Meaning of material objects through space and time 62

4.0 Introduction

In unit 3, the rate of a reaction is defined and factors that affect the rate enumerated and discussed. Factors like temperature, concentration, nature and catalyst will affect the rate of all homogeneous reactions.

Other factors like pressure, light and surface area are specific for some types of reaction. One method of expressing reaction rate is the rate of formation (production) of products. The rate at which a product forms from a reaction is of economic importance in industries and factors are always made to bear on reactants (raw materials) not only to improve quality or yield alone but also to increase production rate and reduce time of labour.

In this unit a theory called the collision theory will be discussed and used to explain why some of the enumerated factors in unit 3 have so much effect on the rates of reactions. The concept of activation energy of a reaction and its effect on the rate will also be discussed.

4.1 Objectives

By the end of this unit you should be able to:

• Explain the concept of activation energy.

• Draw energy level diagrams to explain the concept of activation energy.

• Explain and illustrate the effect of catalyst on the activation energy.

• Explain the effects of factors on rate using collision theory.

• Explain why chemical reactions occur at different rates.

4.2 Activation Energy and the Reaction Rate

In a chemical reaction, bonds are broken and formed. An initial energy input is always required to start the reaction. This input of energy activates the reactant molecules to start reacting. This minimum energy that is needed to start a reaction is called the activation energy.

When the reactant molecules acquire the activation energy they form a high energy particle called the activated complex. The activated complex is relatively more unstable than the reactant or product particles because of its high energy. It readily decompose to give the product or the reactant. All chemical ^reactions whether exothermic or endothermic require the activation energy. Reactions with fairly high activation energies are usually very slow at ordinary temperatures while those with small activation energies are very fast. When the activation energy is very high the reaction may not occur except an external source of energy is supplied to activate the reactant molecules. The reaction of hydrogen and oxygen gas is an example of

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cti ation energy

AH

Lion energy

All

catalysed atalised

enthalpy change

such reactions with relatively high activation energy. The reaction occurs only after a spark is introduced into the gas mixture. When a catalyst is added to a reaction mixture the activation energy is lowered and the rate of the reaction increases. The figures 4.1 (a, b, c) illustrate the concept of activation energy for the exothermic and endothermic reaction and the effect of catalyst on the activation energy of the reaction.

Fig. 4.1

(a)

Reaction co-ordinate

Reaction profile for the exothermic reaction

(b)

Reaction co-ordinate Reaction profile for the endothermic reaction

(c⁾

Reaction co-ordinate

Reaction profile for a reaction with and without a catalyst 116

4.3 The Collision Theory 4.3.1 The collision theory discussed

The collision theory assumes that for a reaction to occur the reactant particles must collide. Not all collisions will lead to reaction but only collisions of particles that have sufficient energy to overcome the activatiol.

energy. The collisions that lead to reaction are called effective collisions . The reaction rate is determined by the rate of collision as well as the fraction of effective collisions. The rate of a reaction increases with, number of collisions as well as the fraction of effective collisions. The collision theory can be used to explain the effects of the factors enumerated earlier on the reaction rate.

4.3.2 The collision theory and the nature of the reactant

The reactant nature determines the type of bonds that must be broken before products can be formed. The type of bond depends on the compound. A reactant with very strong bonds in its molecules will not react as fast as one with weak bonds because it will require a higher activation energy. You will recall that the higher the activation energy the slower the reaction and vice versa.

4.3.3 The collision theory and the reactant concentration

When the reactant concentration is increased, the number of colliding particles per unit volume increases.

This will lead to more collisions and therefore increase in reaction rate.

4.3.4 The collision theory and temperature

An increase in temperature of the reacting molecules has a two fold effect.

(i) The particles now have higher average kinetic energies. This will increase the fraction of effective collisions because particles collide with higher energies.

(ii) Because of the higher average kinetic energy, there is increase in random motion which will result in more collisions.

The net effect is that the number of collisions and fraction of effective collisions increase. This will lead to a higher rate of reaction. Fig. 42 gives the normal distribution of kinetic energies between particles at two different temperatures

T, and T2 with T2 > T i

Activation Energy (E.) Fig 4.2 Kinetic energy

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Enthalpy distribution among reacting particles at two temperatures T 1 and T2 . The shaded area under each curve represents the proportion of reactant that have sufficient energy to cause reactions. There are more molecules at T2 that have sufficient energy. Recall that only collisions between molecules with energy equal to or greater than Ea can lead to reaction.

4.3.5 The collision theory and pressure

As the pressure of the gas increases the volume decreases. The particles are now closer together and will make more collisions. This will lead to higher rate of reaction.

4.3.6 The collision theory and catalyst

The catalyst lowers the activation energy of the reaction. More molecules will now have sufficient energy to overcome this small activation energy. The fraction of effective collisions increases and rate of reaction also increases. The catalyst effect on the activation energy is illustrated in Fig. 4.1

4.4 Conclusion

All reactions, whether exothermic or endothermic require activation energy. The very fast nature of some reactions, suggest that their activation energies are very small. Slow reactions such as that between hydrogen and oxygen, nitrogen and oxygen require very high activation energies and do not even occur unless they are given energy to start. The collision theory is able to account for the effect of many factors on the rate of the reaction.

4.5 Summary

The concept of activation energy is discussed. The activation energy of a reaction has a bearing on its rate.

Fast reactions have small and slow ones have very high activation energies.

• The collision theory is discussed. The theory is used to explain the effect of nature of reactant, concentration of reactant, temperature, catalyst and gas pressure on the rate of reaction.

• The effect of temperature as explained by the theory is two fold. Temperature increases the number of collisions and the fraction of effective collisions. This may explain in part why rate experiments are so sensitive to temperature change. You will recall that reaction rate may double for every 10°C rise in temperature.

4.6 Tutor-Marked Assignments

1. What do you understand by the following: reaction rate, energy level diagram, activation energy and activated complex?

2. (a) Explain the effect of a catalyst on the rate of a reaction and draw energy level diagram to illustrate its effect on the reaction profile.

(b) List three properties of a good catalyst.

4.7 References

Bajah, S. T., Teibo, B. 0., Onwu, G. and Obikwere A. (2002). Senior Secondary Chemistry Textbook 2.

Lagos. Longman Publishers.

Osei Yaw Ababio (2002). New School Chemistry. Onitsha. Africana-FEP Publishers.

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Unit 5