Standard Grade Chemistry
Summary Notes
Topic 2 : Speed Of Reactions
Learning Outcomes
By the end of the unit you should be able to :
 describe how the following affect the speed of a reaction :
 particle size
 concentration
 temperature
 give everyday examples of reactions affected by :
 particle size
 concentration
 temperature
 state that catalysts are substances which :
 speed up some reactions
 are not used up during the reaction
 can be recovered chemically unchanged
 give everyday example of uses of catalysts e.g. transition metals in car
exhaust systems
Speed of Reactions
Chemical reactions happen all around us. They also happen in plants and animals and
in our own bodies. Chemical reactions can take place quickly or slowly.
When iron rusts, its atoms join with atoms of oxygen and turn into iron oxide, which
is rust.
iron + oxygen  iron oxide (rust)
However, this happens quite slowly. A car does not turn into rust overnight! It
becomes rusty after several years.
Oil and gas were formed from the dead bodies of tiny sea animals. This did not
happen quickly, it took millions of years for heat and pressure in the earth to make
chemical reactions happen which turned these tiny animals into oil and gas.
Chemical Reactions
Reactions go ahead at different rates or speeds, even though they may be similar
types of reaction.
Chemical reactions can be slow or fast. The speed at which chemical reactions
happen is called the rate of reaction.
Changing the Speed
The speed of a chemical reaction can be increased by:
 increasing the temperature
 increasing the concentration
 decreasing the particle size
 using a catalyst
Understanding Chemical Reactions
To understand why different conditions affect the rate of a chemical reaction we
really need to understand how particles behave. All substances are made of tiny
particles called atoms or molecules. These particles are always moving. Even in a
solid (with its fixed shape and volume) they are always moving – because they can
vibrate to and fro.
Chemicals cannot react together unless their molecules bump into each other! The
molecules have to collide. Anything you can do to make more collisions will speed up
the reaction.
Making More Collisions
If we heat up a substance we make its molecules move faster. In fact, temperature
is just a measure of how fast the molecules are moving! If molecules are moving
faster, there is more chance that they will collide with each other so there is more
chance that they will react. It is just like a busy road. The faster the cars are
moving, the more chance there is of a collision.
Another way of making more collisions is to make the chemicals more concentrated.
If there are more molecules then there is more chance of them colliding. It is like a
busy road and a quiet road. There are more cars on a busy road so there is more
chance of a collision.
If one of the chemicals is a solid, then the only molecules of the solid that can react
are the ones on the surface. If we can make the surface bigger then more molecules
will be able to react. We can make the surface bigger by breaking the lump of solid
into smaller pieces, because then there will be more surface molecules exposed to
react. One gram of a chemical as a single lump has quite a small surface area. If we
crush it down to a fine powder, it has a much bigger surface area and so more
molecules can react.
Catalysts
The rate of some chemical reactions can be changed by adding a substance called a
catalyst.
A catalyst is a substance which speeds up a chemical reaction without being
used up.
Catalysts are important in making chemicals on a large scale. Catalysts are used in
industry to produce chemicals using less energy and, therefore, more cheaply.
Uses Of Catalysts
Catalysts are not only used in a chemical laboratory. They are used in factories and
we use them at home and in the garden.
 nickel - is used in the manufacture of margarine
 iron – is used in the manufacture of ammonia
 platinum – is used in the manufacture of nitric acid
 platinum – is used in catalytic converters in car exhaust systems