Question

Manufacture of ammonia from the elements is represented by
$N_2\left(g\right)+3H_2\left(g\right)\rightleftharpoons 2{NH}_3\left(g\right)+22.4kcal$
The maximum yield of ammonia will be obtained when the process is made to take place:

• A. At low pressure and high temperature
• B. At low pressure and low temperature
• C. At high pressure and high temperature
• D. At high pressure and low temperature

Answer 1

The correct option is D At high pressure and low temperature

Manufacture of ammonia from the elements is represented by:
$N_2\left(g\right)+3H_2\left(g\right)\rightleftharpoons 2{NH}_3\left(g\right)+22.4kcal$
The maximum yield of ammonia will be obtained when the process is made to take place at high pressure and low temperature.
High pressure is maintained because the moles of the product are less than the moles of reactants.
$N_2\left(g\right)+3H_2\left(g\right)\rightleftharpoons 2{NH}_3\left(g\right)$
$\Delta n_g=2-(1+3)=-2$
$\Delta n_g<0$
When pressure is increased, the equilibrium will shift to product side (which contains less number of moles of gaseous species). This nullifies the effect of increase in pressure. Hence, more and more ammonia will be formed.
According to Le Chatelier's principle, when a system at equilibrium is disturbed, the position of the equilibrium shifts in a direction so that the effect of the change is nullified.
Low temperature favours exothermic reaction. Heat is evolved during the reaction (positive value of enthalpy change).