Chemistry, Third edition

Speed of Chemical

Reactions

Objectives

Discusses the factors affecting reaction rate

Defines the terms rate constantandorder of reaction

Introducesactivation energyandreaction mechanism

Looks at first-order reactions in detail

Explains the role of catalysts

Reaction rate

Chemical reactions take place at different speeds. Rusting, the reaction of iron with

oxygen and water is a slow process, whereas the reaction of potassium metal with

water is explosively fast. Reaction speeds are more properly referred to as ‘reaction

rates’. The study of the rate of reactions (and of the factors controlling reaction

rates) is known as chemical kinetics.

A knowledge of the rate at which a reaction takes place is often crucial. In industry,

reactions are economically profitable only if the yield of product is sufficient andif

the products are made in a short enough time. In the chemistry of pollution, the rates

at which a pollutant is formed and destroyed are important factors in assessing the

hazard posed by the pollutant.

Definition of reaction rate

The word ‘rate’ is part of everyday language (Table 14.1). The average rate

of the chemical reaction:

M + N MN

(where M and N are reactants and MN is the product) is defined as the rate

at which the reactantconcentrationfallsbetween two times:

rate of reaction 

change in concentration of reactant

change in time



[M]

t

14.1

Contents

14.1Reaction rate 241

14.2Factors affecting

reaction rate 245

14.3Reaction rate

expressions 250

14.4Examples of rate

expressions found

by experiment 252

14.5Calculations using

rate expressions 253

14.6More about first-

order reactions 257

14.7Reaction

mechanisms 260

14.8Catalysis 262

14.8Revision questions 263

Table 14.1Examples of ‘rates’

Rate Units

Oil production rate barrels day^1

Speed m s^1

Plant growth cm month^1

Population growth people year^1

Reaction rate concentration s^1

The units are read as barrels per day,

metres per second etc.

14

UNIT