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KS4 Chemistry
Rates of Reaction
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Contents
Rates of Reaction
Introduction
Effect of temperature
Effect of concentration
Effect of surface area
Effect of catalysts
Summary activities
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Rates of reactions
The speed of different chemical reactions varies hugely.
Some reactions are very fast and others are very slow.
The speed of a reaction is called the rate of the reaction.
What is the rate of these reactions?
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Reaction
Rate
rusting
slow
explosion
very fast
chemical weathering of rocks
very slow
sodium and water
fast
rotting fruit
slow
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Reactions, particles and collisions
Reactions take place when particles of reactants collide with
a certain amount of energy.
This energy is called activation energy, and is different for
each reaction.
The rate of a reaction depends on two things:
 the frequency of collisions between particles;
 the energy with which particles collide.
If particles collide with less energy than the activation energy,
they will not react. The particles will just bounce off each
other.
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Changing the rate of reactions
Anything that increases the number of successful collisions
between reactant particles will speed up a reaction.
What factors speed up reactions?
 Increased temperature;
 increased concentration of
dissolved reactants, and increased
pressure of gaseous reactants;
 increased surface area of solid
reactants;
 use of a catalyst.
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Measuring rates of reaction
Measuring the rate of a reaction means measuring the
rate of change over a period of time.
This means measuring the change in the amount of a
reactant or the amount of a product.
What can you measure to calculate the rate of reaction
between magnesium and hydrochloric acid?
magnesium
+
hydrochloric
magnesium

acid
chloride
+
hydrogen
 The amount of magnesium used up (g/min).
 The amount of hydrochloric acid used up (cm3/min).
 The amount of magnesium chloride produced (g/min).
 The amount of hydrogen produced (cm3/min).
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Slower and slower!
Reactions do not proceed at a steady state. They start off at
a certain speed, then get slower and slower until they stop.
As the reaction progresses, the concentration of reactants
decreases. This reduces the frequency of collisions between
particles and so the reaction slows down.
fast
slower
very slow
stopped
0%
25%
75%
100%
reactant A
reactant B
product
percentage completion of reaction
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Rate of reaction and graphs
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Graphs and reactant-product mix
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Particles and rates of reaction
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Contents
Rates of Reaction
Introduction
Effect of temperature
Effect of concentration
Effect of surface area
Effect of catalysts
Summary activities
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The effect of temperature on collisions
How does temperature affect the rate of particle collision?
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Temperature
The higher the temperature, the faster the rate of a reaction.
In many reactions, a rise in temperature of 10°C causes the
rate of reaction to approximately double.
Why does increased temperature increase the rate of
reaction?
 At a higher temperature, particles
have more energy. This means
they move faster and are more
likely to collide with other particles.
 When the particles collide, they do
so with more energy, and so the
number of successful collisions
increases.
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Temperature and particle collisions
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Temperature and food
Food goes off because chemical reactions take place.
Why does food remain usable for much longer if it is kept in
a freezer?
The low temperature in the freezer means that particles will
move much slower and with less energy than if they were at
room temperature. This means that there are fewer
successful collisions and so a slower rate of reaction.
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Temperature and cooking
Before microwave ovens were common, many people used
pressure cookers to cook food more quickly.
In a pressure cooker, water
doesn’t boil until it reaches about
115°C. How does this help
cooking?
The higher temperature means that particles move more
quickly and with more energy. This means that there are
more successful collisions between particles, and the food
cooks more quickly.
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Temperature and rate of reaction
The reaction between sodium thiosulfate and hydrochloric
acid produces sulfur.
sodium
sodium
hydrochloric

