Factors affecting rate of reaction
There are several factors that can speed up or slow down a reaction, and some of these factors are:
- Concentration of the reactants
- Temperature of the reactants
- Surface area or particle size of the reactants
- Presence of a catalyst
- Pressure of the reactants if they are gaseous
How concentration of the reactants affects rate of reaction
If the concentration of the reactants is increased, the rate of reaction increases.
For example, in the reaction between dilute hydrochloric acid and magnesium,
- if the acid is dilute, there are fewer acid molecules in the reacting mixture. Therefore there is a lesser chance of an molecules particle colliding with a magnesium atom.
- if the acid is more concentrated, there are more acid molecules in the reacting mixture, and therefore more chance of a successful collision.
The more successful collisions there are, the faster the reaction.
How temperature of the reactants affects rate of reaction
If the temperature of the reactants is increased, the rate of reaction increases.
For example, in the reaction between hydrochloric acid and magnesium,
- when heated, the reacting particles take in heat energy.
- this makes the acid molecules move faster and therefore collide more often with magnesium particles.
The extra heat energy provided by increasing the temperature increases the speed of the reacting particles, which results in more successful collisions. So the reaction rate increases.
How surface area or particle size of the reactants affects rate of reaction
The higher the surface area (smaller particle size) of the reactants, the faster the rate of reaction.
For example, in the reaction between magnesium and hydrochloric acid,
- If the magnesium is in form of a ribbon, the acid molecules can collide only with the magnesium atoms in the outer layer of the metal ribbon. This results in fewer magnesium atoms being exposed to the acid. So there is less chance of successful collisions.
- If the magnesium is in powdered form, many more atoms are exposed to the acid. So the chance of a successful collision increases.
Smaller particle size means larger surface area and larger particle size means smaller surface area.
How the presence of a catalyst affects rate of reaction
A catalyst is a substance that increases the rate of a chemical reaction without itself being used up in the reaction.
The table below shows some examples of catalysts and the reactions in which they are used.
|Catalyst||Where it is used|
|Iron||manufacture of ammonia from hydrogen and nitrogen |
N2 + 3H2 → 2NH3
|Vanadium (V) oxide||manufacture of sulphuric acid |
2SO2 + O2 → 2SO3
Another example of the use of a catalyst is in the decomposition of hydrogen peroxide. Hydrogen peroxide is a colourless liquid that breaks down very slowly to water and oxygen as follows:
hydrogen peroxide → water + oxygen
2H2O2(l) → 2H2O(l) + O2(g)
Adding manganese(IV) oxide to hydrogen peroxide increases the rate of decomposition of hydrogen peroxide.
As we said earlier on, for a reaction to proceed, the reacting particles must collide with enough activation energy. However a catalyst works by lowering the activation energy of the reaction so that successful collision occur even when the particles have less energy. This means that more collisions now have enough energy to be successful and the reaction speeds up.
At the end of the reaction, the catalyst itself is unchanged.
What are enzymes?
Enzymes are proteins that act as biological catalysts.
One example of an enzyme is amylase in saliva. Amylase speeds up the breakdown of the starch in food. Enzymes make it possible for the reactions that take place in the body to be quick at body temperature.
How the pressure of the reactants affects rate of reaction
Increasing pressure increases the rate of reaction if the reactants are gases.
Increasing the pressure of two reacting gases increases the number of gas molecules in the given space. This increases the chance of
How light affects rate of reaction
Some chemical reactions need light energy to proceed. These type of reactions are called photochemical reactions. One good example of a photochemical reaction is photosynthesis.
Photosynthesis is the reaction between carbon dioxide and water, in the presence of chlorophyll to produce glucose using sunlight as a source of energy.
The rate of a photochemical reaction can be imcreased by increasing the intensity of the light.
Lumps of calcium carbonate (marble chips) react with hydrochloric acid as follows:
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)
- Name the gas that is released in this reaction?
- Describe a laboratory method that could be used to investigate the rate of the reaction.
- How will this affect the rate of the reaction?
- Explain each of the effects in (c) in terms of collisions between reacting particles.
- If the lumps of marble are crushed first, will the reaction rate change? Explain your answer.
i. increasing the temperature
ii. adding water to the acid
You can read more on factors that affect rate of chemical reactions here