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Effect Of Catalyst On Rate Of Reaction And Collision Theory

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Effect Of Catalyst On Rate Of Reaction And Collision Theory - Lesson Summary

The rate of a reaction depends on the concentration of its reactants and the temperature.

Along with the concentration and temperature, a catalyst also has a major effect on the rate of a reaction.

A catalyst is defined as a substance that alters the rate of a reaction without itself undergoing any permanent chemical change. A catalyst, thus, can enhance (or) retard the rate of a reaction. A catalyst that enhances the rate of a reaction is called a positive catalyst, while a catalyst that retards the rate of a reaction is called a negative catalyst.

Positive catalyst:

Increases the rate of a chemical reaction

Is not consumed by the reaction

Does not change the equilibrium constant for the reaction, it shortens the time required to establish equilibrium, but doesn't affect the final position of the equilibrium.

Ex: The hydrogenation of ethene in the presence of nickel yields ethane.

Catalysts increase the rate of reactions by providing a new pathway (or) mechanism that has a lower activation energy.

A larger proportion of the collisions that occur between the reactants have enough energy to overcome the activation energy for the reaction. Thus, a large number of collisions become effective and the rate of the reaction increases.

The collision theory was proposed individually by Max Trautz in 1916 and William Lewis in 1918.

According to this theory "Chemical reactions occur when the hard, spherical molecules of reactants collide with each other". Also, the reactant particles must collide not only with sufficient energy, but also with the correct orientation. Such collisions are referred to as effective collisions. These collisions lead to the formation of the product.

Thus according collision theory, the activation energy and proper orientation of the molecules together determine the criteria for an effective collision.

Arrhenius equation for a reaction can be modified to

                                A + B → Products
                       Rate Constant , k = ZAB x e-Ea/RT
ZAB = Collision frequency of reactants A and B
e-Ea/RT = Fractions of the molecules with activation energy greater than Ea


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