Rates Solution Page 6

Remedial Problem Set Part B2 Solution

  1. Hydrogen peroxide reacts with iodide according to the equation

2H+ + 2I + H2O2 = 2H2O + I2

The mechanism suggested for this reaction is

(1) H+ + I + H2O2 = HOI + H2O (slow)

(2) H+ + I + HOI = I2 + H2O (fast)

(a) Do the elementary processes add up to the overall equations? Show this.

Yes they do

H+ + I- + H2O2 = HOI + H2O (slow)

H+ + I- + HOI = I2 + H2O (fast) adding the two equations gives the original equation

2H+ + 2I- + H2O2 = 2H2O + I2

(b) According to the mechanism, which elementary process is the rate determining step? Explain.

The second step or equation 4 is the rate determining step as it is the slowest step.

(c) What is the rate law expression for this reaction? Rate = k[H+] [I][H2O2]

(d) How would you expect the rate to be affected if the [I] concentration is doubled?

The rate of the reaction would double if the concentration of I alone is doubled.

  1. For the reaction between gaseous chlorine and nitric oxide

2NO + Cl2 = 2NOCl (all gases)

It is found that doubling the concentration of Both reactants increases the rate by a factor of 8, but by doubling the chlorine concentration alone the rate only doubles

(a) What is the rate law expression with respect to chlorine gas?

With respect to chlorine the order of the reaction is one

Then the order with respect to NO should be 2 as the overall order is 3

Rate = k[NO]2[Cl2]

(b) What is the overall order of the reaction?

Overall order n = 3 only then doubling the concentration of the reactants will increase the rate by a factor of 8

  1. The reaction between nitrogen dioxide and fluorine is

2NO2 + F2 = 2NO2F (all gases)

The rate law data is tabulated as follows

Expt # [NO2]






1 1 1 2
2 1 2 4
3 1 3 6
4 3 1 6
5 5 1 10
6 6 1 12

(a) Determine the rate law expression. Give appropriate reasoning.

(b) The overall order is Rate = k [NO2] [F2]

(c) The units of k are L mol-1 time -1

(d) Evaluate the following mechanism for this reason. Give reasons for acceptance or rejection of each.

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