell = E(right) - E(left). When hydrogen electrode is used as the left-hand electrode, E(right, the unknown electrode) = overall e.m.f. of the cell
The sign of the electrode potential of the unknown half cell is the same as the polarity of the right hand electrode (unknown half cell) in the cell diagram.
In some cases, the solution will be saturated with the salt and with some insoluble salt present. e.g. Ag(s)|AgCl(s)|Cl-(aq)
e.g. Pt(s) | Fe2+(aq) , Fe3+(aq)
e.g. C(graphite) | 2I-(aq) , I2(aq)
Platinized = covered with platinum black = covered with platinum powder
An inert electrode, e.g. Pt, is immersed in a mixture of the reduced and oxidized forms of the reagent concerned. e.g. Fe3+(aq) /Fe2+(aq) , I2(aq)/I-(aq)
The metal concerned will be used as the electrode immersed in a solution containing the salt of the metal. e.g. Cu(s) | CuSO4(aq)
By definition, the electrode potential of an unknown half-cell is the e.m.f. of an electrochemical cell measured at standard condition where a standard hydrogen electrode (SHE) is used as the left-hand electrode and the unknown electrode is used as the right-hand electrode.
Involving metal
Not involving metal
Standard hydrogen electrode
Pt(s) | H2(g) | 2H+(aq)
Use a potentiometer
Use a voltmeter with high impedance
To measure the e.m.f. of a cell, the current flowing through the external circuit should be kept as minimal as possible.
The use of porous partition will create a junction potential across the partition and the voltage measured will be different from the case if a salt bridge is used.
The salt bridge is used to provide cations and anions to balance the negative and positive charge cumulated in the two half cells during the discharge of the cell.
Only KNO3 and NH4Cl are used in the salt bridge since the migration speed of the cation and anion are similar and the voltage reading will not be affected.
The electrodes of the two half cells are connected together by an external wire.
The electrolytes of the two half cell are connected together by using a salt bridge.
In some electrochemical cell, a porous partition is used to separate the two electrolytes instead of using a salt bridge.
Measurement of e.m.f of an electrochemical cells.
Phase
Definition : region with identical properties. Phase is not the same as physical state. Usually, there are only 3 physical states i.e. solid, liquid and gas. e.g. Diamond and graphite are both in solid state but they are two different phases of carbon. Consider an ideal gas obeying ideal gas law PV=nRT, for a given amount of gas, P is a function of V and T. i.e. P(V,T).
By plotting a graph using P as the z-axis, V as the x-axis and T as the y-axis, a PVT surface of an ideal gas can be constructed.
PVT surface of of a real substance e.g. water
The isotherms of an ideal gas shows that no matter how low the temperature is, the ideal gas is still a gas and the volume is always inversely proportional to pressure.
Isotherms
The projection along the temperature axis (Isotherms)
Ideal gas
Isotherms
Real substance e.g. water
Real gas doesn't obey Boyle's law at low temperature. Real gas starts condensing at a temp. lower than the critical temp. of the substance.
Phase Equilibrium I : One-component system Isochors
PVT surface
As it is difficult to study a 3-dimensional surface, the projections along the volume axis and the temperature axis are usually studied separately.
Ideal gas
The isochors of an ideal gas shows that no matter how low the temperature, the ideal gas is still a gas and has zero volume at 0 K.
The diagram is known as phase diagram. critical temp : above this temperature, gas cannot be compressed into liquid without cooling. AB : sublimation curve - conditions that solid and vapour can coexist at equilibrium. BC : vaporization / boiling curve - conditions that water and vapour can coexist at equilibrium. BD : fusion / melting curve - conditions that solid and liquid can coexist at equilibrium. B : triple point - the conditions that solid, liquid and vapor can coexist at equilibrium. BE : supercooling curve - the condition that a liquid exists at a temp. lower than its melting point and the liquid is at a metastable state. This is because freezing also requires activation energy. If a liquid is cooled very calmly, the liquid may become supercooled without solidification. Real gas condenses at low temperature and becomes liquid and solid.
