PHYSICAL CHEMISTRY IN BRIEF

CHAP. 9: CHEMICAL KINETICS [CONTENTS] 318

For the temperature dependence of the reaction rate constant (9.163) we then have

k= (cst)(n−1)

RT

NAh

e

∆S#

R e−

∆H#

RT , (9.164)

wherecst= 1 mol dm−^3 is the standard concentration,nis the order of reaction,NAis the Avo-
gadro constant, andhis the Planck constant. The activation enthalpy ∆H#and the activation
entropy ∆S#are the reaction enthalpy and reaction entropy of a hypothetical chemical reaction
reactants ⇀↽ activated complex.In the first approximation they are considered independent
of temperature.

Example

The reaction

2 HI→H 2 +I 2

was studied at two temperatures. At T 1 = 556 K, the rate constant k 1 = 3.517 ×

10 −^7 dm^3 mol−^1 s−^1 , and atT 2 = 781 K , the rate constantk 2 = 3.954× 10 −^2 dm^3 mol−^1 s−^1.

Calculate the values of∆H#and∆S#.