# Applications of Le-Chatelier's Principle

I. In the Study of Physical Equilibrium

(A) Effect of temperature on solubility: Some solids absorb heat while some evolve heat on dissolution. Hence according to this principle solubility of the former class of solids increases with rise of temperature.

For example:

(i) $KNO_3 + aq. \leftrightharpoons KNO_3 (aq.) - Q \hspace{3mm}kcal \hspace{3mm}mol^{-1} (\text{Endother} . )$

(ii) $Ca(OH)_2 + aq. \leftrightharpoons Ca(OH)_2 (aq.) + Q \hspace{3mm}kcal \hspace{3mm}mol^{-1} (\text{Exother}.)$

With rise of temperature solubility of $KNO_3$ increases while that of $Ca(OH)_2$ decreases.

(B) Effect of pressure on solubility: Since on dissolution of gas volume decreases, hence on increasing pressure, solubility of gas increases. On the other hand, if volume of the solution increases on dissolution of solid then solubility of the solid decreases with rise of pressure.

On operating carbonated water (soft drinks) $CO_2$ comes out, due to decrease of pressure, it’s solubility decreases.

(C) Effect of temperature and pressure on melting of ice: Since ice melts with absorption of heat and decreases in volume, hence both temperature and pressure effect the melting of ice. Since the change of ice into water is an endothermic process hence with rise of temperature ice melts into water. Since volume of ice is more than that of water so increase of pressure favour melting.

$\text{Ice} \leftrightharpoons \text{Water}- Q kcal \hspace{3mm} mol^{-1}$

(D) Effect of temperature and pressure on vapourization of water: When water changes into vapour heat is absorbed hence rise of temperature is favourable but in this process volume of vapour increases hence rise of pressure is not favourable for forward reaction.

$\text{Water} \leftrightharpoons \text{Vapours}- Q kcal \hspace{3mm} mol^{-1}$

II. In the study of Chemical Equilibrium

(A) Formation of ammonia (Haber's Process): The reaction is as follows:

$\underset{1 vol.}{N_2 (g)} + \underset{3 vol.}{3H_2 (g)} \leftrightharpoons \underset{2 Vol.}{2NH_3 (g)} + 23 kcal \hspace{3mm} mol^{-1}$

Favourable conditions for forward reaction are:

(i) high pressure

(ii) low temperature and

(iii) excess of $N_2 \text{and} H_2$.

(B) Formation of nitric oxide:

$\underset{1 Vol.}{N_2 (g)} + \underset{1 Vol.}{O_2 (g)} \leftrightharpoons \underset{2 Vol.}{2NO}- 43.2 \hspace{3mm}kcal \hspace{3mm} mole^{-1}$

Favorable conditions for forward reaction are :

(i) No effect of pressure

(ii) high temperature and

(iii) excess of $N _2 \text{and} _O2$.

(C) Formation of sulphur trioxide:

$\underset{2 Vol.}{SO_2 (g)} + \underset{1 Vol.}{O_2 (g)} \leftrightharpoons \underset{2 Vol.}{2SO_3}- 42.0 \hspace{3mm} kcal \hspace{3mm} mole^{-1}$

Favourable conditions for forward reaction are:

(i) High pressure

(ii) low temperature and

(iii) excess of $SO_2 \hspace{3mm} \text{and} \hspace{3mm} O_2$.

(D) Formation of nitrogen peroxide:

$\underset{2 vol.}{NO (g)} + \underset{1 vol.}{O_2 (g)} \leftrightharpoons \underset{2 Vol.}{2NO_2(g)} + 27.8 \hspace{3mm} kcal \hspace{3mm} mol^{-1}$

Favourable conditions for forward reach” on are as in the case (C).

(E) Oxidation of carbon monoxide by steam:

$CO (g) + H_2O (g) \leftrightharpoons CO_2 (g) + H_2 (g) + X \hspace{3mm} kcal \hspace{3mm} mol^{-1}$

Favourable conditions for forward reaction are:

(i) no effect of pressure

(ii) low temperature and

(iii) excess of steam and CO.

(F) Dissociation of dinitrogen tetraoxide:

$\underset{1 vol}{N_2 O_4 (g) } \leftrightharpoons \underset{2 Vol}{2NO_2 (g)} - 14 \hspace{3mm} kcal \hspace{3mm} mol^{-1}$

Favourable conditions for forward reaction are:

(i) low pressure

(ii) high temperature and

(iii) excess of $N_2O_4$.

(G) Dissociation of phosphorus pentachloride:

$\underset{1 vol.}{PCl_5 (g)} \leftrightharpoons \underset{1 vol.}{PCl_3 (g)} \underset{1 Vol.}{Cl_2 (g)} + X \hspace{3mm} kcal \hspace{3mm} mol^{-1}$

Favourable conditions for forward reaction are :

(i) low pressure

(ii) high temperature and

(iii) excess of $PCl_5$.

Continuous removal of gaseous product favors formation of more products.

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