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Monday, March 20, 2017

Temperature dependence of the rate of a reaction

 1. Effect of temperature on reaction rate:

As the temperature increases, the average energy of the reaction increases, hence rate of reaction increases.

 As a common rule, for every 10° rise of temperature, the rate of reaction will increase by 2 (or) 3 times of initial value.

 According to Arrhenius concept:

η = η₀ e -Eₐ/RT

η₀ = total number of molecules

η = total number of molecules activated

Eₐ = Activation energy

It can be written in the following way.

K = A e -Eₐ/RT

K = Rate Constant of reaction

log K₂/K₁ = -Eₐ/R ([1/T₂] – [1/T₁])


2. Effect of catalyst on reaction rate:

 A catalyst alters the reaction velocity by changing the activation energy by changing the reaction mechanism, without getting consumed during the reaction.

 The physical state of the catalyst may or may not change.

 A catalyst works by providing an alternate path to the reaction

a. Positive catalysts: A catalyst which increases the rate of reaction is called positive catalyst. Such catalyst decreases activation energy by accepting a smaller path, so rate of reaction is increased.


Eg: 

b. Negative catalysts (Inhibitors): A catalyst which decreases or retards the rate of reaction is called negative catalysts.

It is because a -ve catalyst increase activation energy by taking a longer alternative path.

Eg: 

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