Q: A reaction has a rate constant $$k = 1.8 \times 10$$^{-5}$$ \ s$$^{-1}$$ at 20°C and is first order. How long will it take for 90% of the reactant to decompose?

$$t = \frac{2.303}{k} \log_{10}(10) = 1.28 \times 10^5$ \ s ($$about 35.5 hours)

Question 1

Which of the following elements, as shown in the periodic table, is classified as a metalloid?

Accepted Answer

Boron (B)

Question 2

Refer to the concentration vs. time graph for three reactions. Which curve (black, blue, or red) represents a zeroth-order reaction?

Accepted Answer

The black curve

Question 3

The reaction progress diagram shown depicts a single-step reaction. What does the peak of the curve represent?

Accepted Answer

The transition state

Question 4

In the series of boxes shown, each containing shaded and unshaded spheres, which experiment (I, II, III, or IV) has reached equilibrium for the reaction $$A \rightleftharpoons B$$?

Accepted Answer

Experiment IV

Question 5

Which of the following best describes the effect of a catalyst on the reaction progress diagram?

Accepted Answer

It lowers the activation energy, $$E_a$$, providing an alternate pathway.

Question 6

Given the equilibrium mixture diagrams for $$A_2$$ + $$X_2$$ \rightleftharpoons 2AX$$, which reaction is product-favored (i.e., has K_c$$ \u003e 1$$)?

Accepted Answer

Reaction I only

Question 7

For the reaction $$A \rightleftharpoons B$$, the concentration vs. time plot shows that the concentrations of A and B become constant and nonzero after some time. What does this indicate?

Accepted Answer

The system has reached dynamic equilibrium.

Question 8

Which of the following is the correct equilibrium constant expression for the reaction $$2CO(g) + 4H_2O(g) \rightleftharpoons 2CH_4(g) + 3O_2(g)$$?

Accepted Answer

$$K_c = \frac{[CH_4$]^2[O_2$]^3}{[CO]^2[H_2O]^4}$$

Question 9

If the rate law for a reaction is $$	ext{Rate} = k[A]^2$$, which of the following statements is FALSE?

Accepted Answer

Doubling [A] will double the rate.

Question 10

A reaction is studied and a plot of $$\ln[A]$$ versus time is linear. What is the order of the reaction?

Accepted Answer

First order

Question 11

For a zeroth-order reaction, what happens to the rate if the concentration of the reactant is doubled?

Accepted Answer

The rate remains the same.

Question 12

Which part of the Arrhenius equation $$k = Ae$$^{-E_a/RT}$$ accounts for the fraction of collisions with the correct orientation?

Accepted Answer

The frequency factor, $$A$$

Question 13

Given the following equilibrium: $$N_2O_4(g) \rightleftharpoons 2 NO_2$(g)$$ with $$K_c = 0.11 at 25$$°C, what is $$K_c$$ for $$6 NO_2$(g) \rightleftharpoons 3 N_2O_4(g)$$?

Accepted Answer

$$K_c = 0.0013$$

Question 14

Which of the following statements about chemical equilibrium is correct?

Accepted Answer

At equilibrium, the rates of the forward and reverse reactions are equal.

Question 15

In a reaction mechanism, what is the significance of the slowest step?

Accepted Answer

It determines the overall rate law for the reaction.

Question 16

Which of the following is a correct statement about intermediates in a reaction mechanism?

Accepted Answer

They are produced in one step and consumed in a subsequent step.

Question 17

A reaction has a rate constant $$k = 1.8 	imes 10$$^{-5}$$ \ s$$^{-1}$$ at 20°C and is first order. How long will it take for 90% of the reactant to decompose?

Accepted Answer

$$t = \frac{2.303}{k} \log_{10}(10) = 1.28 	imes 10^5$ \ s ($$about 35.5 hours)

Question 18

Which of the following is true for a reaction at equilibrium?

Accepted Answer

The concentrations of reactants and products remain constant over time.

Question 19

Which of the following best explains why the equilibrium constant $$K_c is $$unitless for some reactions?

Accepted Answer

The exponents in the equilibrium expression sum to zero, canceling out the units.

General Chemistry Exam 4 Practice Guidance: Chemical Kinetics & Equilibrium

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