Cliffs AP Chemistry, 3rd Edition

Samples: Multiple-Choice Questions

1. Acetaldehyde, CH 3 CHO, decomposes into methane gas and carbon monoxide gas. This

is a second-order reaction. The rate of decomposition at 140°C is 0.10 mole/liter ⋅sec

when the concentration of acetaldehyde is 0.010 mole/liter. What is the rate of the

reaction when the concentration of acetaldehyde is 0.50 mole/liter?

A. 0.50 mole/liter ⋅sec

B. 1.0 mole/liter ⋅sec

C. 1.5 mole/liter ⋅sec

D. 2.0 mole/liter ⋅sec

E. 2.5 mole/liter ⋅sec

Answer: E

Begin this problem by writing a balanced equation representing the reaction.

CH 3 CHO(g) →CH 4 (g) + CO(g)

Next, write a rate expression.

rate = k(conc. CH 3 CHO)^2

Because you know the rate and the concentration of CH 3 CHO, solve for k, the rate-specific
constant.

..

k. .liters

001

(^01010)
conc CH CHO
rate
mole/liter
mole/liter sec / mole sec
3
22 "
==: :
^^hh
Finally, substitute the rate-specific constant and the new concentration into the rate expression.

..
1 ./

10

1

rate^05025

mole sec

liter

liter

mole^2 moles liter sec

==: #:cm

2. The rate of the chemical reaction between substances A and B is found to follow the
   rate law

rate = k[A]^2 [B]

where kis the rate constant. The concentration of A is reduced to half its original value.

To make the reaction proceed at 50% of its original rate, the concentration of B should be

A. decreased by^1 ⁄ 4

B. halved

C. kept constant

D. doubled

E. increased by a factor of 4

Kinetics