Factors that influences the rate of reaction

The rate of reaction depends on the following factors:

• Concentration of the reactants

• Surface area of the reactants

• Temperature

• Catalyst

• Light

Effect of Concentration

The rate of a reaction generally increases with increase in concentration of the reactants. For a reaction to take place between two molecules, they must collide or come into contact. The probability of such collisions increase with the increase in the concentration of the reacting molecules.

Different reactants can affect the rate of a particular reaction to different extent.

For example, consider the reaction

The rate of this reaction doubles when concentration of oxygen is doubled. On the other hand, when the concentration of nitric oxide is doubled, the rate of reaction increases four times.

In systems where the reactants and the products are in different states, the area of contact between the reacting substances influences the rate considerably. It is found that magnesium powder reacts much more rapidly than magnesium ribbon with dilute H₂SO₄.

Activity to Study the Effect of Concentration

Requirements: 6g of granular zinc

10 mL of 1 M HCl, 10 mL of 2 M HCl, A graduated syringe and a boiling tube.

The syringe is attached to the boiling tube and the reaction is carried out with 3g of zinc and 5 mL of 1 M HCl and next with 3g of zinc and 5 mL of 2 M HCl.

The volume of the gas at time intervals of 20s, 40s and 60s is recorded and the results are plotted.

Data Plot Demonstrating Concentration Effects at 1 M and 2 M HCl, at constant Temperature

Effect of Surface Area of the Reactants

The rate of the reaction increases with increase in surface area of the reactants.

It is a common observation that small pieces of wood burn more rapidly than large logs of wood.

As mentioned earlier, magnesium powder reacts with dilute sulphuric acid more rapidly than magnesium ribbons. Thus smaller the size of reacting particles, the greater is the total surface area exposed for reaction and consequently greater is the rate of the reaction.

Effect of Temperature

Generally, the rates of chemical reactions increase with increase in temperature. It is a common observation that perishable foods like milk, cooked vegetables decay more quickly in summers than in winter.

Consider the chemical reaction:

At 273 K, the reaction is extremely slow and the amount of CO₂ collected is very small. When the temperature is raised to 323 K (50^o C) the evolution of CO₂ is more rapid.

Showing Effect of Temperature on the Rate of Reaction

The reaction between zinc and hydrochloric acid can be performed at two different temperatures: 293 K and 308 K.

It will be found that the volume of hydrogen formed during the same time interval will be more at 308K then at 298 K.

Data Plot Demonstrating Temperature Effects at 293 K and 308 K for 1M HCl

Effect of Catalyst

Catalysts are substances, which alter the rate of chemical reactions without undergoing any overall chemical change themselves.

In the manufacture of ammonia, iron is used as a catalyst and it enhances the rate of this reaction.

as a catalyst.

Sometimes catalysts are used to retard or slow down the rates of reaction. For example., glycerol is used to slow down the decomposition of hydrogen peroxide. Such catalysts are known as negative catalysts.

Many industrial processes like the manufacture of polythene and polystyrene require catalysts.

In the living bodies, a large number of complex chemical reactions occur which are catalysed by complex organic molecules called enzymes. For example., enzyme amylase present in saliva catalyses the decomposition of starch into maltose.

Effect of Light

There are many reactions, which are influenced by light. For example., photosynthesis and photography. Such reactions are known as photochemical reactions.

Other examples are:

Light supplies the necessary energy for the reaction to take place. Photographic films which are normally coated with silver bromide (and a very small amount of silver iodide) undergo chemical reaction when exposed to sunlight. This basic reaction is used in photography.