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

**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 = k_{0}t à a/2 = k_{0}t_{1/2} = a/2k_{0}

t_{1/2} =a/2k_{0}

a=(2k_{0})(t_{1/2})

y = mx

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

m = 2k_{0}

**Half-life for first order reactions**:

t = 2.303/k_{1} log a/(a-x)

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

t _{½} = 2.303/k_{1} log a/(a-a/2) --> t1/2 = 2.303/k_{1} log2

t _{½} = 2.303/k_{1 }X 3.010

t _{½} = 0.693/ k_{1}

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