Factors Affecting the Rate of a Chemical
Reaction
The following events must occur
before a reaction can proceed:
1. The reactant particles must collide with each
other.
2. The collisions must be of enough energy to
overcome the energy barrier, called the
activation energy (more about this on the next
slide).
3. The reactants must form new bonds to produce
This presentation is useful as a refresher for those of
products.
you starting Unit 2 of the A-level chemistry course.
Activation
Energy 1
Defined as:
The minimum energy required
to bring about a chemical
reaction.
If there were no such thing as
activation energy life would be very
difficult:
Gasoline for your car would ignite as soon as it came into
contact with air.
You would burst into flames.
Trees would spontaneously combust.
Activation energy is why these things do not happen,
there is an energy barrier so most reactions need to be
started off by putting in some energy.
Activation Energy 2
Activation energy for a reaction is shown on
reaction profile diagrams (do you remember
these?).
energy
Activated
intermediate
Activatio
n energy
Reactant
s
H
Product
s
Changing the Rate of a Chemical
Reaction
To change the rate of a reaction one or more
of the following things must happen:
1. Increase the number of collisions
between the reactant particles
2. Increase the energy of the collisions.
3. Decrease the activation energy.
This is all very well but
how can we follow the
progress of a chemical
reaction?
Following a Chemical
Reaction 1
To find the rate of a chemical reaction
we must be able to follow its progress
with time.
We have two choices:
1. Record the increase in product
concentration as the reaction progresses.
2. Record the decrease in reactant
concentration as the reaction progresses.
Following a Chemical Reaction
2
As an example consider the reaction between calcium
carbonate and hydrochloric acid.
You should already know the equation but here it is:
CaCO3 + 2HCl CaCl2 + H2O + CO2
We can follow this reaction by measuring the volume of
carbon dioxide produced as the reaction proceeds.
Gas being
collected
Dilute
acid
Marble
chips
This apparatus can be
used to measure the gas
as it is formed. It is not
the only way, look in your
text book for more
details.
Following a Chemical
Reaction
3
If you collect data for the total amount of gas produced as
the reaction progresses then plot this data on a graph you
should get a curve similar to that shown below.
Slowing
down.
Volume
of
gas/cm3
All very well, but what
does the graph tell
you?
Reaction
finished.
Reaction fastest
at the beginning.
The gradient or slope of the
graph shows the rate of the
reaction.
Time/sec.
Effect of Surface
Area 1
When solids take part in chemical reactions only the
surface particles are exposed so they are the only ones
that can collide with particles of other reactants.
Inner particles are protected
and cannot collide with other
particles until they become
exposed.
The surface
particles are
exposed and can
react.
Effect of Surface
Area 2 If we break up this lump into
smaller pieces the number of
particles has not changed but the
there are now more surface
particles.
There is now a greater surface
area with more exposed
particles so more collisions
can occur, hence faster
reaction.
Larger surface
area = faster
reaction.
Effect of
Consider theConcentration
reaction between zinc and hydrochloric
acid:
Zn + 2HCl H2 +
(How could you follow the progress of this reaction?
Click to find
ZnCl2
out)
Acid Particles
Zinc
2M hydrochloric acid
1M hydrochloric acid
There are more particles of acid per unit volume in the 2M
acid than there are in the 1M acid. So, there will more
collisions between the acid and zinc particles in the stronger
acid, giving a faster reaction.
Higher concentration = faster
reaction
Gas
Reactions
The rate of
reaction between gases is
increased by increased pressure.
In effect pressure is the gas equivalent of
concentration.
These two gas jars
contain the same
number of gas
particles.
The higher pressure jar
has more particles per
unit volume which
means a higher
concentration, hence
faster reaction.
Low pressure,
particles far
apart.
Higher pressure,
particles closer
together.
Higher pressure = faster
Effect of
Temperature
According to kinetic theory (do you remember
this?) as the temperature increases the particles in
a substance move about more quickly.
Reaction at 300C
Reaction at 500C
As the temperature increases the number of collisions increases as well
as the energy of the collisions. So temperature has a big effect on the rate
of reaction. For every 100C increase the rate approximately doubles.
Higher temperature = faster reaction
Effect of a
Catalyst
A catalyst
is a substance 1
that increases the speed of a
reaction, without being used up. A catalyst can be
recovered at the end of a reaction and used again.
A catalyst reduces the
activation energy of a reaction.
This is easier to understand with a diagram
see next slide.
Effect of a Catalyst 2
Activation energy
without catalyst.
energy
The lower activation energy in the
presence of a catalyst means the
reaction will be faster. More of the
collisions have enough energy to react.
There is a lower energy barrier.
Activation energy
with catalyst.
Catalyst = faster reaction.
More About Rate
Graphs
Slowing
down.
Volume
of
gas/cm3
1. Why is the reaction
fastest at the beginning?
Reaction
finished.
Reaction fastest
at the beginning.
2. Why does the reaction
slow down?
3. Why does the reaction
eventually stop?
Time/sec.
1. This is where the concentration of the reactants is highest, therefore
fastest reaction.
2. As the reactants are used their concentration decreases so the rate of
reaction decreases.
3. One of the reactants is used up, so there can be no further reaction.
Special
Note
Some exothermic reactions speed up shortly after they
start, this might be unexpected, but think about it!
The temperature increases and this overcomes, at least
to begin with, the effect of reducing the concentration.
So, in some cases the reaction will speed up then slow
down and eventually stop.
Do not get caught out by this. Questions
related to this effect are very common!
Summary
1. Increasing the surface area gives a faster reaction
because more particles are exposed to the other
reactant.
2. Increasing the concentration increases the rate of
reaction because there are more collisions between the
reactant particles.
3. Increasing the temperature increases the rate of
reaction because the particles move move quickly and
so collide more often and with greater energy.
4. A catalyst increases the rate of a reaction because it
reduces the activation energy so more of the collisions
have enough energy to react.
The End
Constructed by:
Rob Dickens, Doha College Science Department.
