Rate of Reaction and Factors Affecting Rate of Reaction

Rate of reaction: The rate of reaction can be defined as the decrease in concentration of reactants or increase in concentration of product per unit time.

Let us consider a general reaction A giving product, then the rate of reaction in terms of decrease in concentration of reactant.

$$i.e.,{\text{A} \rightarrow \text{Product}}$$

During the time interval ${\Delta}t$, the rate of reaction in terms of decrease in concentration of reactants during the interval of time is given by:

$$\text{Rate} = -\frac{{\Delta}\text{A}}{{\delta}\text{t}}$$

The rate of reaction in terms of increase in concentration if product during the time interval  of time is given by:

$$\text{Rate} = +\frac{{\Delta}\text{B}}{{\delta}\text{t}}$$

Average rate and instantaneous rate:

Average rate of reaction is defined as the rate of reaction during the time interval Δt,  whereas instantaneous rate of reaction is defined as the rate of reaction at particular instant.

For this let us consider a reaction between hydrogen peroxide (H2O2) and potassium iodide (KI).

$$\ce{2H2O2 + KI ->2 H2O + O2 + KI}$$

The rate of this reaction can be followed by monitoring the increase in concentration of iodine or decrease in concentration of hydrogen peroxide.

$$\text{Rate} = +\frac{{\Delta} \text{[I]}_2}{{\Delta}\text{t}} = -\frac{{\Delta} \text{[H}_2 \text{O}_2]}{{\Delta}\text{t}}$$

This equation only gives average rate of reaction. But for instantaneous rate of reaction at particular time ${\Delta}\text{t}$ should be infinitely small tending to zero.

So for the instantaneous rate of reaction, it is expressed mathematically by the expression.

$$\text{Rate} = \frac{\text{dx}}{\text{dt}}$$

Where dt is the small interval  of time and dx is the change in concentration at time interval dt.

Factors affecting the rate of chemical reaction:

The major factors which affect the rate of chemical reaction are as follows:

1. Nature of reactants: The rate of chemical reaction is affected by the nature of reacting substance or reactants. For example: Inorganic reaction i.e., Ionic reactions are very fast whereas organic reactions are slow in nature.

2. Concentration of reactants: The rate of reaction also increases with the increase in concentration of reactant. Increase in concentration increases the collision between the molecules, and the increase in effective collision enhances the rate of reaction.

3. Surface Area: The rate of reaction increases with the increase in surface area. Lumps of lime stone react slowly with dilute hydrochloric acid (dil. HCL) but when when it it reacted with the powered limestone, the reaction proceeds faster. This is because more number of molecule come in contact with dil. HCL i.e., exposed area is higher.

4. Temperature: With the increase in temperature , the rate of chemical reaction is also increased. In most cases, the rate of reaction is also increased and becomes double for every 10 degree rise in temperature. When temperature is increased, the kinetic energy increased, the KE of the molecule is increased which increase the effective collision between the molecule and hence enhance the rate of reaction.

5. Use of catalyst: The rate of reaction is also affected by the addition 3rd substance called the catalyst. For example: The decomposition of H2O2 takes place faster when catalyst is added.

6. Effect of radiation: The rate of photochemical reaction is affected by radiation. For example: Reaction of methane and chlorine takes place slowly in the absence of sunlight, but in the presence of sunlight, reaction rate increases.

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