A catalyst is a substance which alters the rate of a chemical reaction but remains chemically unchanged at the end of the reaction.

The catalyst does this by lowering the activation energy making it possible for many more reactant particles to attain the towered value on collision. The presence of a catalyst therefore increases the rate of chemical reactions. For example, in the thermal decomposition of potassium chlorate, manganese (iv) oxide acts as a catalyst.

The reaction rate increases tremendously on adding a small amount of manganese (iv) oxide.

• Experimental investigation of the effect of catalyst on reaction rate

Example

Decomposition of hydrogen peroxide to give water and oxygen in the presence of manganese (v) oxide as catalyst.

Procedure

Weigh out exactly 1 g of manganese (IV) oxide into a weighing bottle. Pour 50cm3 of distilled water into the conical flask and add 1 0cm3 of hydrogen peroxide solution. Add the manganese (iv) oxide to the conical flask and cork quickly. At the same time, start the stop-watch. The volume of oxygen given off is measured at regular intervals. (The gas can be shown to be oxygen because it will re-light a glowing splint).

Record the volume of oxygen evolved at one-minute intervals. Repeat the experiment without the manganese (IV) oxide.

Plot the rate curves on the same axes. Rate curves similar to the following are obtained.

A catalyst that works negatively (reduces the rate of a chemical reaction) is called an inhibitor.

Glycerin or phosphoric acid is sometimes added to hydrogen peroxide to slow down its rate of decomposition. A catalyst can be described as homogeneous. That is, the catalyst is in the same phase as the reactants and the products of the reaction. For example, Nitrogen monoxide gas can be used as a catalyst in the conversion of sulphur dioxide to sulphur trioxide in the presence of oxygen

Characteristics of a catalyst

• It is specific. That is a given catalyst can alter the rate of only one particular or a specific type of reaction. For example copper(II) oxide catalyses the thermal decomposition of potassium chlorate but no decomposition of hydrogen peroxide.

• It remains chemically and quantitatively unchanged at the end of the reaction.

• It may be poisoned by certain substances (impurities) and hence need constant replacement e. g. platinum in the contact process is easily poisoned by arsenic compounds.

• Increasing its surface area increases its efficiency. For example finely divided iron is used in the Haber process.

• A catalyst speeds the rate for a chemical reaction by lowering the activation energy of the reaction

The negative catalyst (inhibitor) on the other hand alters the rate by making the activation energy of the reaction higher.

• A catalyst does not affect the type of product Light

1. Light

Some reactions called photochemical reactions are influenced by light. For example the reaction between chlorine and methane does not occur in the d but it is explosive in bright sunlight. In most photochemical

reactions, investigations have shown that When the reactant molecules absorb light, free radicals are formed which then initiate a series of chain reactions eventually leading to the formation of the products.