Chapter 17 Laboratory: Photochemistry 309

Laboratory:

Photochemistry

Many chemical reactions occur spontaneously at normal ambient temperatures and

pressures, but some chemical reactions require energy from an external source, called

activation energy, to initiate the reaction. Once the reaction is initiated, it may proceed

spontaneously to completion, or it may cease if external energy is not continuously supplied.

(Technically, all reactions have an activation energy; however, for a spontaneous reaction at

ambient temperature, that amount of energy is present in the system already.)

For example, the gasoline used to fuel an automobile does not react spontaneously with

the oxygen in the air. (Well, actually it does, but not quickly enough to be useful.) The spark

plug supplies the activation energy required to initiate the reaction. Once the reaction

between the gasoline and oxygen begins, it is sufficiently exothermic to supply the energy

required to sustain the reaction, which proceeds spontaneously to completion. Such

reactions are called self-sustaining.

Conversely, when you bake a cake, the baking powder or baking soda in the cake mixture

reacts to release carbon dioxide, which causes the cake to rise. But that reaction, once

initiated, does not proceed spontaneously to completion, because the reaction does not

produce enough energy to be self-sustaining. A continuing source of external energy is

required to force the reaction to completion, and that energy is supplied by the heat of the

oven as the cake bakes. (If the baking powder reaction were self-sustaining, you could bake a

cake just by touching a match to it. Yee haw.)

Activation energy can take many forms. In an automobile engine, it’s provided by an

electrical spark. In many routine laboratory reactions, activation energy is supplied by

heating a test tube or flask. In a firearm, activation energy is supplied mechanically when the

firing pin strikes the primer. But there is a separate class of reactions in which the activation

energy is supplied by visible or ultraviolet light, or some other form of electromagnetic

radiation. A reaction of this class is called a photochemical reaction. The study of

photochemical reactions is called photochemistry.

The laboratory session in this chapter demonstrates various aspects of photochemical

reactions.

17