Exam-style Questions: Reaction Kinetics

1. Nitrogen (II) oxide reacts with oxygen in the following way:

2NO(g) + O2(g) = 2NO2(g)

Experiments were carried out to determine the orders of reaction with respect to NO and O2. The results of these experiments are shown below.

Experiment

[NO(g)] / moldm-3

[O2(g)] / moldm-3

Rate / moldm-3 s-1

1

1.5 x 10-5

0.5 x 10-5

2.1 x10-7

2

4.5 x 10-5

0.5 x 10-5

1.9 x10-6

3

1.5 x 10-5

2.0 x 10-5

8.4 x10-7

 

a) Using the data, deduce the orders of reaction with respect to NO and O2.

b) Write down the rate equation for this reaction and use it to calculate k.

c) Explain the meaning of the term 'rate determining step'?

Answer

 

Answer outline and marking scheme for question: 1

a) For NO - examine experiments 1 and 2. A 3-fold increase in the concentration of NO leads to a 9-fold increase in rate. (1)

This means that the reaction must be second order with respect to NO. (1)

For O2 - examine experiments 1 and 3. A 4-fold increase in O2 concentration leads to a 4-fold increase in rate. (1)

This means that the reaction must be first order with respect to O2. (1)

b) Rate = k[NO]2[O2] (1)

2.1 x10-7 = k (1.5 x10-5)2 (0.5 x10-5)

k = (2.1 x10-7) / (1.5 x10-5)2 (0.5 x10-5)

k = 1.9 x 108 mol-2dm6s-2

c) The rate-determining step is the slowest section of the mechanism of the reaction when compared with the other steps.

 

 

2. Carbon dating using 14C is a highly important tool in archaeology. Using this technique it is possible to determine the length of time since an organism has died. The half-life of 14C is 5600 years.

a) Define the term 'half-life'.

b) A sample of wood taken from an archaeological dig is analysed and found to have a radioactive count of 4 counts per minute per gram of carbon. A new sample of the same type of wood has a count of 16 c.p.m.g-1. What is the approximate age of the archaeological sample?

c) Which order of kinetics does radioactive decay follow?

(Marks available: 3)

Answer

 

Answer outline and marking scheme for question: 2

a) The half-life of a substance is the time taken for its concentration to decrease by 50%.

b) After 5600 years the count form the piece of new wood would drop to 8 c.p.m, after a further 5600 years it will drop to 4 c.p.m.

The old wood is therefore approximately two half-lives old = 11200 years.

c) First order

(Marks available: 3)