The document discusses several factors that affect the rate of a chemical reaction: temperature, concentration, pressure, surface area, orientation of molecules, nature of reactants, and catalysts. It explains that increasing temperature, concentration, pressure, or surface area increases the number and energy of collisions between reactant particles, speeding up reactions. Catalysts also increase reaction rates by lowering the activation energy required for reactions to occur.

1. Factors Affecting the Rate of
a Chemical Reaction
• 1. temperature
• 2. concentration
• 3. pressure (gases)
• 4. surface area
• 5. orientation (molecules must collide
in a specific manner to form a chemical
bond)
• 6. nature of the reactants (solid, liquids, gas)
• 7. catalyst

3. Activation Energy
Defined as:
The minimum energy required to
bring about a chemical reaction.
Gasoline for your car would ignite as soon as it came into contact with air.
You would burst into flames.
If there were no such thing as ‘activation
energy’ life would be very difficult:
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.

4. Activation Energy 2
Activation energy for a reaction is
shown on reaction profile diagrams
Reactants
Products
Activated intermediate
Activation
energy
H
energy

5. 1. 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

6. Acid Particles
Zinc
2. Concentration
Consider the reaction between zinc and hydrochloric acid:
Zn + 2HCl  H2 + ZnCl2
1M hydrochloric acid 2M 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

7. 3. Pressure: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.
Low pressure,
particles far apart.
Higher pressure,
particles closer
together.
The higher pressure jar has
more particles per unit
volume which means a
higher concentration,
hence faster reaction.
Higher pressure = faster reaction

8. 4. Surface Area
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.
The surface particles
are ‘exposed’ and
can react.
‘Inner’ particles are protected and
cannot collide with other particles
until they become ‘exposed’.

9. 4. More on Surface Area
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.

10. 3. Orientation and Efficiency of
Molecular Collisions
A2 + B2 → 2AB

11. 7. Effect of a Catalyst
• A catalyst is a substance 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.
• An enzyme is a catalyst
• A catalyst reduces the activation energy of a
reaction
.
.

12. 7. Effect of a Catalyst 2
Activation energy
without catalyst.
Activation energy
with 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’.
Catalyst = faster reaction.

13. 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.
4. Increase the speed of the rate determining
step
This is all very well but how
can we follow the progress of
a chemical reaction?

14. Following a Chemical Reaction
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.

15. Following a Chemical Reaction 2
As an example consider the reaction between calcium carbonate and
hydrochloric acid.
CaCO3 + 2HCl  CaCl2 + H2O + CO2
We can follow this reaction by measuring the volume of carbon
dioxide produced as the reaction proceeds.
Measure the amount of CO2
collected
Dilute acid
Marble
chips
Gas being collected

16. 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.
All very well, but what
does the graph tell you?
The gradient or slope of the graph
shows the rate of the reaction.
Steeper slope = faster reaction.
Volume
of
gas/cm3
Time/sec.
Reaction fastest at
the beginning.
Slowing down.
Reaction
finished.

17. More About Rate Graphs
Volume
of
gas/cm
3
Time/sec.
Reaction fastest at
the beginning.
Slowing down.
Reaction
finished.
1. Why is the reaction
fastest at the beginning?
2. Why does the reaction
slow down?
3. Why does the reaction
eventually stop?
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.

18. Summary: Factors that Affect
Reaction Rate
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 faster and collide more often
with greater energy.
4. If the reactants are both of the same nature, ex. All gases, the
reaction will proceed at a faster rate. (homogeneous)
5. A catalyst increases the rate of a reaction because it reduces
the activation energy so more of the collisions have enough
energy to react.