Chemistry Assignments

1. The Born–Landé equation  is a means of calculating the lattice energy of energy of a crystalline ionic compound.. In 1918[1] Max Born and compound and Alfred  Alfred and! proposed and! proposed that the lattice energy could "e deri#ed from the electrostatic potential of the ionic lattice and a repulsi#e potential energy term.[$]

%here& •

 ' A#ogadro  A#ogadro N  A '

constant( constant(

•

 ' M  '

Madelung constant) constant) relating to the geometry of the crystal(

•

z * '

charge num"er of cation

•

z + '

charge num"er of anion

•

e '

elementary charge) charge) 1.,-$$1-+19 /

•

ε - '

permitti#ity of free space

0ε - ' 1.11$1-+1- /$2345m6 •

r - '

distance to closest ion

•

n '

Born exponent) typically a num"er "et%een 7 and 1$) determined experimentally " y

measuring the compressi"ility compressi"ility of  of the solid) or deri#ed theoretically

Limitation of the Radius Ratio Rule  Although radius is a useful concept of determining coordination num"er and in understanding the structure of ionic compounds yet there are a fe% limitations to this rule. 3i6

ach ion is considered as a hard hard sphere for determining the optimum arrangement of ions in the crystal

lattice. This is far form reality and serious errors can "e made if anions get polaried and the "onding the intermediate 3partially co#alent6 in character. 3ii6

:ome compounds compounds may crystallie crystallie in more than one modification %ith different coordination num"ers. In

such case) anion;anion repulsions and hence intern clear distance %ould "e different. 3iii6

ffecti#e radius of a cation cation is greatly greatly influenced "y the anions anions %ith the conse
changes. =or example) Ag= and >a= crystallie out in > a/I type of structure %ith coordination >o. , and if %e assume that the sie of =; remains constant than Ag* is "igger than >a*. ?n the other hand) for a gi#en sie for a chloride ion in Ag/I and >a/I) the sies of cations are re#ersed) that is) >a* is "igger than Ag*. It is also true for their "romides. This is understanda"le "ecause Ag* is softer than >* and introduces relati#ely more co#alent character %ith @olaria"le anions lie /I ; and Br ;

•

 Metal Excess Defects. The Colour Centres.

It has been observed that if a crystal of NaCl is heated in sodium vapour, it acquires a yellow colour.

This yellow colour is due to the formation of a non-stoichiometric compound of sodium chloride in which there is a slight

excess of sodium ions.

hat

sodium

happens

in

this

case

is

that

some

metal

gets

doped

into

sodium

chloride crystal which, due to the crystal energy, gets ionised into Na! and e". This electron occupies a site that would otherwise be filled by a chloride ion, as illustrated in figure.

There is evidently an excess of metal ions although the crystal as a whole is neutral. # little reflection would show that there are six Na! sites ad$acent to the vacant site occupied by the electron.

The extra electron in thus shared between all the six Na! ions which implies that this electron is not localised at the vacant Cl" site. %n the other hand, this electron is similar to the delocalised p electrons present in molecules containing con$ugate double bonds.

&ight is absorbed when this delocalised electron ma'es an easy transition from its ground state to an excited state. #s a result, the non " stoichiometric form of sodium chloride appears coloured  . (ecause of this, the sites occupied by the extra electrons are 'nown as colour centres. These are also called F-centres. This name comes from the )erman word Farbe meaning colour.

The

non-schiometric

sodium

chloride

may

be

represented

by

the

formula

Na*+!dCl

where d is the excess sodium metal doped in the crystal because of its exposure to sodium vapour.

#nother coloured

common example

of

metal

non-stoichiometric

excess defects

compound

crystals of Cl to  metal vapour.

of

is

potassium

the formation of chloride

by

a

magenta

exposing

the

The coloured compound contains an excess of ! ions, the vacant Cl" sites being filled by electrons obtained by the ioniation of the excess  metal doped in to the crystal.

Uses of Acetylene: Acetylene is used as a key component in low pressure carburizing. It provides the precise carbon source automotive manufacturers need to create a strong, wear-resistance surface layer on steel parts.