2. The collision
theory of how chemical reactions occur
Brown's Chemistry KS4 science GCSE/IGCSE/O level Revision Notes - Factors
affecting the Speed-Rates
of Chemical Reactions - Doc Brown's
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& ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old
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reaction notes INDEX
GCSE/IGCSE Multiple choice
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Advanced A Level KINETICS
The theory of how reactions happen
MORE COLLISIONS INCREASE THE RATE OF A REACTION
(the more the particles hit each other the
greater the probability of reaction!)
MORE ENERGETIC COLLISIONS INCREASE THE RATE OF A
(the more kinetic energy the particles have,
the more likely they are to break bonds and form products, so the
faster the reaction goes!)
WHAT CAUSES A CHEMICAL REACTION?
WHAT MUST HAPPEN FOR A CHEMICAL REACTION TO TAKE
CAN WE MAKE PREDICTIONS ABOUT HOW THE SPEED OF A
REACTION MAY CHANGE IF THE REACTION CONDITIONS ARE CHANGED?
PARTICLE COLLISION THEORY
Reactions can only happen when the reactant particles
collide, but most collisions are NOT successful in forming product
molecules despite the incredible high rate of collisions between ALL the
particles in ANY liquid or gas.
The collision frequency is about 109 per
second between air molecules at room temperature!
It means even in the air around
you, although no chemical reactions are usually taking place, each oxygen,
nitrogen and any other molecule is undergoing around a 1000 million collisions are
So, if there are so many
collisions, even in a reacting mixture, why doesn't every reaction go at an
The reason is that particles
have a wide range of kinetic energy BUT only a small fraction of particles have enough
kinetic energy to break bonds and bring about chemical change.
The diagram above tries to give
you an idea about the concepts of fruitful collisions (minority) leading to
products and the vast majority of collisions are unfruitful, producing no
product, the molecules just bounce of each other.
minimum kinetic energy required for a reaction to take place is known
as the activation energy (shown in the diagrams below).
This 'activation' kinetic energy
is needed and to be sufficient to break bonds in the reactant molecules so
new bonds are created when the reaction products are formed.
The majority of particle collisions do
NOT form products - otherwise all reactions would be superfast!
The minority high kinetic
energy collisions between particles which do produce a chemical change are
called 'fruitful collisions', those that don't produce products are
called 'unfruitful' collisions.
The reactant molecules must collide with enough
kinetic energy to break the original bonds
to enable new bonds to form in the product molecules.
Basically reaction rates are
controlled by the frequency of collision of reactant particles AND the
kinetic energy the particles have.
The more collisions there are
in a given time AND the greater the kinetic energy the particles have, the faster the
reaction goes, and each rates factor requires a particular interpretation of
these concepts and ideas.
Collision frequency means the
'rate of particle collision' or 'the number of collisions in a given time'.
ALL the rate-controlling factors
described in sections 3a to 3e are to do with either ...
(a) [sections 3a, 3b and
3c] the collision frequency (chance of collision)
to give a fruitful collision and products,
so increasing the reactant
concentration of solutions, increasing gaseous reactant pressure or reducing
particle size of a solid reactant (increasing surface area) all favour
increasing the rate of fruitful collisions,
(b) [section 3d and 3e] the combined
kinetic energy of reactant particle collision
(>= activation energy) to give a fruitful collision and products,
so, increasing temperature
increases the KE of particles giving more fruitful energetic collisions,
AND, using a catalyst to
decrease the activation energy means more molecules already have enough
kinetic energy to overcome the activation energy and react without having to increase the temperature.
both these explanations are all about
the 'chance of a fruitful collision' leading to reactant bonds
breaking product formation via new bonds forming.
In the case of temperature, the energy of the collision is even more important than the frequency
In each of the sections 3a to
3e the collision theory is applied in more detail to that particular factor
affecting the speed/rate of a reaction, so read on!
particle theory of gases and liquids and the
particle diagrams and their explanation will also help you understand
or describe in your coursework what is going on.
For more details on
activation energy see GCSE/IGCSE/O Level notes on
For A Level students
Advanced Level Chemistry Theory pages on "CHEMICAL
KINETICS" covers all advanced theoretical aspects of rates of
- More details of laboratory investigations
('labs') involving 'rates of reaction' i.e. experimental methods for
observing the speed of a reaction and including the effect of a catalyst are given in
picture for a solid catalyst
Concept picture for a solid
reaction notes INDEX
GCSE/IGCSE MULTIPLE CHOICE
on RATES of reaction
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