Factors Affecting the Rate of a Chemical Reaction

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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.
3. The reactants must form new bonds to produce
   products.

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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.
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Activation Energy 2
Activation energy for a reaction is shown on
reaction profile diagrams (do you remember
these?).
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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?
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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
• Record the increase in product concentration as
  the reaction progresses.
• Record the decrease in reactant concentration as
  the reaction progresses.

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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
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.
Dilute acid
Marble chips
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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.
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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.
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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.
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Following the reaction between zinc and
hydrochloric acid
Space bar or left click to continue.
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Effect of Concentration
Consider the reaction between zinc and
hydrochloric acid
Zn 2HCl ? H2 ZnCl2
(How could you follow the progress of this
reaction? Click to find out)
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
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Gas Reactions
The rate of reaction between gases is increased
by increased pressure. In effect pressure is the
gas equivalent of concentration.
Low pressure, particles far apart.
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.
Higher pressure, particles closer together.
Higher pressure faster reaction
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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 500C
Reaction at 300C
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
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Effect of a Catalyst 1
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.
A catalyst reduces the activation energy of a
reaction.
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Effect of a Catalyst 2
Activation energy without catalyst.
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.
energy
Activation energy with catalyst.
Catalyst faster reaction.
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More About Rate Graphs

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.

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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.
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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.