Q: For the reaction $$N_2(g) + 3H_2(g$$) \rightleftharpoons 2NH_3(g)$$ at 472°C, K_c = 5.1$$ \times $$10^{-4}. If$$ $$[N_2$] = 4.0 \times 10$$^{-2}$$ mol L$$^{-1}$$, $$[$$H_2$$] = 8.5 \times $$10^{-1}$$ mol L$$^{-1}$$, and $$[NH_3$] = 3.1 \times 10$$^{-3}$$ mol L$$^{-1}$$, what is the value of Q_c$?

$$Q_c = \dfrac{[NH_3$]^2}{[N_2$][H_2$]^3} = \dfrac{(3.1 \times 10$$^{-3}$$)^2$$}{(4.0 \times $$10^{-2})(8.5$$ \times $$10^{-1})^3$} = 4.0 \times 10$$^{-4}$$

Question 1

For the reaction $$aA + bB \rightleftharpoons cC + dD$$, which of the following correctly expresses the equilibrium constant $$K_c$$?

Accepted Answer

$$K_c = \dfrac{[C]^c [D]^d}{[A]^a [B]^b}$$

Question 2

A reaction at equilibrium has $$K_c = 2.5 	imes 10^3$ at a certain temperature. Which statement best describes the composition of the equilibrium mixture?

Accepted Answer

The mixture contains mostly products.

Question 3

For the reaction $$2NO_2(g) \rightleftharpoons N_2O_4(g)$$, $$K_c is $$measured at a certain temperature. If the equation is reversed, what is the new equilibrium constant $$K'_c$$?

Accepted Answer

$$K'_c = \dfrac{1}{K_c}$$

Question 4

Given the reaction $$H_2(g$$) + $$I_2(g$$) \rightleftharpoons 2HI(g)$$ at 472°C, with $$[$$H_2$$] = 1.2$$ mol L$$^{-1}$$, $$[$$I_2$$] = 1.2$$ mol L$$^{-1}$$, and $$[HI] = 0.35$$ mol L$$^{-1}$$ at equilibrium, what is the value of K_c$?

Accepted Answer

$$K_c = \dfrac{[HI]^2}{[H_2$][I_2$]} = \dfrac{(0.35)^2$}{(1.2)(1.2)} = 0.085$$

Question 5

Which of the following statements about the equilibrium constant $$K_c is $$FALSE?

Accepted Answer

$$K_c$$ includes the concentrations of pure solids and liquids.

Question 6

For the reaction $$N_2(g) + 3H_2(g$$) ightleftharpoons 2NH_3(g)$$ at 472°C, K_c = 5.1$$ 	imes $$10^{-4}. If$$ $$[N_2$] = 4.0 	imes 10$$^{-2}$$ mol L$$^{-1}$$, $$[$$H_2$$] = 8.5 	imes $$10^{-1}$$ mol L$$^{-1}$$, and $$[NH_3$] = 3.1 	imes 10$$^{-3}$$ mol L$$^{-1}$$, what is the value of Q_c$?

Accepted Answer

$$Q_c = \dfrac{[NH_3$]^2}{[N_2$][H_2$]^3} = \dfrac{(3.1 	imes 10$$^{-3}$$)^2$$}{(4.0 	imes $$10^{-2})(8.5$$ 	imes $$10^{-1})^3$} = 4.0 	imes 10$$^{-4}$$

Question 7

If $$Q_c \u003c K_c$$ for a reaction mixture, what will happen as the system moves toward equilibrium?

Accepted Answer

The reaction will shift to produce more products.

Question 8

Which of the following correctly describes how $$K_c$$ changes if the coefficients of a balanced chemical equation are doubled?

Accepted Answer

The new $$K_c is $$the original $$K_c$$ squared.

Question 9

Consider the following two reactions:

$$\mathrm{CO}(g) + \frac{1}{2}O_2$(g) \rightleftharpoons \mathrm{CO}_2(g)$$, $$K_{c1}$$

$$2\mathrm{CO}_2(g) \rightleftharpoons 2\mathrm{CO}(g) + O_2$(g)$$, $$K_{c2}

If $$these two reactions are added together, what is the equilibrium constant for the overall reaction?

Accepted Answer

$$K_{c1} 	imes K_{c2}$$

Question 10

Which of the following is TRUE regarding the calculation of $$K_c$$ for a heterogeneous equilibrium?

Accepted Answer

Only the concentrations of gases and aqueous species are included in the $$K_c$$ expression.

Question 11

A saturated solution of sodium acetate is prepared at 110°C and then cooled to room temperature, resulting in a supersaturated solution. Which statement best describes the system in terms of $$Q_c$$ and $$K_c$$?

Accepted Answer

$$Q_c \u003e K_c$$; the system will precipitate sodium acetate to reach equilibrium.

Question 12

Which of the following changes will NOT affect the value of $$K_c$$ for a given reaction?

Accepted Answer

Changing the concentrations of reactants or products.

Question 13

For the reaction $$A(g) + B(g) \rightleftharpoons C(g)$$, $$K_c = 0.10 at a $$certain temperature. If $$[A] = 0.50$$ mol L$$^{-1}$$, $$[B] = 0.50$$ mol L$$^{-1}$$, and $$[C] = 0.10$$ mol L$$^{-1}$$, is the system at equilibrium?

Accepted Answer

Yes, because $$Q_c = K_c.$$

Question 14

Which of the following best explains why pure solids and liquids are omitted from equilibrium constant expressions?

Accepted Answer

Their concentrations are constant and do not affect the equilibrium position.

Question 15

If the equilibrium constant $$K_c$$ for a reaction is much less than 1, what does this indicate about the reaction at equilibrium?

Accepted Answer

The reaction mixture contains mostly reactants.

Question 16

A chemist wants to calculate the concentration of a reactant at equilibrium, given $$K_c$$ and the concentrations of all other species. Which of the following steps is necessary?

Accepted Answer

Rearrange the $$K_c$$ expression to solve for the unknown concentration.

Question 17

Which of the following is a correct statement about the relationship between $$Q_c$$ and $$K_c$$?

Accepted Answer

$$Q_c$$ can be any positive value, but at equilibrium $$Q_c = K_c.$$

Question 18

If two reactions are added together, how is the equilibrium constant for the overall reaction determined?

Accepted Answer

By multiplying the individual $$K_c$$ values.

Chemical Equilibrium: The Equilibrium Constant and Its Applications

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