V. Bonding in compounds
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Topic
V.
Bonding in compounds
Part 1
Reference Reading
Integrated Chemistry Today (2nd Ed.), L.H.M Chung, Book 1A, pg 72\u201374
Objectives
1.5.0.1, 1.5.1.1 \u2013 plan and describe experiments to establish that (a) compounds made of metals and non-metals do not conduct electricity in the solid state but may do s when melted or dissolved in water with decomposition occurring at the electrodes (b) compounds made of non-metals are in general non-conductors of electricity even when melted or dissolved in water \u2013 deduce the existence of charged particles called ions in solutions of compounds made of metals an (electrolytes) from electrolysis experiments \u2013 recognise the existence of neutral particles called molecules in compounds made of non-metals only fact that these compounds cannot conduct electricity even when melted or dissolved in water (nonelectrolytes)
Notes
V. Bonding in compounds
According to particle theory of matter, all matters are made up of tiny particles. And the properties of a Particles substance is depending on the interactions (attractions) among the particles. For example, at a certain temperature, if the attractions among the particles are very strong, the substance will have a solid structure with various properties. It will be hard, strong, with high melting point and high boiling point.
Attractions
Properties and Structure
Obviously, if the substances consist of different particles and involve different attractions, the properties and structure of the substance will also be different.
However, particles and the attractions among them cannot be observed directly, the presence of different kin particles and attractions can only be deduced through measuring the properties of different substances.
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Through these observations, chemists found that basically all substances are made up of atoms, molecules or ions. And they are all comprised of fundamental substomic particles. Order of discovery Atom \u2013 smallest unit of an element. \u2013 1803 by Dalton John Molecule \u2013 group of atoms / a particle which Atom can exist Molecule \u2013 1858 by Stanislao Cannizzaro on its own. Ion \u2013 1833 by William Whewell Ion \u2013 charged atom or molecule. Electron \u2013 1897 by Joseph John Thomson
If the presence of different particles are taken into consideration, the attractions among different particles w become more complicate.
Particles
atom ion molecule
Attractions
atom - atom (covalent bond or metallic bond) ion - ion (ionic bond) molecule - molecule (van der Waals' forces)
Properties and Structure
This also leads to the discovery of different kinds of attractions among different particles. Some of them are e.g. covalent bond, ionic bond and metallic bond. Some of them are relatively weaker e.g. van der Waals' for
1. 2. 3. 4.
Covalent bond e.g. in water H2O, in hydrogen H2; in chlorine Cl2 Ionic bond e.g. in sodium chloride (common salt) Na+ClMetallic bond e.g. in copper Cu Van der Waals' forces e.g. among water molecules
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Electrostatic nature of bondings
Normally, two table tennis balls don't attract or repel each other. However, if both are charged up negatively usin the static charge generator, they will repel each other. Conversely, if one is charged up negatively and another is charged up positively, they will attract each other.
Unlike charges attract each other and like charges repel each other . Basically, all bondings (attractions) studied in chemistry are electrostatic in nature. Conduction of electricity
- an incident leads to discovery of ion (charged particle) and molecule (uncharged particle)
Indeed, electrostatic charge and electric current are the same thing. If the charges (either positive or negati not moving, they are called electrostatic charges. If the charges are moving, it is known as electric current.
Among all the substances in the world, only very few are conductors of electricity. Some of them are in solid (e.g. metal and graphite) and some of them are in liquid form (e.g. sea water) But why do they conduct electricity and what do a solid conductor and a liquid conductor share in common ?
A good conductor must allow electric current to flow through, i.e. allowing electrical charges, either positive negative, to flow through. A particle carrying charges is called a charge carrier. e.g electron. If charge carriers can flow through a substance, the substance is called a conductor. In order to conduct electricity, the charge carriers inside the substance must be free to move. Just the presence of charge carriers would not be enough, for example, all substances contain electrons which are negatively charged but not all substances are conductors.
Electric current (Flowing of charges)
Charges (Mobile charge carriers, Conductor The particles which are free to move and carrying either positive or negative) charges are called mobile charge carriers. e.g. free electron in metal, mobile ions (charged particle in liquid electrolyte).
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A. Electrolyte and non-electrolyte
In metal, the mobile charge carriers are known to be free electrons. Besides metal only very few solids cond electricity, graphite (a kind of carbon) is one of these exceptions. Graphite also contains free electrons.
In spite of this, a lot of compounds conduct electricity in solution form or molten form only. e.g. salt solution is a kind of good conductor of electricity. However, salt conducts electricity only in aqueous or in molten state but not in solid state. Scientists called this class of compounds electrolytes. (electro - electricity, lyte - liquid)
In contrast, the compounds which don’ t conduct electricity in aqueous state or in molten state are called no electrolytes.
The process of conduction in electrolyte is also called electrolysis (electro - electricity, lysis - breaking down electrolyte because the electrolyte will decompose at the same time. Electrolyte - A compound which conducts electricity only in aqueous state or in molten state. Non-electrolyte - A compound which does not conduct electricity in any state.
The followings are examples of electrolyte and non-electrolyte. Electrolyte Substance
Non-electrolyte
Constituent elements
sodium chloride (commonsodium, chlorine salt) lead(II) bromide lead, bromine copper(II) chloride
Substance
Constituent elements
sugar
carbon, hydrogen, oxygen
water (pure)
hydrogen, oxygen
copper, chlorine
In general, electrolyte consists of metal and non-metal while non-electrolyte consists of only non-metals.
N.B.
1. 2.
Not all electrolytes consist of metal and non-metal only, for example, hydrogen chloride (HCl) is electrolyte but it consists of only non-metals. (will be studied in Form 4) Pure water is a poor conductor of electricity, water conducts electricity only by dissolving an electrolyte in it e.g. salt in salt solution.
Electrolyte and non-electrolyte consist of different kinds of particles.
Scientists called the particles responsible for conductivity of electricity in electrolyte - ions (charged particle Since the ions are free to move in aqueous or molten electrolyte, they are also known as mobile ions. And the particles in non-electrolyte are called molecules (uncharged particle). Conclusively, different conductors may contains different mobile charge carriers. Conductor
Mobile charge carrier
All kinds of metal
free electron
Graphite
free electron
Salt solution
mobile positive and negative ion
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Conductivity of an electrolyte and its physical state
Aqueous or molten sodium chloride is a conductor but solid sodium chloride crytal is not. No matter in aqueo state, molten state or solid state, sodium chloride consists of the same kinds of particles (ions).
Therefore, the difference in conductivity must be caused by the difference in attractions. In solid state, the io attracted strongly with each other and not able to move. This does not allow solid sodium chloride to conduc electricity. Solid sodium chloride (non-conductor)
Glossary
Past Paper Questions
Sodium chloride solution (conductor)
Molten sodium chloride (conductor)
particle theory of matter particles attractions properties structure atom ion molecule fundamental subatomic particles covalent bond metallic bond ionic bond van der Waals' forces conductivity electric current mobile charge carrier electrolyte non-electrolyte electrolysis mobile ions
