Question

Dihydrogen gas is obtained from natural gas by partial oxidation with steam as per following endothermic reaction:

CH_4 (g) + H_2O (g) \rightleftharpoons CO (g) + 3H_2 (g)

(a) Write as expression for Kp for the above reaction. (b) How will the values of Kp and composition of equilibrium mixture be affected by (i) increasing the pressure (ii) increasing the temperature (iii) using a catalyst?

Accepted Answer

(a) Kₚ = p CO × (p H 2 )³p CH 4 × p H 2O (b i) Increasing the pressure shifts the equilibrium in the reverse direction because the number of moles of gas increases in the forward direction. The value of Kp remains unchanged. (b ii) Since the reaction is endothermic, increasing the temperature shifts the equilibrium in the forward direction. The value of Kp increases. (b iii) Using a catalyst does not affect the equilibrium composition or the value of Kp. It only helps in attaining equilibrium faster. — The expression for Kp is derived from the partial pressures of the gaseous products and reactants. According to Le Chatelier's principle, increasing pressure shifts equilibrium towards the side with fewer gaseous moles (reverse direction here). For an endothermic reaction, increasing temperature adds heat, acting as a reactant, thus shifting equilibrium forward and increasing Kp. A catalyst speeds up both forward and reverse reactions equally, leaving Kp and equilibrium composition unaffected.

Explanation

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