Physical Chemistry Third Edition

516 11 The Rates of Chemical Reactions

Light source

Monochromator

Reservoir

Light beam syringes

Stop

syringe Device for

driving

syringes

Trigger

switch

To oscilloscope

Mixing unit

Photomultiplier Constant-temperature bath

Cuvette

Drain

valve

block

Mixing

jet

Drive

syringes

A

B

Figure 11.7 A Stopped-Flow Apparatus (Schematic).

reactants and products at equilibrium at a fairly low pressure. On the other side is a

“driver” gas at a higher pressure. When the diaphragm is ruptured the driver gas moves

quickly into the low-pressure chamber. Collisions of the driver gas molecules with the

other molecules produce a shock wave that propagates through the low-pressure gas

and heats it. The reacting system will then relax to the equilibrium state for the new

temperature. The concentration of a reactant or product is monitored spectrophotomet-

rically during this relaxation. This method is applied to reactions that have half-lives in

the range from 1 millisecond to 1 microsecond, but it is limited to gas-phase reactions.

The Flash Photolysis Method

In theflash photolysis method^7 a brief burst of light irradiates the system. If this light is

absorbed it can quickly change the temperature of the system and possibly its compo-

sition. The concentration of a reactant or product is measured spectrophotometrically

as a function of time as the system relaxes to its new equilibrium. Figure 11.8 shows

schematically an apparatus for flash photolysis. Flash photolysis differs from the shock-

wave technique in that the radiation can produce new species as well as change the

temperature of the system.

The Temperature-Jump and Pressure-Jump Methods

In thetemperature-jump(“T-jump”) and thepressure-jump(“P-jump”) methods a

gaseous or liquid system is subjected to a rapid heating or a rapid change in pressure.

A heating pulse can be delivered by a burst of microwave radiation or by the passage of

a brief pulse of electric current if the system is electrically conductive. A rapid change

(^7) See G. Porter,J. Chem. Soc., Faraday Trans., 2, 82 , 2445 (1986) for a historical survey.