O Level Notes : Chemistry - Speed of Reaction

The speed at which a chemical reaction proceeds can vary, and is called the rate of reaction. We will study about this in detail in this chapter.


I). Measuring Speed of Reaction:

The speed of reaction can be found by measuring these quantities at regular time intervals, when the chemical reaction is in progress.

  • The volume of a gas produced by the reaction.
  • The mass of the reactant that remains.


Either of the mentioned is measured, and a graph of that against time is plotted. The speed of reaction at any instant is measured by drawing a tangent against that point on the graph, and calculating its gradient.


II). Factors Affecting Speed of Reaction:


There are various factors that affect the speed of a chemical reaction, the major ones being:

  • The temperature at which the reaction is occurring
  • The concentration of the substances used (reactants)
  • The pressure on the reaction (if the reactants are gaseous)
  • The particle size or surface area of the reactants.


For the reaction to occur between two particles, the reacting particles must collide with each other, and they must collide with a certain minimum amount of energy, known as activation energy.

The more the collisions of particles, the faster the rate of reaction. In other words – as we are talking about the factors that affect the speed of reaction – when any factor increases the rate of effective collisions between reacting particles, it will also increase the speed of reaction.



The higher the temperature, the faster the rate of reaction. You see, when the temperature is raised, the reactant particles have a greater heat energy, causing them to move about more and with a greater kinetic energy. They, therefore, stand a better chance of colliding into another reactant molecule with sufficient energy to convert into product molecules.



The more concentrated the reactants, the faster the speed of a chemical reaction. This is because at a higher concentration, there is a greater likelihood that reacting molecules will collide with one another with sufficient energy to form products.



In the reactions that involve gases, the speed of reaction increases if the pressure is increased. Again, this is really the effect of concentration as higher pressures force the particles close together and so their concentration within a certain volume increases. More collisions therefore occur, and the speed of reaction increases.


Particle Size:

The greater the surface area, the faster the rate of a chemical reaction. Smaller particles like powders have a much greater surface area than larger lumps or crystal. With a greater surface area, the other reactant can attack it more easily, and thus the speed of reaction increases.


III). Catalysts and Enzymes:


A catalyst is a substance which increases the rate of a chemical reaction by lowering its activation energy, without itself being chemically changed at the end of the reaction. It works by providing an alternate, more direct route from reactants to products.

Enzymes are biological catalysts; they are present in all living organisms and are responsible for breakdown of foodstuffs.


As I mentioned, catalyst speed up chemical reactions. So that is another factor that affects the rate of chemical reactions: the presence of catalysts.