Unformatted text preview: We determine the rate equation
(we determine the exponents)
by the “Method of initial Rates” For a reaction of aA + bB ——> cC + dD
rate = k[A]'“[B]n
If we run an experiment, we will find an initial rate. Then we run a second experiment, and hold the concentration of B constant
(same concentration as in the ﬁrst run) but double the concentration of A before starting.
We find the initial rate here and compare it to the initial rate of the first experiment. If
the rate doubled as [A] doubled, then the rate is first order with respect to A. If the rate
was four times as fast when [A] was only doubled, the rate with respect to A is second order and if the rate was eight times as fast when [A] was doubled, then the rate is third
order with respect to [A]. BP=2,BF=4,BP=8 Or we run a third (another/different) experiment, and hold the concentration of B
constant (same concentration as in the first run) but triple the concentration of A before
starting. We find the initial rate here and compare it to the initial rate of the ﬁrst
experiment. if the rate tripled as [A] tripled, then the rate is ﬁrst order with respect to A.
If the rate was 9 times as fast when [A] was tripled, then the rate with respect to A is second order an dif the rate is 27 times as fast when [A] was tripled, the the rate is third
order with respect to [A]. [3]1 = 3, [3]2 = 9, [313 = 27 ...
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 Fall '08
 dockery
 Chemistry

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