Rate of Chemical Reaction

Basic introduction

The rate of reaction may be defined as the speed at which the reaction proceeds. It is the rate at which products are formed or reactants are consumed during a reaction. The rate of reaction for products generally increases as the reaction proceeds. The amount of products increases and the amount of reactants thus decreases. It can be said that the rate of reaction is equal to concentration of product for unit time period. The rate of reaction is affected by different parameters. The rate of reaction gives a brief idea about the nature of reaction whether it is spontaneous or nonspontaneous.The number of factors which affect the rate of reaction:-

Factors Affecting Rate of Reaction

1) Effect of Concentration

In addition to more concentration of reactants, then according to collision theory , there will be more collisions and thus the rate of formation of products will be more. Also in addition to more concentrated products the according to law of mass action the formation of products in a reaction will be more respectively.

2) Order of Reaction

The order of reaction also determines the occurrence of reaction. A reaction which is of second order will proceed more efficiently than the reaction which is of first order. The order of reaction thus determines the rate of reaction.

3) Effect of Catalyst

The catalyst which is used in the reaction also plays a major role in determining the rate of reaction. A reaction in which there is no role of catalyst will be slower than the reaction in which the catalyst is being used.

• Positive catalyst

A chemical in which the rate of reaction increases in addition to the catalyst is said to be a positive catalyst. In addition to positive catalysts, the rate of reaction increases respectively.

• Negative catalyst

A chemical in which the rate of reaction decreases in addition to the catalyst then the catalyst is said to be a negative catalyst. In addition to negative catalysts, the rate of reaction decreases respectively.

4) Surface area

The rate of reaction depends on the surface area of reactant molecules. When the size of molecules is small and surface area is larger then the rate of reaction will be more respectively.

5) EFFECT OF TEMPERATURE AND PRESSURE

The effect of temperature and pressure on the rate of reaction is defined by the Le-Chatelier’s Principle. According to gaseous reactions the reaction will move forward(product formation) when pressure increases and the number of moles of reactants is more than the number of moles of products.

On increase in temperature the exothermic reaction will move forward(product) and thus rate of reaction increases.

6) Activation Energy

The activation energy of molecules also plays a major role in determining rate of reaction. The activation energy is different for different molecules. The molecules which have lower activation energy are easily converted into products and thus have high reaction rate as compared to molecules which have higher activation energy.

MATHEMATICAL REPRESENTATION

The rate of reaction is represented by r. The rate of reaction can also be defined as the rate of appearance of products and the rate of disappearance of reactants.  For chemical reaction,

The rate of reaction can be written as the rate of appearance of products and the rate of disappearance of  reactants. It can be represented as,

The rate of appearance of products is a positive number as it is increasing while the rate of disappearance of products is a negative number as it is reducing. Here the positive sign indicates the increment in products and the negative sign indicates the decrement in concentration of reactants. The rate of reaction thus can be calculated by dividing the coefficient with the concentration of reactant.

Unit of rate of reaction

The rate of reaction is equal to concentration of molecules for a unit interval of time. The rate of reaction is the ratio of concentration of reactants or product and time period. The unit of concentration is mole/litre and the unit of time is ‘s’. 

CONCLUSION

The rate of reaction, the speed of reaction at which the products are formed and reactants are consumed during a reaction. The rate of reaction can also be defined as the rate of appearance of products or the rate of disappearance of the reactants in the reaction. The rate of reaction depends on a number of factors. It may include the temperature, surface area of reactant molecules, the activation energy and the pressure. When a molecule has lower activation the formation of a product is much easier than higher activation energy molecules. The rate of reaction also depends on concentration of the reactant molecules. When the concentration of reactant molecules increases then the rate of appearance of products increases and thus rate of reaction also increases.