Question 1

Which of the following statements best describes the rate-limiting step in a multi-step reaction mechanism?

Accepted Answer

It is the slowest step in the reaction mechanism, corresponding to the highest activation energy barrier.

Question 2

Consider the following reaction mechanism:
Step 1: $$A + B \rightarrow C ($$slow)
Step 2: $$C \rightarrow D ($$fast)
Which species is an intermediate in this mechanism?

Accepted Answer

C

Question 3

A catalyst in a chemical reaction:

Accepted Answer

Lowers the activation energy and is regenerated at the end of the reaction.

Question 4

Which of the following best describes dynamic equilibrium in a chemical system?

Accepted Answer

The rates of the forward and reverse reactions are equal, so macroscopic concentrations remain constant.

Question 5

For the reaction $$2A(g) + B(g) \rightleftharpoons 3C(g)$$, what is the correct expression for the equilibrium constant $$K_c$$?

Accepted Answer

$$K_c = \frac{[C]^3}{[A]^2[B]}$$

Question 6

Which of the following is true about the equilibrium constant $$K$$ for a reaction?

Accepted Answer

A large $$K$$ value means the reaction favors products at equilibrium.

Question 7

For the reaction $$H_2(g$$) + $$I_2(g$$) \rightleftharpoons 2HI(g)$$, which of the following is the correct equilibrium constant expression in terms of partial pressures (K_p$)?

Accepted Answer

$$K_p = \frac{P_{HI}^2}{P$$_{H_2}$$P_{I_2$}}$$

Question 8

A reaction mixture initially contains only reactants. As the reaction proceeds toward equilibrium, what happens to the concentrations of reactants and products?

Accepted Answer

Reactant concentration decreases, product concentration increases until equilibrium is reached.

Question 9

Which of the following is omitted from the equilibrium constant expression for a heterogeneous equilibrium?

Accepted Answer

Pure solids and pure liquids

Question 10

For the reaction $$2H_2S(g) ightleftharpoons 2H_2(g) + S_2$(g)$$, $$K_p = 2.4 	imes 10$$^{-4}$$ at 1073 K. At equilibrium, which gas will have the highest partial pressure?

Accepted Answer

$$H_2S$$

Question 11

Which of the following best explains why a catalyst does not affect the equilibrium position of a reaction?

Accepted Answer

A catalyst lowers the activation energy for both the forward and reverse reactions equally.

Question 12

Given the reaction $$PCl_5(g$$) \rightleftharpoons $$PCl_3(l$$) + $$Cl_2(g$$)$$, which is the correct equilibrium constant expression?

Accepted Answer

$$K_p = \frac{P_{Cl_2$}}{P_{$$PCl_5$$}}$$

Question 13

Which of the following statements about the law of mass action is correct?

Accepted Answer

It can be derived from both kinetic and thermodynamic principles.

Question 14

A student claims that a reaction with $$K_c = 1.2 	imes 10^5$ will reach equilibrium very quickly. What is the flaw in this reasoning?

Accepted Answer

A large $$K_c$$ only indicates that products are favored at equilibrium, not the speed of the reaction.

Question 15

For the reaction $$MgCO_3(s$$) \rightleftharpoons $$Mg^{2+}(aq$$) + $$CO_3$$^{2-}(aq)$$, what is the correct equilibrium constant expression?

Accepted Answer

$$K_c = [Mg$$^{2+}$$][CO_3$^{2-}]$$

Question 16

Which of the following best describes the effect of adding a catalyst to the reaction shown in the energy diagram for catalyzed and uncatalyzed pathways?

Accepted Answer

The catalyst lowers the activation energy, providing an alternative pathway for the reaction.

Question 17

In a reaction mechanism, which of the following is true about an intermediate?

Accepted Answer

It is produced in one step and consumed in a subsequent step.

Question 18

For the reaction $$N_2(g) + 3H_2(g$$) \rightleftharpoons 2NH_3(g)$$, which of the following is the correct K_p$ expression?

Accepted Answer

$$K_p = \frac{P_{NH_3$}^2}{P$$_{N_2}$$P_{H_2$}^3}$$

Question 19

A reaction mixture at equilibrium has $$K_c = 0.002. $$Which statement is true about the composition of the mixture?

Accepted Answer

The mixture contains mostly reactants.

L23 Chemical Equilibrium and Reaction Mechanisms: Study Notes

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