. Rate of a chemical reaction: A rate is a change in a particular variable per unit time. You have already learnt in physics that change in the displacement of a particle per unit time gives its velocity. Similarly in a chemical reaction, the change in the concentration of the species involved in a chemical reaction per unit time gives the rate of a reaction.
Let us consider a simple general reaction B The concentration of the reactant ([A]) can be measured at different time intervals. Let the concentration of A at two different times t and t , (t >t ) be [A ] and [A ] respectively. The rate of the reaction can be expressed as Rate= – [Change in the concentration of the reactants] (Change in time) i.e., Rate t -t - [A] t = - ([ ] [ ]) - ∆ ∆ ... ( .
) During the reaction, the concentration of the reactant decreases i.e. [A ] < [A ] and hence the change in concentration [A ] [A ] - gives a negative value. By convention the reaction rate is a positive one and hence a negative sign is introduced in the rate expression (equation . ) If the reaction is followed by measuring the product concentration, the rate is given by ∆ ∆ [B] t since [B ]>[B ] , no minus sign is required here.
XII U7 kinetics - Jerald XII U7 kinetics - Jerald - - - - Unit of rate of a reaction: unit of rate = unit of concentration unit of time Usually, concentration is expressed in number of moles per litre and time is expressed in seconds and therefore the unit of the rate of a reaction is mol L s - - . Depending upon the nature of the reaction, minute, hour, year etc can also be used. For a gas phase reaction, the concentration of the gaseous species is usually expressed in terms of their partial pressures and in such cases the unit