Chemical

Rate of appearance of P Increase in concentration of P P Time taken = + ∆ ( . ) Since, D [R] is a negative quantity (as concentration of reactants is decreasing), it is multiplied with – to make the rate of the reaction a positive quantity. Equations ( . ) and ( .

) given above represent the average rate of a reaction , r av . Average rate depends upon the change in concentration of reactants or products and the time taken for that change to occur (Fig. . ) .

Fig. . : Instantaneous and average rate of a reaction Units of rate of a reaction From equations ( . ) and ( .

), it is clear that units of rate are concentration time – . For example, if concentration is in mol L – and time is in seconds then the units will be mol L - s – . However, in gaseous reactions, when the concentration of gases is expressed in terms of their partial pressures, then the units of the rate equation will be atm s – . From the concentrations of C H Cl (butyl chloride) at different times given below, calculate the average rate of the reaction: