 ## Q. 13.16

Applying Le Châtelier’s Principle to Temperature Changes

The following pictures represent the composition of the equilibrium mixture at 600 K and 650 K for the combination of two A molecules, $2 \mathrm{~A}(g) \rightleftharpoons \mathrm{A}_2(g)$:

Is the reaction endothermic or exothermic? Explain using Le Châtelier’s principle.

STRATEGY

We can determine the direction of net reaction on raising the temperature by counting the number of $A and \mathrm{A}_2$ molecules at each temperature. According to Le Châtelier’s principle, if the net reaction converts reactants to products on raising the temperature, heat is on the reactant side of the chemical equation and the reaction is endothermic. Conversely, if the net reaction converts products to reactants on raising the temperature, heat is on the product side and the reaction is exothermic ## Step-by-Step

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Two A and five $A_2$ molecules are present at 600 K, and six A and three $A_2$ molecules are present at 650 K. On raising the temperature, the net reaction converts products to reactants so heat is on the product side of the chemical equation:

$2 \mathrm{~A}(g) \rightleftharpoons \mathrm{A}_2(g)+\text { heat }$

The reaction is therefore exothermic, as expected for a reaction in which a chemical bond is formed. Note that Le Châtelier’s principle predicts that net reaction will occur in the direction that uses up the added heat.

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