chloride
thiosulfate +
acid
Na2S2O3
(aq)
+
2HCl
(aq)
2NaCl

(aq)
+
sulfur
dioxide
+
SO2
(g)
+
sulfur
+
S
(s)
+ water
+
H2O
(l)
Sulfur is solid and so it turns the solution cloudy.
The effect of increasing temperature on the rate of reaction
can be measured by comparing how long it takes the
solution to turn cloudy at different temperatures.
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Sodium thiosulfate and hydrochloric acid
To run the experiment investigating the effect of temperature
on the rate of reaction:
1. Mark a cross on a piece of paper.
2. Add a known amount of sodium thiosulfate to a beaker,
and place it on the piece of paper.
3. Add a known amount of hydrochloric acid to the beaker
and immediately start a stop-clock. The solution will begin
to turn cloudy.
4. As soon as the cross can no longer be seen, stop the
clock and note the time.
5. Repeat the experiment at different temperatures using the
same volume of reactants. Compare how long it takes the
cross to disappear.
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Sodium thiosulfate and hydrochloric acid
When looking down into the beaker, the cross will become
fainter over time:
increasing time
The time taken for the cross to disappear can be used as the
time of the reaction.
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Time of reaction vs. temperature graph
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True or false?
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Contents
Rates of Reaction
Introduction
Effect of temperature
Effect of concentration
Effect of surface area
Effect of catalysts
Summary activities
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Increasing concentration
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Concentration
The higher the concentration of a dissolved reactant, the
faster the rate of a reaction.
Why does increased concentration increase the rate of
reaction?
At a higher concentration, there are more particles in the
same amount of space. This means that particles are more
likely to collide with other particles.
low concentration
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high concentration
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Concentration and particle collisions
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Reaction between acid and metal
Reactive metals such as magnesium react with acid to
produce hydrogen gas.
magnesium
+
Mg (s)
+
magnesium
hydrochloric

chloride
acid
2HCl (aq)
 MgCl2 (aq)
+
hydrogen
+
H2 (g)
The effect of increasing concentration on the rate of reaction
can be measured by comparing how quickly hydrogen is
produced using different concentrations of hydrochloric acid.
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Mg + HCl: experiment set-up
What equipment do you need for the experiment investigating
the effect of concentration on the rate of reaction?
glass
tube
conical
flask
rubber
connector
gas syringe
rubber bung
hydrochloric
acid
magnesium
ribbon
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Magnesium and hydrochloric acid
To run the experiment investigating the effect of concentration
on the rate of reaction:
1. Measure out a fixed volume of hydrochloric acid into
the conical flask.
2. Add a known mass of magnesium to the flask, immediately
attach the gas syringe and start a stop-clock.
3. Measure the volume of hydrogen collected in the syringe
at regular intervals until no more gas is produced.
4. Repeat the experiment using a different concentration of
hydrochloric acid but using the same volume of acid and
the same mass of magnesium. Compare the rate at which
hydrogen is produced.
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Mg + HCl: different concentrations
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Pressure
Why does increasing the pressure of gaseous reactants
increase the rate of reaction?
As the pressure increases, the space in which the gas
particles are moving becomes smaller.
low pressure
high pressure
The same
number of
particles but in a
smaller space.
The gas particles become closer together, increasing the
frequency of collisions, and so increasing the rate of reaction.
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Contents
Rates of Reaction
Introduction
Effect of temperature
Effect of concentration
Effect of surface area
Effect of catalysts
Summary activities
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Surface area
Any reaction involving a solid can only take place at the
surface of the solid.
slow
rate
If the solid is split into several
pieces, the surface area increases.
This means that there is an increased area for
the non-solid reactant particles to collide with.
The smaller the pieces, the larger the surface
area. This means more collisions and a faster
rate of reaction.
fast
rate
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Surface area and particle collisions
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Reaction between a carbonate and acid
Marble chips are made of calcium carbonate. They react with
hydrochloric acid to produce carbon dioxide.
calcium
carbonate
CaCO3
(aq)
+
+
hydrochloric

acid
2HCl
(aq)

