# 10 multiple choice kinetics questions

Please can you provide an explanation for the following questions:

For the rate law, Rate = k[A]2[B], which of the following is false about the chemical reaction having this rate law?

A) Doubling [A] quadruples the rate.

B) The reaction is third order overall.

C) The chemical reaction must be: 2 A + B â?' C.

D) Doubling [B] doubles the rate.

The reaction, A + 2 B â?' C, was found to have the rate law: Rate = k[A][B]2. While holding the concentration of A constant, the concentration of B was increased from x to 3x. Predict by what factor the rate of reaction will increase.

A) 6

B) 27

C) 9

D) 3

At 45°C, the rate constant for the reaction C â?' D is 5.17 x 10-5 s-1. What is the half-life of C at this temperature?

A) 165 minutes

B) 126 minutes

C) 198 minutes

D) 223 minutes

The reaction A â?' B is first order with a rate constant of 7.16 x 10-4 s-1 at 25°C. If the initial concentration of A is 5.29 x 10-4 M, what is the concentration of A after 1815 seconds?

A) 5.26 x 10-5 M

B) 1.44x 10-4 M

C) 3.39 x 10-5 M

D) 6.26 x 10-4 M

For the reaction, 2 H2S + O2 â?' 2 S + 2 H2O, we can say that the

A) reaction is second order with respect to H2S and first order respect to O2.

B) reaction is second order overall.

C) rate law cannot be determined from the information given.

D) rate law is: Rate = k[H2S]2[O2].

The reaction A â?' B is second order with a rate constant of 7.50 x 10-3 M-1s-1 at 25°C. If the initial concentration of A is 2.50 M, how long will it take for the concentration of A to decrease to 0.500 M?

A) 893 seconds

B) 663 seconds

C) 457 seconds

D) 213 seconds

The reaction A â?' B is first order with a rate constant of 1.50 x 10-3 s-1 at 25°C. If the initial concentration of A is 7.25 x 10-3 M, what is the concentration of A after 329 seconds?

A) 4.43 x 10-3 M

B) 1.09 x 10-4 M

C) 7.29 x 10-5 M

D) 5.46 x 10-4 M

The following rate data were obtained at 25°C for the reaction below. What is the rate law for the reaction?

2A + B â?' C

A) rate = k[A][B]2

B) rate = k[A]

C) rate = k[A][B]

D) rate = k[A]2

A first-order reaction is 72.8% complete in 1612 seconds. How long will it take for the reaction to go to 15.5% completion?

A) 156 seconds

B) 447 seconds

C) 208 seconds

D) 343 seconds

Kinetics data were collected for the following reaction at 75°C: A â?' B. A plot of ln[A] vs time gave a straight line. What is the overall reaction order?

A) second

B) zero

C) third

D) first

#### Solution Preview

1) C, the only time the rate law expression will give you stoichiometry is when you have an elementary reaction (A+B--->C). Anything more complex will be too ambiguous to tell.

2) C, since you will square B, when you triple the amount of B, the rate should increase quadratically, so 3^2, or 9, will be the factor you increase the rate by

3) Using the formula: ln2=kt1/2, where t1/2 is the half-life

0.693/k=t1/2

0.693/5.17x10^-5s = t

t=13404 seconds

=223 minutes

D

4)

k=7.16x10^-4

A0=5.29x10^-4

t=1815

Using this formula:

A=A0e^(-kt)

We can plug in numbers and move ...

#### Solution Summary

Have you been feeling confused about kinetics? In this solution, 10 questions covering a variety of topics related to rate laws and integrated rate laws are answered. We cover how to find the rate law, how to use the integrated rate law to find how much material remains after a given time, how to use half-life, and we briefly consider how to figure out the order of the kinetics based on the graph of rate vs. concentration. Don't miss this one!