CHAPTER 3. REACTION RATES 3.3
theory.
For a reaction to occur,the particles that are reacting must collide withone another.
Only a fraction of all the collisions that take place actually cause a chemical change.
These are called ’successful’ collisions. Whenthere is an increase in the concentra-
tion of reactants, the chance that reactant particleswill collide with each other also
increases because thereare more particles in that space. In other words,the collision
frequency of the reactants increases. The number of successful collisions will therefore
also increase, and so will the rate of the reaction. In the same way, if the surface area
of the reactants increases, there is also a greaterchance that successful collisions will
occur.
DEFINITION: Collision theory
Collision theory is a theory that explains how chemical reactions occur
and why reaction ratesdiffer for different reactions. The theory states
that for a reaction to occur the reactant particlesmust collide, but that
only a certain fractionof the total collisions, the effective collisions,
actually cause the reactant molecules to changeinto products. This is
because only a small number of the molecules have enough energy
and the right orientationat the moment of impact to break the existing
bonds and form new bonds.When the temperature of the reaction increases, the average kinetic energy of the re-
actant particles increases and they will move around much more actively. They are
therefore more likely tocollide with one another (Figure 3.1). Increasing the temper-
ature also increases thenumber of particles whose energy will be greater than the
activation energy for thereaction (refer section 3.5).
B
A
B
A
B A
B
B
A
A
A
B
Low TemperatureB
A
B
A
B A
B
B
A
A
A
B
High TemperatureFigure 3.1: An increasein the temperature of areaction increases the chances that the
reactant particles (A and B) will collide because the particles have more energy and
move around more.