Magnitude of the Equilibrium Constant (College Board AP® Chemistry)

C₂H₄ (g) ⇌ C₂H₂ (g) + H₂ (g)                K_P = 1.54 x10^-7

Pure C₂H₄ (g) is placed in a 1.0 L rigid, sealed flask at 700 K and allowed to react according to the equation shown above. Which of the following statements correctly compares the partial pressures of C₂H₄ (g) and H₂ (g) after equilibrium is established?

1.  , because the smaller the K_P value, the greater the partial pressure of the reactants at equilibrium

2.  , because at equilibrium the partial pressures of the products and reactants must be constant and equal

3.  , because the initial rate of the forward reaction is greater than that of the reverse reaction leading to an increase in the partial pressure of the products at equilibrium

4.  , because the smaller the K_P value, the faster the rate of reaction and the greater the partial pressure of the products at equilibrium

Answer:

• At equilibrium, the partial pressures of the products and reactants must remain constant, but they do not necessarily have to be equal

• A very small K_P value indicates that the forward reaction barely proceeds and that the reactants are favored at equilibrium

• So at equilibrium, the flask will container a much greater proportion of C₂H₄ (g) than H₂ (g)

• As the gases occupy the same volume and are held at the same temperature, the partial pressure of C₂H₄ (g) at equilibrium will be greater than that of H₂ (g)

Into two separate flasks, a solution containing 1.0 M OH^-(aq) is combined with a solution containing 1.0 M of the appropriate metal ion as shown in the table below. The ions react via an equilibrium reaction to form a complex ion.

 After reaching equilibrium at 25°C, which flask contains a higher concentration of its respective complex ion?

Answer:

• A very large K_c value indicates that the forward reaction essentially proceeds to completion

• The larger the K_c value, the greater the proportion of products in the equilibrium mixture

• As both flasks initially contain equimolar amounts of the reactants and the reaction in flask A has a larger K_c value, the concentration of the complex ion in flask A at equilibrium will be greater than that of flask B

If you forget which side of a reaction is favored based on the value of K you can perform a quick test using simple concentrations. This is summarized in the table below.