A catalyst is a substance, which increases the rate of a reaction without itself being consumed at the end of the reaction, and the phenomenon is called catalysis. There are some catalysts which decrease the rate of reaction and such catalysts are called negative catalyst. Obviously, the catalyst accelerating the rate will be positive catalyst.

However, the term positive is seldom used and catalyst itself implies positive catalyst. Catalyst are generally foreign substances but sometimes one of the product formed may act as a catalyst and such catalyst is called ” auto catalyst ” and the phenomenon is called auto catalysis. Thermal decomposition of KClO3 is found to be accelerated by the presence of MnO2. Here MnO2 (foreign substance) acts as a catalyst.

2KClO3 + [MnO2] –> 2KCl + 3O2 + [MnO2]

MnO2 can be received in the same composition and mass at the end of the reaction. In the permanganate titration of oxalic acid in the presence of bench H2SO4 (acid medium), it is found that the titration in the beginning there is slow discharge of the colour of permanganate solution but after sometime the discharge of the colour become faster. This is due to the formation of MnSO4 during the reaction which acts as a catalyst for the same reaction. Thus, MnSO4 is an ” auto catalyst ” for this reaction.
This is an example of auto catalyst.

2KMnO4 + 3H2SO4 + 5H2C2O2 –> K2SO4 + 8H2O + 10CO2

General characteristics of catalyst

(i) A catalyst does not initiate the reaction. It simply fastens it.

(ii) Only a small amount of catalyst can catalyse the reaction.

(iii) A catalyst does not alter the position of equilibrium i.e. magnitude of equilibrium constant and hence ΔGo. It simply lowers the time needed to attain equilibrium. This means if a reversible reaction in absence of catalyst completes to go to the extent of 75% till attainment of equilibrium, and this state of equilibrium is attained in 20 minutes then in presence of a catalyst also the reaction will go to 75% of completion before the attainment of equilibrium but the time needed for this will be less than 20 minutes.

(iv) A catalyst drives the reaction through a different route for which energy barrier is of shortest height and hence Ea is of lower magnitude. That is, the function of the catalyst is to lower down the activation.

Ea = Energy of activation in absence of catalyst.
E’a = Energy of activation in presence of catalyst.
Ea − E’a = lowering of activation energy by catalyst.
If k and kcat be the rate constant of a reaction at a given temperature T, and Ea and E’a are the activation energies of the reaction in absence and presence of catalyst, respectively, the

$\large \frac{k_{cat}}{k} = \frac{A e^{-E_a’/RT}}{A e^{-E_a/RT}}$

$\large \frac{k_{cat}}{k} = A e^{(E_a – E_a’)/RT}$

Since Ea, Ea’ is + so kcat > k. the ratio gives the number of times the rate of reaction will increase by the use of catalyst at a given temperature and this depends upon Ea − Ea1.
Greater the value of Ea − Ea1, more number of times kcat is greater than k.

The rate of reaction in the presence of catalyst at any temperature T1 may be made equal to the rate of reaction in absence of catalyst but for this sake we will have to raise the temperature. Let this temperature be T2 this

$\large e^{-E_a’/RT_1} = e^{-E_a/RT_2} $

$\large \frac{E_a’}{T_1} = \frac{E_a}{T_2}$ ….(9)

Studying the progress of reaction

In order to follow the progress of reaction, the concentration of any of the reactants or products which ever is convenient may be determined at various time intervals using some suitable chemical method. At each time interval the progress of the reaction is arrested by immersing the reaction vessel in freezing mixture.

The temperature of the freezing mixture being very low (less than 0°C), the rate of reaction is reduced to almost nothing. It is not necessary to determine always the concentration but one can determine any other parameter that changes during the reaction with time and which directly proportional to the concentration of the reactant or product

Also Read :

→ Chemical Kinetics
→ Rate of Reaction
→ Determination Of rate of reaction
→ Molecularity
→ Order of Reaction
→ Order Of Reaction
→ Difference between Order and Molecularity
→ Kinetics of First order Reaction
→ Half-life period of a first order reaction
→ Graphical Representation for order reaction
→ Effect of Temperature on the Reaction Rate
→ Arrhenius Equation

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