Page 178 - Instant notes
P. 178

Physical chemistry     164






        The order, α, of the reaction with respect to A is obtained from the slope of the graph of
        logarithm of  initial rate  of reaction of A against the logarithm of the corresponding
        starting concentration [A] 0 whilst values of [B] 0,…are held constant. Similarly, the value
        of β is obtained by varying only [B] 0 and measuring the initial rate of reaction of B.
           In the simpler situation in which the rate law involves only a single species:




        the logarithmic form (for all times) is:




        and it is possible to obtain both the order and the rate constant for the rate law directly by
        plotting log|d[A]/dt| against log[A] values derived from a single experimental graph of
        variation in [A] with time (Fig. 1).




















                              Fig. 1. (a) Derivation of rates of
                              reaction from tangents to the plot of
                              concentration against time. (b) A plot
                              of log rate vs. log concentration gives
                              a straight line with gradient equal to
                              the order, and intercept equal to log
                              rate constant
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