Class 12 Chemistry Chemical Kinetics Half life reactions

Half life reactions : The time elapsed in reduction of the concentration of a reactant participating in a chemical reaction to one half of its original concentration is called half-life reaction and is represented by t1/2.

Half-life for zero order reactions:

A --> B

At time t = 0 concentration of A (reactant) is a and B (product) is 0. At time t = t the concentration of A (reactant) is (a-x) and that of B (product) is x.

x = k0t à a/2 = k0t1/2 = a/2k0

t1/2 =a/2k0


y = mx

t = Independent variable. It is taken at x-axis.

m = 2k0

Half-life for first order reactions:

t = 2.303/k1 log a/(a-x)

For half-life x = a/2; t = t 1/2

t ½ = 2.303/k1 log a/(a-a/2) --> t1/2 = 2.303/k1 log2

t ½ = 2.303/k1 X 3.010

t ½ = 0.693/ k1

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