PHYSICAL CHEMISTRY IN BRIEF

CHAP. 9: CHEMICAL KINETICS [CONTENTS] 314

Solution

The energy of one photon was

=hc/λ= 6. 626 · 10 −^34 · 2. 993 · 108 /(299. 3 · 10 −^9 ) = 6. 626 · 10 −^19 J.

The number of the absorbed photons was

N=

10



=

10

6. 626 · 10 −^19

= 1. 509 · 1019 photons.

9.5.11.2 Quantum yield of reaction.

is defined by the relation

φ=

number of molecules changed

number of photons absorbed

. (9.156)

In simple reactions the quantum yield is in the intervalφ∈[0,1]. In chain reactions, if a
photon initiates the formation of a radical, it may beφ >1.

9.5.11.3 Rate of a photochemical reaction

The rate of reaction of the type

A + photon

k

→ B (9.157)

is
dcB
dτ

=kIabs, (9.158)

whereIabsis the energy absorbed per unit time in unit volume.