MP Board · Class 12 · Chemistry · Chemical KineticsDefine the rate of a chemical reaction and explain the difference between average rate and instantaneous rate of a reaction.

The rate of a chemical reaction can be defined as the change in concentration of any one of the reactants or products per unit time. Mathematically, it is expressed as the rate of decrease in concentration of reactants or the rate of increase in concentration of products divided by the respective time interval.

1. Average Rate: The average rate is defined as the change in concentration of reactants or products divided by a large time interval ($\Delta t$). It represents the rate of reaction over a specific, measurable period of time and is given by the formula $\text{Rate}_{\text{avg}} = -\frac{\Delta [R]}{\Delta t} = +\frac{\Delta [P]}{\Delta t}$.

2. Instantaneous Rate: The instantaneous rate is defined as the rate of a chemical reaction at a particular, exact moment of time. It is obtained by taking the smaller time interval ($\Delta t$ approaching zero) of the average rate, converting it into a derivative. Mathematically, it is expressed as $\text{Rate}_{\text{inst}} = -\frac{d[R]}{dt} = +\frac{d[P]}{dt}$. Graphically, it is determined by drawing a tangent at a specific point on the concentration-time curve and calculating its slope.