calcium
chloride
CaCl2
(aq)
+
water
+
H2O
(aq)
+
carbon
dioxide
+
CO2
(g)
The effect of increasing surface area on the rate of reaction
can be measured by comparing how quickly the mass of the
reactants decreases using marble chips of different sizes.
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CaCO3 + HCl: experiment set-up
What equipment do you need for the experiment investigating
the effect of surface area on the rate of reaction?
conical
flask
calcium
carbonate
chips
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cotton wool
‘plug’
hydrochloric
acid
weighing
scales
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Calcium carbonate and hydrochloric acid
To run the experiment investigating the effect of surface area
on the rate of reaction:
1. Measure out a fixed volume of hydrochloric acid into a
conical flask and place the flask on weighing scales.
2. Add a fixed mass of calcium carbonate chips to the flask,
and place a cotton wool plug in the neck. This stops the
liquid from spitting while allowing the CO2 to escape.
3. Begin taking mass readings straight away, and continue
until there is no further change in mass.
4. Repeat the experiment using the same mass of calcium
carbonate but of a smaller chip size, and the same
volume of hydrochloric acid. Compare the rate at which
the mass of reactants decreases.
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CaCO3 + HCl: different surface areas
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Contents
Rates of Reaction
Introduction
Effect of temperature
Effect of concentration
Effect of surface area
Effect of catalysts
Summary activities
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What are catalysts?
What are catalysts?
Catalysts are substances that change the rate of
a reaction without being used up in the reaction.
Catalysts are very important in industry because products
can be made more quickly, saving time and money.
They can also avoid having to use high temperatures, so
they can save fuel and reduce pollution.
Catalysts are also very important in living cells. Biological
catalysts are special types of protein called enzymes.
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Examples of catalysts
Many catalysts are transition metals or their compounds.
Different reactions use different catalysts. For example:
 Nickel is a catalyst in the production of margarine
(hydrogenation of vegetable oils).
 Iron is a catalyst in the production of ammonia from
nitrogen and hydrogen (the Haber process).
 Platinum is a catalyst in the
catalytic converters of car
exhausts. It catalyses the
conversion of carbon monoxide
and nitrogen oxide into the less
polluting carbon dioxide and
nitrogen.
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How do catalysts work?
How do catalysts work?
For a chemical reaction to take place:
 energy is needed to break existing bonds, so new
bonds can be formed;
 the reacting parts of particles need to be brought together.
Different catalysts work in different ways, but most solid
catalysts work by lowering the amount of energy needed for
the reaction to take place.
Catalysts work by lowering the
activation energy of a reaction.
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How a platinum catalyst works
When hydrogen and oxygen are mixed in a jar, there is no
reaction. If a platinum wire is added, the gases react instantly
with a loud pop, producing water.
How does platinum catalyse this reaction?
platinum
wire
oxygen
molecule
hydrogen
molecule
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The gas molecules are brought
together onto the surface of the
platinum. They are adsorbed.
The molecules are much closer
together and their bonds are
weakened, lowering the
activation energy of the reaction.
The larger the surface area
of the platinum, the quicker
the reaction.
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Decomposition of hydrogen peroxide
Hydrogen peroxide decomposes into water and oxygen.
hydrogen
peroxide

water
+
oxygen
2H2O2 (aq)

2H2O (l)
+
O2 (g)
Without a catalyst, this reaction is very slow, and can take
months. With a catalyst such as manganese (IV) oxide, the
reaction takes minutes.
Catalysts never produce more product – they
just produce the same amount but quicker.
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Contents
Rates of Reaction
Introduction
Effect of temperature
Effect of concentration
Effect of surface area
Effect of catalysts
Summary activities
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Glossary
 activation energy – The amount of energy needed for a
reaction to begin.
 adsorption – The formation of a layer of molecules on the
surface of a solid.
 catalyst – A substance that changes the rate of a reaction
without being used up.
 concentration – The amount of particles in a given
volume.
 enzyme – A biological catalyst.
 rate of reaction – The speed with which a particular
chemical reaction progresses.
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Anagrams
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Increase or decrease?
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Stages of a reaction
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Multiple-choice quiz
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