Question

Explain why high pressure is used in the manufacture of ammonia by Haber's process. State the law or principle used.

Answer 1

In Haber's process ammonia is manufactured using the reversible reaction:
$\begin{array}{cccccc}{N}_2& +& 3H_2& \rightleftharpoons & 2N{H}_3& \\ a& & b& & c& \text{(initial no. of moles)}\\ (a-x)& & (b-3x)& & 2x& \text{(No. of moles at equilibrium)}\\ \underset{–}{a-x}& & \underset{–}{b-3x}& & \underset{–}{2x}& \text{(Molar Conc. at equilibrium)}\\ \nu & & \nu & & \nu & \end{array}$
Applying law of Mass Action
$K_c=\frac{[N{H}_3{]}^2}{\left[N_2\right][H_2{]}^3}$
$=\frac{{\left(\frac{2x}{v}\right)}^2}{\left[\left(\frac{(a-x)}{v}\right)\right]\times {\left[\left(\frac{(b-3x)}{v}\right)\right]}^3}$

$=\frac{4x^2{v}^2}{(a-x)(b-3x{)}^3}$
On increasing pressure volume $v$ will decrease, hence, $x$ will increase to maintain value of $K_c$ constant. Or in other words an increase in pressure will favour formation of ammonia. This is in accordance with Le Chateliers principle the law states.
'If a system at equilibrium is subjected to a change which displaces it from equilibrium, then equilibrium shifts itself in such a direction that the effect of change is reduced or nullified.'