Topic 2: Speed of Reactions
Changes in speeds of reaction can be explained using collision theory.
Collision theory states
" before any reaction can take place the reacting particles must collide with enough energy"
eg H2 + O2  H2O
If insufficient energy is present the reacting particles will not react.
Chemical reactions can be speeded up by
1. Decreasing particle size
2. Increasing temperature
3. Increasing concentration
4. Using a catalyst
1. Decreasing particle size
As the particle size decreases for a given mass the surface area increases, collisions must occur
on the surface, so the greater the surface area, the more collisions that will occur, this will increase
the reaction rate.
e.g. iron filings react faster than iron rods.
potatoes cook quicker if cut up in to small pieces.
in flour mills fans extract powdered flour to prevent explosions.
in mines the air is kept damp to stop coal dust catching fire.
powdered metals are used in fireworks to give bright colours.
2. Increasing temperature
As the temperature of the reacting particles increase, the energy of the particles increase. The
number of successful collision will also increase as more collisions will have enough energy to
react. e.g. foods go bad faster at room temperature than in a fridge
3. Increasing concentration
When the concentration is increased the number of particles increase, this increases the number of
collisions and hence the rate increases
e.g. cooking food in a pressure cooker cuts down the time taken.
4. The presence of a catalyst
Catalysts lower the energy needed for successful collisions resulting in the formation of products,
this speeds up the reaction
e.g. manganese dioxide speeds up the decomposition of hydrogen peroxide
vanadium oxide in the Contact Process i.e. making sulphuric acid
iron in the Haber Process i.e. making ammonia
platinum in the Ostwald Process i.e. making nitric acid
platinum and rhodium in catalytic converters
Many catalysts such as manganese dioxide, vanadium oxide, iron and platinum contain transition
elements
Enzymes are biological catalysts.
Enzymes are used in making cheese, yoghurt, bread, wine, beer, lager, whiskey and biological
detergents.
Amylase breaks down starch into glucose.The activity and efficiency of enzymes are influenced by
various factors, including temperature and pH conditions.
Enzymes have many medical and industrial uses, from washing powders to drug production, and
as research tools in molecular biology. They can be extracted from bacteria and moulds, and
genetic engineering now makes it possible to tailor an enzyme for a specific purpose.