Chemical Kinetics

The study of rates of chemical reactions mechanisms and factors affecting rates of reactions is called chemical kinetics.

Spontaneous chemical reaction means the reaction which occurs on its own without any external reagent. A spontaneous chemical reaction may be slow or fast (instantaneous).

For spontaneous reaction ΔG = -ve.

Based on the speed of reactions, the chemical reactions are of three types:

  1. Very slow reactions. Example: Rusting of iron, Formation of H2O from H2 and O2 at room temperature
  2. Very fast reactions (instantaneous reactions)
  3. Slow reactions: These reactions occur with optimum speeds.

Rates of very slow and very fast reactions can not be determined. Rate of the slow reactions can be measured.

Reaction Rate or Rate of Reaction

The change in concentration per unit time is called rate of reaction. Rate of reaction can be expressed with respect to reactants or products. The rate of reaction changes with time.

The decrease in the concentration of the reactant per unit time or increase in the concentration of the product per unit time is called rate of the reaction.

The rates of chemical reactions differ form one another, since the number and the nature of the bonds are different in the different substances (reactants products or both).

Factors influencing rate of reaction

1. Nature of the reactants

The rate of reaction depends on the nature of reactants. Reactants which require less activation energy react faster than the reactants which require higher activation energy. Ionic reactions and neutralisation reactions are very fast in solution as they involve simple exchange of ions.

Reactions between covalent molecules are generally slow as they involve shuffling of bonds. The reactions which involve greater bond rearrangements are slow and the reactions involving lesser bond rearrangements are fast.

2. Effect of concentration of reactants

According to law of mass action, the rate of a chemical reaction is directly proportional to the product of concentration of reactants. In case of gaseous reactions, rate is directly proportional to the product of partial pressures of reactants.

If concentration is higher, the number of molecules per unit volume is more and the number of active collisions increases. According to collision theory, rate of reaction increases with increase in the number of collisions.

3. Effect of temperature

With increase in temperature rate of reaction increases because the number of active collisions or activated molecules increases. 

4. Effect of catalyst

Catalyst alters the speed of reaction by changing the activation energy. Catalyst alters the activation energy by changing the path of the reaction or mechanism of reaction.

A positive catalyst increases the rate of reaction by decreasing the activating energy. In case of some reactions, rate is proportional to the concentration of catalyst. A negative catalyst decreases the rate of reaction by increasing the activation energy.