Phase diagram of water
Real substance e.g. water
The projection along the volume axis (isochors)
Special features of the phase diagram of water
Special features of the phase diagram of carbon dioxide
Special features of the phase diagram of carbon
21.Phase Equilibrium I . One-component system.mmap - 2005/5/9 -
The fusion curve of water has a negative slope. This implies that the melting point of water decreases with increasing pressure. When a great pressure is applied to ice, the open structure of ice will collapse and the ice will be turned to water. According to the Le Chatelier's principle, when a pressure is applied to an equilibrium system, the system will react in a way to minimize the effect of the change imposed. For ice, it will turn to water which has a smaller volume to lower the increase in pressure.
Like most substances, the fusion curve of carbon dioxide has a positive slope. This implies that the melting point of carbon dioxide increases with increasing pressure. The triple point pressure is higher than atmospheric pressure. This implies that dry ice will sublime directly to gaseous carbon dioxide without going through the liquid state.
It is possible to convert graphite to diamond by applying pressure to graphite. However, the rate of conversion is very slow as this involves rearrangement of atoms in solid state. If graphite is heated to molten state and allowed to crystallize under high pressure, artificial diamond can be made. Moreover, the crystallization is very fast comparing with the natural process of formation of diamond, the diamond crystal obtained will be small and may not be suitable for making jewelry.
The components cannot be separated by fractional distillation completely.
Positive deviation
Extreme negative deviation (a solution of involatile solute)
Negative deviation
Extreme positive deviation (2 immiscible liquids)
Azeotropic mixture (or constant boiling mixture) - A mixture with a constant composition when it is being boiled continuously. i.e. the composition of the liquid mixture = the composition of the vapour The 2 components vaporize independently and give a total vapour pressure higher than that of either component.
The lowering of vapour pressure will cause depression in melting point and elevation of boiling point of the solvent.
22. Phase Equilibrium II . Two-Component System.mmap - 19/5/2005 -
Deviation from Raoult's Law (non-ideal solutions)
Difference between 1-component and 2-component systems
Phase Equilibrium II : Two-Component System
Raoult's Law
For a 1-component system, the (saturated) vapour pressure (P) of a substance is a constant at a given temperature.
In a 1-component system, the properties of a substance is a function of P and T. This requires a 3-dimensional space to represent. Instead of studying the problem in a 4-dimensional space, the study will be limited to the case in which a liquid and its saturated vapour coexist in equilibrium.
In a 2-component system, the properties of a substance will be depending on P, T and X (mole fraction) which will need a 4-dimensional space to represent.
For a 2-component system, the (saturated) vapour pressure at a given temperature will be a function of X (mole fraction) of the component present.
The boiling point of an ideal solution varies linearly with the composition approximately.
Partition of a solute between 2 phases
Phase Equilibrium III : Three-Component Systems
23. Phase Equilibrium III . Three-Component Systems.mmap - 2005/5/23 -
Solvent Extraction (between 2 immiscible solvents)
Partition of a solute between 2 immiscible solvents
Partition Law - At a given temp. and at a state of equilibrium, the ratio of concentrations of a solute in two immiscible solvents is constant.
At equilibrium, the rates of diffusion of the solute between two immiscible solvents in opposite directions are the same.
As the unit of concentration will be canceled out in the calculation, the unit can be expressed in any unit and partition coefficient will have no unit eventually.
Most common organic solvent are lighter than water except chloroform(CHCl3), tetrachloromethane(CCl4) and 1,1,1-trichloroethane
e.g. The partition law is not applicable to the case where ethanoic acid is partitioned between hexane and water as ethanoic acid exists as dimmer in hexane (non-polar solvent) and as monomer in water (polar solvent)
If the density of the solvents are not known, when we say solute A is partitioned between solvent B and C, it usually means
The law is only application to the case if the solute has the same molecular structure in the two solvents.
Partition of a solution between a stationery phase and a mobile phase
Mobile phase
Stationery phase
e.g. different kind of solvent or mixture of solvents
e.g. silica gel on an aluminium foil
e.g. the microscopic water film on the surface of the chromatography paper
It would be more effective to carry out a solvent extraction in successive extractions, a small portion at a time, for a given amount of solvent.
Paper Chromatography (between a stationery phase and a mobile phase)
At a given temperature, with a given chromatography paper and a given solvent, the solute being analyzed is characterized by its Rf value
Analysis of amino acids - A mixture of colourless amino acids can be separated by paper chromatography. The positions of amino acids spot in the chromatograph can be detected by spraying with ninhydrin (a developer), which reacts with amino acids to yield highly coloured products (usually purple in colour).
