Question

When the following reactions come to equilibrium, does the equilibrium mixture contain mostly reactants or mostly products? (a) ${\mathrm{N}}_2(g)+{\mathrm{O}}_2(g)\rightleftharpoons 2\mathrm{NO}(g);K_c=1.5\times {10}^{-10}$ (b) $2{\mathrm{SO}}_2(g)+{\mathrm{O}}_2(g)\rightleftharpoons 2{\mathrm{SO}}_3(g);K_p=2.5\times {10}^9$

Short answer

For reaction (a), the equilibrium mixture will contain mostly reactants (N₂ and O₂), as $K_c=1.5\times {10}^{-10}<1$. For reaction (b), the equilibrium mixture will contain mostly products (SO₃), as $K_p=2.5\times {10}^9>1$.

Step-by-step solution

Understand the equilibrium constant

The equilibrium constant (K) represents the ratio of the concentration of products to the concentration of reactants at equilibrium. If K > 1, the equilibrium mixture will contain mostly products; if K < 1, the equilibrium mixture will contain mostly reactants. (a) $N_2(g)+O_2(g)\rightleftharpoons 2NO(g);K_c=1.5\times {10}^{-10}$

Evaluate K for Reaction (a)

Since the equilibrium constant $K_c=1.5\times {10}^{-10}<1$, the equilibrium mixture will contain mostly reactants. In this case, the mixture will primarily consist of N2 and O2. (b) $2S{O}_2(g)+O_2(g)\rightleftharpoons 2S{O}_3(g);K_p=2.5\times {10}^9$

Evaluate K for Reaction (b)

For the second reaction, the equilibrium constant $K_p=2.5\times {10}^9>1$. This means that the equilibrium mixture will contain mostly products. In this case, the equilibrium mixture will mostly consist of SO3.