5 Steps to a 5 AP Chemistry

Therefore, the reaction is first order with respect to O 2 and the rate equation can be writ-

ten as:

Rate =k[NO]^2 [O 2 ]

The rate constant can be determined by substituting the values of the concentrations

of NO and O 2 from any of the experiments into the rate equation above and solving for k.

Using experiment 1:

0.05 M/s =k(0.01 M)^2 (0.01 M)

k=(0.05 M/s)(0.01 M)^2 (0.01 M)

k= 5 × 104 /M^2 s

Sometimes, because of the numbers’ complexity, you must set up the equations

mathematically. The ratio of the rate expressions of two experiments will be used in deter-

mining the reaction orders. The equations will be chosen so that the concentration of only

one reactant has changed while the others remain constant. In the example above, the ratio

of experiments 1 and 2 will be used to determine the effect of a change of the concentra-

tion of NO on the rate, and then experiments 1 and 3 will be used to determine the effect

of O 2. Experiments 2 and 3 cannot be used, because both chemical species have changed

concentration.

Remember: In choosing experiments to compare, choose two in which the concentra-

tion of only one reactant has changed while the others have remained constant.

Comparing experiments 1 and 2:

Canceling the rate constants and the [0.01]nand simplifying:

m=2 (use logarithms to solve for m)

Comparing experiments 1 and 3:

Canceling the rate constants and the [0.01]nand simplifying:

Writing the rate equation:

Rate =k[NO]^2 [O 2 ]

Again, the rate constant kcould be determined by choosing any of the three experi-

ments, substituting the concentrations, rate, and orders into the rate expression, and then

solving for k.

1

2

1

2

1

=

⎛

⎝

⎜

⎞

⎠

⎟

=

n

n

005 001 001

010 001 002

.[.][.]

.[.][.]

M/s

M/s

=

=

k mn

mn

1

4

1

2

=

⎛

⎝

⎜

⎞

⎠

⎟

m

005 001 001

020 002 001

.[.][.]

.[.][.]

M/s

M/s

=

=

k mn

mn

200  Step 4. Review the Knowledge You Need to Score High

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