Chemistry, Third edition

242 14 · SPEED OF CHEMICAL REACTIONS

where[M] (‘delta bracket M’) is read as ‘change in concentration of M’ and t

(‘delta t’) as ‘change in time’. The minus sign shows that [M] is falling.

The dimensions of reaction rate are

units of concentration

units of time

The usual units of reaction rate used in this book are mol dm^3 s^1.

Alternatively, the average rate of a reaction may be defined as the rate at which the

productconcentrationrisesover a period of time:

rate of reaction 

change in concentration of product

change in time

 

[MN]

t

Summarizing, the rate of reaction for reaction M + N MN is given by the

expressions

 [M] [N] [MN]

 ——— ——— ————

t t t

where the minus sign signifies that the concentration of M (and N) falls as time

passes, and the plus sign signifies that the concentration of MN increases as time

passes.

These equations need to be modified when we are dealing with chemical reactions

in which the ratio of reactants to products is not 1:1. For example, consider the

decomposition of dinitrogen pentoxide (N 2 O 5 ) into nitrogen dioxide (NO 2 ) and

oxygen:

2N 2 O 5 (g)4NO 2 (g) O 2 (g)

The equation shows that for every two molecules of N 2 O 5 that decompose, four NO 2

molecules and one oxygen molecule are produced. This means that NO 2 is produced

Example 14.1

The reaction of hydrogen and iodine to make hydrogen iodide at

a particular temperature,

H 2 (g)+ I 2 (g)2HI(g)

was studied at various times by stopping the reaction (by cooling

the mixture) and titrating any unreacted iodine with sodium

thiosulfate. At 100.0 s after the start of the reaction, the iodine

concentration had fallen from 0.010 mol dm^3 to

0.0080 mol dm^3. What is the average rate of reaction during

this period?

Answer

average rate (0.010 0.0080)/100.02.0 10 ^5 mol dm^3 s^1

Therefore, the average rate of reaction in the first 100 s was

2.0 10 ^5 mol dm^3 s^1.