Introduction:

We are now very much familiar with the rate of reaction and factors which affect the rate enumerated. Factors such as concentration, temperature, nature and catalyst will influence the rate of all the homogeneous reactions.

Other factors such as pressure, light and surface area are particular for a few kinds of reaction. One process of expressing reaction rate is the rate of formation (that is, production) of products. The rate at which a product forms from a reaction is of economic significance in industries and factors are for all time made to bear on reactants (that is, raw materials) not just to enhance quality or yield alone however as well to increase the production rate and reduce time of labor.

Activation Energy and Reaction Rate:

In a chemical reaction, the bonds are broken and formed. An initial energy input is for all time needed to start the reaction. This input of energy activates the reactant molecules to begin reacting. This minimum energy which is required to start a reaction is known as the activation energy.

If the reactant molecules obtain the activation energy they make a high energy particle known as the activated complex. The activated complex is relatively very unstable than the reactant or product particles due to its high energy. It readily decomposes to provide the product or the reactant. All chemical reactions whether endothermic or exothermic need the activation energy. Reactions having fairly high activation energies are generally very slow at ordinary temperatures whereas those having small activation energies are very fast. Whenever the activation energy is very high the reaction might not take place apart from an external source of energy is supplied to activate the reactant molecules. The reaction of hydrogen and oxygen gas is an illustration of such reactions having relatively high activation energy. The reaction takes place only after a spark is introduced to the gas mixture. Whenever a catalyst is added to a reaction mixture the activation energy is lowered and the rate of the reaction increases. The figure shown below describes the concept of activation energy for the exothermic and endothermic reaction and the effect of catalyst on the activation energy of the reaction.

Fig: Activation Energy and the Reaction Rate

The Collision Theory:

The collision theory supposes that for a reaction to take place, the reactant particles should collide. Not all collisions will lead to reaction however only collisions of particles which encompass adequate energy to overcome the activation energy. The collisions that lead to the reaction are known as the effective collisions. The reaction rate is found out by the rate of collision and also the fraction of effective collisions. The rate of a reaction rises by, number of collisions and also the fraction of effective collisions. The collision theory can be employed to describe the effects of the factors on the reaction rate.

The collision theory and the nature of the reactant:

The reactant nature finds out the kind of bonds which should be broken before products can be formed. The kind of bond based on the compound. A reactant having very strong bonds in its molecules will not react as fast as one by weak bonds as it will need higher activation energy. We are familiar that, the higher the activation energy the slower the reaction and vice-versa.

The collision theory and the reactant concentration:

Whenever the reactant concentration is increased, the number of colliding particles per unit volume rises. This will lead to more collisions and thus increase in the reaction rate.

The collision theory and temperature:

The rise in temperature of the reacting molecules consists of a twofold effect.

1) The particles now encompass higher average kinetic energies. This will raise the fraction of effective collisions as particles collide by the higher energies.

2) Due to the higher average kinetic energy, there is an increase in arbitrary motion that will outcome in more collisions.

The total effect is that the number of collisions and fraction of effective collisions increase. This will lead to the higher rate of reaction. The figure illustrated below represents the normal distribution of kinetic energies between the particles at two different temperatures.

Fig: Kinetic energy

The Enthalpy distribution is among the reacting particles at two temperatures T₁ and T₂.  The shaded area under each and every curve symbolizes the part of reactant which has adequate energy to cause reactions. There are more molecules at T₂ that encompass adequate energy. Remember that only collisions between the molecules having energy equivalent to or greater than Ea can lead to the reaction.

The collision theory and pressure:

As the pressure of the gas rises the volume reduces. The particles are now closer altogether and will make more collisions. This will lead to the higher rate of reaction.

The collision theory and catalyst:

The catalyst lowers the activation energy of the reaction. More molecules will now encompass adequate energy to overcome this small activation energy. The fraction of effective collisions rises and rate of reaction as well increases.

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