Cliffs AP Chemistry, 3rd Edition

6. The reaction 2 X + Y →3 Z was studied and the following data were obtained:

Experiment X Y Rate (mole / liter ⋅sec)

1 3.0 1.5 1.8

2 1.5 3.0 0.45

3 1.5 1.5 0.45

What is the proper rate expression?

A. rate = k[X][Y]

B. rate = k[Y]^2

C. rate = k[X]

D. rate = k[X]^2 [Y]

E. rate = k[X]^2

Answer: E

Examine experiments 2 and 3, wherein [X] is held constant. Note that as [Y] doubles (from 1.5
to 3.0), the rate does not change. Hence, the rate is independent of [Y] and the order is 0 for Y.

Now examine experiments 1 and 3, wherein [Y] is held constant. Note that as [X] doubles, the
rate is increased by a factor of 4. In this case, the rate is proportional to the square of the con-
centration of the reactant. This is a second-order reactant.

Combining these reactant orders in a rate equation gives

rate = k[X]^2 [Y]^0 = k[X]^2

For Samples 7 and 8, refer to the following diagram.

Path of Reaction

A

B

C

Energy

Part II: Specific Topics