CHEMISTRY

TOPIC: RATE OF REACTION – PART 2

By Kingsley Idiagbor, B.Sc (Hons), PGDCs, NCE, MNSM

In the previous lesson, we dealt with the effect of concentration on the rate of reaction, among other points. We will now proceed to cover the effect of the other factors like catalyst, light, temperature, nature of reactants and surface area:

Effect of Catalyst

Catalyst is simply defined as a substance that alters the rate of reaction but itself does not undergo any permanent change at the end of the reaction.

A catalyst like manganese (IV) oxide exerts positive effect on the preparation of oxygen from potassium trioxochlorate, KClO₃. When no catalyst is used in this reaction, long period of heat is required and time for the end of reaction to be reached is longer than when catalyst is used. {See Figure 1}
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Effect of Light

Light affects only some reactions. Examples include those reactions that involve the halogens like those between hydrogen and chlorine and the decomposition of hydrogen peroxide. The rate of these reactions is very negligible in the absence of light, moderate in daylight and very explosive in direct sunlight or in the presence of ultra-violet (UV) radiation. These types of reactions are said to be photocatalytic

Effect of Nature of Reactants

The rate of chemical reaction is first determined by the chemical nature of the reacting substances since various substances may have different energy contents (or enthalpy). Thus, the reaction between copper and hydrochloric acid is a laboratory curiosity and will never take place. Also, the reaction involving gold and hydrochloric acid shows no evidence of reaction.

Effect of Temperature

Reactions generally are faster at high temperatures and slower at low temperatures. The reason for this can be explained away. By increasing the temperature of a system could lead to increase in the kinetic energy, thus leading to a higher frequency of collisions. As a result of this, the number of successful or effective collisions increases and the reaction proceeds at a faster rate. {See Figure 2}
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Effect of Surface Area of Reactants

By bringing the individual particles into greater contact, the rate of reaction could be increased. In order to facilitate greater contact, the exposed greater surface area of solid must be increased by subdividing or breaking the solid into smaller pieces. Thus, powdered CaCO₃ will react faster with HCl acid than would marble chips. Thus, the greater the surface area of the reactants, the higher is the rate of reaction.

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