Question 15.5: Write the equilibrium expression Kc for (a) CO2(g) + H2(g) ⇌...

Chemistry: The Central Science in SI Units, Expanded Edition

Write the equilibrium expression $K_{c}$ for

(a) $CO_{2} (g) + H_{2} (g) ⇌ CO (g) + H_{2}O (l)$

(b) $SnO_{2} (s) + 2 CO (g) ⇌ Sn (s) + 2 CO_{2} (g)$

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Analyze We are given two chemical equations, both for heterogeneous equilibria, and asked to write the corresponding equilibrium expressions.
Plan We use the law of mass action, remembering to omit any pure solids and pure liquids from the expressions.

Solve
(a) The equilibrium expression is

K_{c} =\frac{[CO]}{[CO_{2}][H_{2}]}

Because $H_{2}O$ appears in the reaction as a liquid, its concentration does not appear in the equilibrium expression.

(b) The equilibrium expression is

K_{c} =\frac{[CO_{2}]^2}{[CO]^2}

Because $SnO_{2}$ and Sn are pure solids, their concentrations do not appear in the equilibrium expression.

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