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General Chemistry Principles, Patterns, and Applications, 2011

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C Because the reaction rate is independent of the N 2O concentration, doubling the concentration<br />

will have no effect on the reaction rate.<br />

3. A The rate law contains only one concentration term raised to the first power. Hence the rate<br />

constant must have units of reciprocal seconds (s −1 ) to have units of moles per liter per second for<br />

the reaction rate: M·s −1 = M/s.<br />

B The only concentration in the rate law is that of cyclopropane, <strong>and</strong> its exponent is 1. This means<br />

that the reaction is first order in cyclopropane. Cyclopropane is the only species that appears in<br />

the rate law, so the reaction is also first order overall.<br />

C Doubling the initial cyclopropane concentration will increase the reaction rate<br />

from k[cyclopropane] 0 to 2k[cyclopropane] 0. This doubles the reaction rate.<br />

Exercise<br />

Given the following two reactions <strong>and</strong> their experimentally determined differential rate laws: determine<br />

the units of the rate constant if time is in seconds, determine the reaction order with respect to each<br />

reactant, give the overall reaction order, <strong>and</strong> predict what will happen to the reaction rate when the<br />

concentration of the first species in each equation is doubled.<br />

1. CH 3N = NCH 3(g) ® C2H 6(g) + N2(g)rate<br />

= -D[CH 3N = NCH 3]Dt = k[CH 3N = NCH 3]<br />

1. 2NO2( g) + F2( g) ® 2NO2F( g)rate<br />

2.<br />

= -D[F2]Dt = -12(D[NO2]Dt) = k[NO2][F2]<br />

Answer:<br />

1. s −1 ; first order in CH 3N=NCH 3; first order overall; doubling [CH 3N=NCH 3] will double the<br />

reaction rate.<br />

2. M −1·s −1 ; first order in NO 2, first order in F 2; second order overall; doubling [NO 2] will double<br />

the reaction rate.<br />

Summary<br />

Reaction rates are reported either as the average rate over a period of time or as the instantaneous<br />

rate at a single time.<br />

The rate law for a reaction is a mathematical relationship between the reaction rate <strong>and</strong> the<br />

concentrations of species in solution. Rate laws can be expressed either as a differential rate law,<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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