**Writing Equilibrium Equations for Gas-Phase Reactions **

Write the equilibrium equation for each of the following reactions:

(a) N_2(g) + 3 H_2(g) \rightleftharpoons 2 NH_3(g)

(b) 2 NH_3(g) \rightleftharpoons N_2(g) + 3 H_2(g)

**STRATEGY **

Put the concentrations of the reaction products in the numerator of the equilibrium-constant expression and the concentrations of the reactants in the denominator. Then raise the concentration of each substance to the power of its coefficient in the balanced chemical equation.

Step-by-Step

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\text { (a) } K_{\mathrm{c}}=\frac{\left[\mathrm{NH}_3\right]^{2 \ \leftarrow \ Coefficient \ of \ NH_3}}{\left[\mathrm{~N}_2\right]\left[\mathrm{H}_2\right]^{3 \ \leftarrow \ Coefficient \ of \ H_2}}

(b) K_{\mathrm{c}}{ }^{\prime}=\frac{\left[\mathrm{N}_2\right]\left[\mathrm{H}_2\right]^{3 \ \leftarrow \ Coefficient \ of \ H_2}}{\left[\mathrm{NH}_3\right]^{2 \ \leftarrow \ Coefficient \ of \ NH_3}} \ \ \ \ \ \ \ \ K_c{ }^{\prime}=\frac{1}{K_c}

Because the balanced equation in part (b) is the reverse of that in part (a), the equilibrium-constant expression in part (b) is the reciprocal of the expression in part (a) and the equilibrium constant K_{c}^{ʹ} is the reciprocal of K_c

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