# The collision theory of reactions

```CI 10.1
The collision theory of reactions
Reactions occur when particles of reactants collide with a certain
minimum kinetic energy. (Activation enthalpy)
Low concentration
Higher concentration
Any factor which increases the number of collisions will increase
the rate of reaction.
As particles approach and collide, K.E. is converted to P.E. and
the P.E. of reactants rises.
X
E
n
t
h
a
l
p
y
Activation enthalpy
Reactants
H
Products
Progress of reaction
In the above exothermic reaction, existing bonds start to stretch
and break and new bonds form. Only Reactants with sufficient
K.E. will overcome the activation enthalpy and form products.
At higher temperatures, more of the colliding particles have
enough energy to react.
Enthalpy profiles
X
E
n
t
h
a
l
p
y
Activation enthalpy
Reactants
H
Products
Progress of reaction
The curved line is the energy pathway for a pair of colliding
molecules (called the energy or enthalpy profile for the reaction).
X corresponds to the arrangement of atoms where old bonds are
stretched and new bonds are starting to form.
Eg. For chlorine atoms and ozone, the oxygen – oxygen bonds in
ozone break and a new bond between oxygen and chlorine is
forming.
Cl + O3  Cl------O------O
O  ClO + O2
At X
(this is a very unstable arrangement and only lasts for a very short
time).
The enthalpy profile is shown as a single curve (this is only true
for single-step reactions). Other reactions have several steps, so
the enthalpy profile would show several steps, too.
 do problems for 10.1 ‘Rates of Reactions’ p. 222-223 questions
1 to 4.
1. For each of the following reactions, say how, if at all, you
would expect its rate of reaction to be affected by the
following factors:
a) Temperature
b) Total pressure of gas.
c) Concentration of solution
d) Surface area of solid
a
b
C
A
Rate will
Rate will
Rate will
increase with increase with increase with
temperature
temperature
temperature
B
Rate of
Rate increases Rate of
forward
forward
reaction not
reaction not
affected
affected
C
Increasing the Solutions not Increasing the
concentration involved
concentration
of acid will
of peroxide
increase the
will increase
rate
the rate
D
The more
The more
Solids not
finely divided finely divided involved
the
the catalyst the
magnesium,
faster the rate
the faster the
rate
2.) Both the acid and the enzyme can act as catalysts for the
hydrolysis of a protein.
3. a) The greater the concentration of reactants, the greater the
rate of collisions and hence the faster the reaction proceeds.
3 b) A change of temperature has little effect. Most collisions
result in a reaction.
4 a) B and C
b) A and D
c) D
d) B
e) B f) D
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