(3) The reaction 2HBre +NOH20w+NO +Brzp) has been studied at 127 C A serics of i

Question

(3) The reaction 2HBre +NOH20w+NO +Brzp) has been studied at 127 C A serics of initial-rate experiments were performed at a constant NO2 pressure of 10 torr but different HBr pressures: PHBetorr4.7 11.0 15.3 20.3 26.1 34.1 v/tom min-1 0.98 235 3.16 3.96 556 6.77 A second series of initial-rate experiments were carried out at a constant HBr pressure of 10 torr while varying the NO2 pressure: PNO /torr 5.0 10.2 15.4 20.3 25.7 u/torrmin1 127 2.37 3.64 494 6.19 (a) Show that the reaction obeys a simple rate law and determine the orders with respect to the two reactants. (b) What is the value of the rate constant? 243 12.3 The vant Hoff method Key ideas and equations . In gencral, the partial orders of reaction can have any value, positive or negative, although * Many reactions have simple rate laws of the form u = ka"I in practice small whole numbers or fractions with small denominators are most common. , van't Holf method: il u ka". InIn k +nIna, which allows you to extract k and n hy a graphical method. Review exercise group 12.3

Explanation / Answer

For the given reaction,

(a) let the rate law be represented as,

rate = k[HBr]^x.[NO2]^y

with, k = rate constant

x and y be order of reaction

when [NO2] = constant

From Ist and IInd data,

(2.35/0.98) = (11/4.7)^x

taking log of both sides,

x = 1

So order with respect to [HBr] is 1.

when [HBr] = constant

From Ist and IInd data set

(2.37/1.27) = (10.2/5)^y

taking log of both sides,

y = 1

So order with respect to [NO2] = 1

the rate law thus becomes,

rate = k[HBr][NO2]

(b) rate constant = 2.36/11 x 10.2 = 0.021 torr-1.min-1

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