Q: For the reaction $CO(g) + 2H_2(g) \rightleftharpoons CH_3OH(g)$, $K_c = 10.2$ at 500 K. If the initial concentrations are $[CO] = 0.50\,M$, $[H_2] = 1.00\,M$, and $[CH_3OH] = 0.10\,M$, what is the value of the reaction quotient $Q_c$ and in which direction will the reaction proceed to reach equilibrium?

$Q_c = \frac{[CH_3OH]}{[CO][H_2]^2} = \frac{0.10}{0.50 \times (1.00)^2} = 0.20$; the reaction will proceed to the right (toward products).

Q: A reaction mixture at equilibrium contains $[A] = 0.20\,M$, $[B] = 0.30\,M$, and $[C] = 0.40\,M$ for the reaction $A(g) + 2B(g) \rightleftharpoons C(g)$. What is the value of $K_c$?

$K_c = \frac{[C]}{[A][B]^2} = \frac{0.40}{0.20 \times (0.30)^2} = 22.2$

Question 1

For the reaction $\mathrm{N_2O_4(g) ightleftharpoons 2NO_2(g)}$, the equilibrium constant $K_c$ at 25°C is $4.6 	imes 10^{-3}$. Which of the following statements best describes the composition of the equilibrium mixture?

Accepted Answer

The mixture contains mostly $N_2O_4$ with very little $NO_2$.

Question 2

Given the reaction $2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$, write the correct expression for the equilibrium constant $K_c$.

Accepted Answer

$K_c = \frac{[SO_3]^2}{[SO_2]^2[O_2]}$

Question 3

For the reaction $H_2(g) + I_2(g) \rightleftharpoons 2HI(g)$, the equilibrium concentrations are $[H_2] = 0.10\,M$, $[I_2] = 0.10\,M$, and $[HI] = 0.60\,M$. Calculate the value of $K_c$.

Accepted Answer

$K_c = \frac{[HI]^2}{[H_2][I_2]} = \frac{(0.60)^2}{(0.10)(0.10)} = 36$

Question 4

Which of the following will NOT affect the value of the equilibrium constant $K$ for a given reaction?

Accepted Answer

Changing the concentrations of reactants or products.

Question 5

For the reaction $CO(g) + 2H_2(g) \rightleftharpoons CH_3OH(g)$, $K_c = 10.2$ at 500 K. If the initial concentrations are $[CO] = 0.50\,M$, $[H_2] = 1.00\,M$, and $[CH_3OH] = 0.10\,M$, what is the value of the reaction quotient $Q_c$ and in which direction will the reaction proceed to reach equilibrium?

Accepted Answer

$Q_c = \frac{[CH_3OH]}{[CO][H_2]^2} = \frac{0.10}{0.50 	imes (1.00)^2} = 0.20$; the reaction will proceed to the right (toward products).

Question 6

Which of the following statements about heterogeneous equilibria is correct?

Accepted Answer

Only the concentrations of gases and aqueous species appear in the equilibrium constant expression.

Question 7

For the reaction $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$, $K_p$ and $K_c$ are related by the equation $K_p = K_c(RT)^{\Delta n}$. What is the value of $\Delta n$ for this reaction?

Accepted Answer

$\Delta n = 2 - (1 + 3) = -2$

Question 8

A reaction mixture at equilibrium contains $[A] = 0.20\,M$, $[B] = 0.30\,M$, and $[C] = 0.40\,M$ for the reaction $A(g) + 2B(g) \rightleftharpoons C(g)$. What is the value of $K_c$?

Accepted Answer

$K_c = \frac{[C]}{[A][B]^2} = \frac{0.40}{0.20 	imes (0.30)^2} = 22.2$

Question 9

If the equilibrium constant $K$ for a reaction is much greater than 1, which of the following best describes the equilibrium mixture?

Accepted Answer

The mixture contains mostly products.

Question 10

For the reaction $2NO_2(g) ightleftharpoons N_2O_4(g)$, $K_c = 4.6 	imes 10^{-3}$ at 25°C. If $[NO_2] = 0.10\,M$ and $[N_2O_4] = 0.50\,M$, is the system at equilibrium? If not, in which direction will the reaction proceed?

Accepted Answer

$Q_c = \frac{[N_2O_4]}{[NO_2]^2} = \frac{0.50}{(0.10)^2} = 50$; $Q_c > K_c$, so the reaction will proceed to the left (toward reactants).

Question 11

Which of the following is a correct statement regarding the use of activities in equilibrium constant expressions?

Accepted Answer

The activity of a pure solid or pure liquid is taken as 1 and is not included in the equilibrium expression.

Question 12

Consider the following two reactions:

1) $A \rightleftharpoons B$, $K_1$

2) $2A \rightleftharpoons 2B$, $K_2$

What is the relationship between $K_1$ and $K_2$?

Accepted Answer

$K_2 = (K_1)^2$

Question 13

A chemist mixes $0.10$ mol of $CO_2(g)$, $0.10$ mol of $CO(g)$, and $0.10$ mol of $O_2(g)$ in a $1.0$ L vessel at $1000$ K. For the reaction $CO_2(g) + H_2(g) \rightleftharpoons CO(g) + H_2O(g)$, $K_c = 0.64$ at $1000$ K. If $[H_2] = 0.10$ M and $[H_2O] = 0.10$ M, is the system at equilibrium?

Accepted Answer

$Q_c = \frac{[CO][H_2O]}{[CO_2][H_2]} = \frac{0.10 	imes 0.10}{0.10 	imes 0.10} = 1$; $Q_c > K_c$, so the reaction will proceed to the left.

Question 14

For the reaction $2SO_2(g) + O_2(g) ightleftharpoons 2SO_3(g)$, if the equilibrium constant $K_c$ is $1.5 	imes 10^5$ at 700 K, what does this indicate about the position of equilibrium?

Accepted Answer

The reaction mixture contains mostly $SO_3$ at equilibrium.

Question 15

If the equilibrium constant for a reaction is $K = 1.0 	imes 10^{-8}$, which of the following best describes the reaction at equilibrium?

Accepted Answer

The reaction mixture contains mostly reactants.

Question 16

For the reaction $H_2(g) + I_2(g) \rightleftharpoons 2HI(g)$, if the equilibrium constant $K_c$ is $50$ at a certain temperature, what is the value of $K_c$ for the reverse reaction?

Accepted Answer

$K_c = 1/50 = 0.02$

Question 17

A reaction has $K_c = 1.0 	imes 10^3$ at 298 K. If the initial concentrations are $[A] = 1.0\,M$, $[B] = 1.0\,M$, and $[C] = 0.01\,M$ for the reaction $A(g) + B(g) ightleftharpoons C(g)$, what will happen as the system moves toward equilibrium?

Accepted Answer

The concentration of $C$ will increase as the reaction proceeds to the right.

Question 18

Which of the following is true regarding the effect of a catalyst on the equilibrium constant $K$?

Accepted Answer

A catalyst has no effect on the value of $K$.

Chapter 15: Chemical Equilibrium – Study Notes

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