Short Answer Type
Long Answer TypeHow can the lattice enthalpy of an ionic NaCl be determined by using Born-Haber cycle?
Calculate the lattice enthalpy of KCl crystal from the following data:
Sublimation enthalpy of pottasium (K) = +89 kJ mol-1
Dissociation enthalpy of 
= +122 kJ mol-1
Ionisation enthalpy of K(g) +425 kJ mol-1
Electron gain enthalpy of Cl(g) = -355 kJ mol-1
Enthalpy of formation of 
Calculate the lattice enthalpy of LiF, given that the enthalpy of
(i) sublimation of lithium is 155.2 kJ mol–1
(ii) dissociation of 1/2 mol of F2is 75.3 kJ
(iii) ionization of lithium is 520 kJ mol–1
(iv) electron gain of 1 mol of F(g) is –333 kJ
(v) ∆f H0 ∆fH0 overall is –795 kJ mol–1
Short Answer TypeCalculate the lattice enthalpy of MgBr2 from the given data. The enthalpy of formation of MgBr2 according to the reaction
Long Answer TypeThe tendency of a system to acquire a state of maximum randomness is the sole criterion for the spontaneity of a process. Comment.
The overall tendency of a process to occur can be expressed on the resultant of two tendencies namely:
(i) the tendency to acquire a state of minimum energy, and
(ii) the tendency to acquire a state of maximum randomness or disorder.
The overall tendency of a process to take place by itself is called the driving force.
It should be noted that:
(i) the two tendencies act independent of each other,
(ii) the two tendencies may work in the same direction or opposite direction in a process and
(iii) the driving force is the resultant of the magnitude of the two tendencies. When the two tendencies act in the opposite direction, the tendency with the greater magnitude determines whether the process is feasible or not. For example,
(a) Evaporation of water. Evaporation of water is endothermic, therefore, energy factor opposes the process. But it is favoured by randomness factor.
Since the process is known to take place, randomness factor must be greater than energy factor.
(b) The reaction between hydrogen and oxygen to form water. It is an exothermic reaction, therefore, favours the process, but randomness factor opposes the reaction.
As the reaction takes place, the energy factor must be greater than the randomness factor.
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