Question

What happens to the half-life period for a first order reaction, if the initial concentration of the reactants is increased?

Answer 1

The half-life period of different order of reactions has a different extent of dependence on the concentration of the reactant. The relationship between the half-life period and concentration must be determined in order to understand how they affect each other.

Complete answer:
Chemical kinetics deals with the rate of reactions i.e. how fast or slow the reactants are being transformed into products. It is associated with rate law expressions that establish a relationship between the concentration of the reactants and the rate at which they are consumed.
A first order reaction is the one with its rate dependent on the first power of a single reactant concentration. This means that the change in the concentration of a single reactant alters the rate of the reaction.
The half-life period of a reaction is defined as the time required by a reaction to consume half of the initial concentration of the reactant. The expression for half-life period of a first order reaction in terms of the rate constant is given as follows:
${t_{\dfrac{1}{2}}} = \dfrac{{0.693}}{{{k_1}}}$
Here, ${k_1}$ is the rate constant for the first order reaction.
The expression reveals the fact that the half-life period is independent of the concentration of the reactant and has a constant value at a particular temperature.
Therefore, the half-life period remains unchanged on increasing the initial concentrations of the reactants.

Note:
The order of a reaction is the numerical power on the concentration term written in a rate law expression that determines the extent of dependence of the rate of the reaction on the concentration of the reactant.