depends upon the concentration of the reactant. Now let us understand how the reaction rate is related to concentration by considering the following general reaction. x y A + B products The rate law for the above reaction is generally expressed as Rate = k [A] [B] m n Where k is proportionality constant called the rate constant. The values of m and n represent the reaction order with respect to A and B respectively.
The overall order of the reaction is given by (m+n). The values of the exponents (m and n) in the rate law must be determined by experiment. They cannot be deduced from the Stoichiometry of the reaction. For example, consider the isomerisation of cyclopropane, that we discussed earlier.
The results shown in table . indicate that if the concentration of cyclopropane is reduced to half, the rate also reduced to half. It means that the rate depends upon [cyclopropane] raised to the first power i.e., Rate = k[cyclopropane] ⇒ Rate [cyclopropane] = k . .
. . . [Cyclopropane] (M) Time (mins) Fig .
Concentration of cyclopropane vs time - graph XII U7 kinetics - Jerald XII U7 kinetics - Jerald - - - - Table . Rate constant for isomerisation Rate mol L min - - [cyclopropane] mol L - . × – . × – .
× – Let us consider an another example, the oxidation of nitric oxide (NO) 2NO(g) + O 2NO (g) ( ) g → Series of experiments are conducted by keeping the concentration of one of the reactants constant and the changing the concentration of the others. Experiment . . .
. . Rate = k [NO] [O ] m n For experiment , the rate law is Rate = k [NO] [O ] m n . X10 = k [ .
] [ . ] - m n ...( ) Similarly for experiment Rate = k