AP Chemistry : Integrated Rate Laws

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

Example Question #1 : Integrated Rate Laws

The rate constant for a second-order reaction is 0.15 M-1s-1.  If the initial concentration of the reactant is 0.30 M, how long does it take for the concentration to decrease to 0.15 M?

Possible Answers:

22.2 seconds

11.1 seconds

44.4 second

88.8 seconds

253 second

Correct answer:

22.2 seconds

Explanation:

\(\displaystyle \frac{1}{[A]_t} = kt + \frac{ 1 }{[A]_o}\)

 

\(\displaystyle t =\frac{ \frac{1}{[A]_t} -\frac{1}{[A]_o}}{k} =\frac{\frac{1}{0.15} - \frac{1}{0.30}}{0.15}= 22.2 seconds\)

Example Question #1 : Integrated Rate Laws

02

Based on the figure above, what is the order of reaction?

Possible Answers:

First order

Third Order

Squared Order

Second Order

Zero Order

Correct answer:

First order

Explanation:

For a first order reaction, the ln [A]t is linear with t.

Example Question #1 : Integrated Rate Laws

A compound decomposes by a first-order process.  If 25.0% of the compound decomposes in 60 minutes, the half-life of the compound is?

Possible Answers:

180 minutes

120 minutes

145 minutes

198 minutes

65 minutes

Correct answer:

145 minutes

Explanation:

\(\displaystyle ln[A]_t=-kt + ln[A]_o\)

 

\(\displaystyle k = - \frac{ln[A]_t - ln[A]_o}{t}= - \frac{ln[75]-ln[100]}{60}= 4.79 x10^{-3}\)

 

\(\displaystyle t_{\frac{1}{2}}=\frac{ ln2 }{k} = \frac{ln2}{4.79x10^{-3}}= 145 minutes\)

Example Question #2 : Integrated Rate Laws

Cyclopentane is unstable and decomposes by a first order reaction.  The rate constant for this reaction is 9.5 s-1.  What is the half life of the reaction?

Possible Answers:

0.0614 seconds

0.0314 seconds

0.0582 seconds

0.0729 second

0.132 seconds

Correct answer:

0.0729 second

Explanation:

\(\displaystyle t_{\frac{1}{2}}=\frac{ln2 }{k} = \frac{ln2}{9.5} = 0.0729 seconds\)

Example Question #3 : Integrated Rate Laws

The half life of a first order reaction is 1.5 hours.  What is the rate constant of this reaction?

Possible Answers:

0.46

0.75

0.21

0.52

1.5

Correct answer:

0.46

Explanation:

\(\displaystyle t_{\frac{1}{2}} = \frac{ln2 }{k}\)

 

\(\displaystyle k =\frac{ln2}{t_{\frac{1}{2}}} = \frac{ln2}{1.5} = 0.46\)

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