Physical Chemistry Third Edition

11.7 The Experimental Study of Fast Reactions 517

Oscilloscope

Monochromator PM

Sample

Analyzing

light source

Flash lamp

Spark

gap

C High-voltage

supply

Trigger

signal

Figure 11.8 A Flash Photolysis Apparatus (Schematic).

in pressure can be achieved by rupturing a diaphragm. After the temperature or pressure

change the system relaxes to its new equilibrium state. The concentration of a reactant

or product is monitored spectroscopically or by some other rapid means.

Consider the reaction:

(1) A+B

k 1




k′ 1

C (11.7-1)

Assume that this reaction is second order overall in the forward direction and first order

in the reverse direction. A temperature jump is suddenly imposed on the system at time

t0. Figure 11.9 shows schematically the concentrations of A, B, and C before and

after a T-jump. The initial concentration[A] 0 was the equilibrium concentration at the

temperature prior to the temperature jump, but because the equilibrium constant of the

reaction depends on temperature, [A] 0 is not equal to the new equilibrium concentration,

denoted by [A]eq. The same is true of [B] and [C]. We now let

∆[A][A]−[A]eq (11.7-2a)

∆[B][B]−[B]eq (11.7-2b)

∆[C][C]−[C]eq (11.7-2c)

From the stoichiometry of the reaction shown in Eq. (11.7-1),

∆[A]∆[B]−∆[C] (11.7-3)

so that we can express the concentrations in terms of∆[C]:

[C][C]eq+∆[C] (11.7-4a)

[A][A]eq−∆[C] (11.7-4b)

[B][B]eq−∆[C] (11.7-4c)