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Question 13.15: Applying Le Châtelier’s Principle to Temperature Changes In ......

Applying Le Châtelier’s Principle to Temperature Changes

In the first step of the Ostwald process for the synthesis of nitric acid, ammonia is oxidized to nitric oxide by the reaction

4 \mathrm{NH}_3(g)+5 \mathrm{O}_2(g) \rightleftharpoons 4 \mathrm{NO}(g)+6 \mathrm{H}_2 \mathrm{O}(g) \quad \Delta H^{\circ}=-901.2 \mathrm{~kJ}

How does the equilibrium amount of NO vary with an increase in temperature?

STRATEGY

Le Châtelier’s principle predicts that the stress of added heat when the temperature is increased will be relieved by net reaction in the direction that absorbs the heat. It’s helpful to include the heat in the balanced equation—on the reactant side if the reaction is endothermic, or on the product side if the reaction is exothermic.

Step-by-Step
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Because the oxidation of ammonia is exothermic, we include the heat (901.2 kJ) on the product side:

4 \mathrm{NH}_3(g)+5 \mathrm{O}_2(g) \rightleftharpoons 4 \mathrm{NO}(g)+6 \mathrm{H}_2 \mathrm{O}(g)+901.2 \mathrm{~kJ}

The stress of added heat when the temperature is increased will be relieved by net reaction from products to reactants, which absorbs the added heat. The equilibrium will therefore shift to the reactant side (K_c will decrease) with an increase in temperature. Consequently, the equilibrium mixture will contain less NO at higher temperatures.

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