1. RATE OF REACTION
CHAPTER 1

2. In this chapter, you will learn to:
Analyse the rate of reaction.
Synthesise factors affecting the rate of reaction.
Synthesise ideas on the collision theory
Learn about practising scientific knowledge to enhance quality
of life.

3. 1.3 SYNTHESISE IDEAS ON
THE COLLISION THEORY

4. LEARNING OUTCOMES
Relate reaction with energy produced by movement and
effective collision of particles,
Describe activation energy
Sketch and describe energy profile diagram,
Relate the frequency of effective collision with the rate of
reaction,
Relate the frequency of effective collision with factors
influencing the rate of reaction,
Describe how a certain factor affects the collision of particles
in a reaction.

5. (1) INTRODUCTION
The five factors which affect the rate of reaction are:
 The size of reactants (or surface area)
 Concentration of reactants
 Temperature
 Catalyst
 Pressure
All these factors can be explained by using the collision theory.
According to the collision theory, a reaction only occurs when:
a) The particles of reactants collide with one another during the
reaction.
b) The reacting particles possess a minimum energy termed activation
energy.
c) The reacting particles collide in the correct orientation.

7. Continue…
Hence chemical reaction only occurs when there are collisions
where particles are correctly oriented and possess a total
energy that is equal to or more than the activation energy for
that reaction.
A collision which results in a chemical reaction is known as an
effective collision.

8. (2) ACTIVATION ENERGY (Ea)
This is the minimum energy that is required by colliding
reactants for a chemical reaction to take place.
The activation energy can be considered as an energy barrier
or resistance that has to be overcome in order to produce a
reaction.
Figure 1.31 shows the energy profile diagram of a reaction
where: A + B  C + D. The reaction produces heat energy and
therefore called an exothermic reaction. In the energy profile
diagram, the x-axis represents the progress of the reaction
while the y-axis indicates the energy level of the reactants and
products of the reaction.
Figure 1.32 is the energy profile diagram for an endothermic
reaction where heat energy is absorbed during a chemical
reaction.

9. Figure 1.31 Figure 1.32

10. (3) RATE OF REACTIONS AND
THE COLLISION THEORY
According to the collision theory, two important factors
determine the rate of a reaction:
 The frequency of collisions
 The magnitude of the activation energy
If the number or frequency of collisions in a reaction is high,
then the frequency of effective collisions also increase. This
leads to an increase in the rate of reaction.
Magnitude of activation energy refers to how high or low the
activation energy is. A chemical reaction with a high activation
energy will progress at a low rate because only small number
of particles will contain enough energy to react. On the other
hand, a reaction with a low activation energy will occur at a
faster rate because a larger number of particles contain
enough energy to overcome the activation energy level of that
reaction. Thus, more particles are able to react affectively.

11. (A) Surface Area and Collision
Theory
For a reaction involving a solid reactant, the rate of reaction is
inversely proportional to the size of the reactant. That is, the
smaller the size of the solid particles, the higher is the rate of
the reaction.
This is because the smaller the size of the particles, the
greater the surface area that is exposed to colliding particles
of the reactants.
Total surface area of cuboid A = 4(2x1) + 2(2x2) = 16 cm2
Total surface area of B, C, D and E = 6(1x1) x 4 = 24 cm2

12. (B) Concentration and Rate of
Reaction
In the reaction between zinc and dilute hydrochloric acid, the
rate of reaction increases if the concentration of the acid is
increased.
This is because when the concentration of the solution
increases, the number of particles per unit volume also
increases.
As a result, the frequency of collisions increase, leading to a
high frequency of effective collisions.
Thus, the rate of reaction increases.

14. (C) Temperature and Rate of
Reaction
 When the temperature of the reactant or reactants is increased, the
rate of reaction increases too. This is because the kinetic energy of
particles increase with an increase in temperature.
 In the reaction between marble chips and hydrochloric acid, the acid
particles absorb the excess energy that is produced when the
temperature of the acid is increased. This causes the kinetic energy
to increase.
 When the kinetic energy increases, the reacting particles move more
rapidly. The number of particles which attain activation energy also
increases.
 This mean that the frequency of collisions between reactants is
higher. Therefore the frequency of effective collisions also increases,
leading to a higher rate of reaction.
 For many reactions, an increase in temperature of 10 oC will result in
an increase in the rate of reaction by two times.

15. (D) Catalyst and Rate of
Reaction
In the decomposition of hydrogen peroxide, a catalyst
(manganese(IV) oxide or Iron(III) oxide) may be used to speed
up the reaction.
During the reaction, the H2O2 particles decompose when they
collide with one another. If there is a catalyst, the H2O2
molecules collide with the catalyst as well as with one
another. Therefore, the particles react in a different manner.
Depending on the type of catalyst used, the activation energy
is either increased or decreased.

16. Continue…
In the presence of a catalyst which increases the rate of
reaction, an alternative pathway is created during the
reaction. This pathway requires lower activation energy than
the reaction without a catalyst.
When the activation energy is lowered, the number of
collisions which are able to overcome the lower activation
energy will increase.
As such, the frequency of effective collisions increase.
Therefore, the rate of reaction too increases.

18. (E) Pressure and Rate of
Reaction
Pressure influences the rate of reactions involving gaseous
reactants and products only.
Pressure increases when the volume of the gases is reduced.
When this occurs, the rate of reaction increases.
This is because when volume decreases, the gas particles
occupy a smaller volume.
Thus, the number of gas particles per unit volume increases or
the concentration of gaseous reactants is higher at higher
pressure.
The frequency of collisions increase and consequently, the
frequency of effective collisions increases too, resulting in a
higher rate of reaction.