Chemistry, Third edition

ABSORPTION SPECTRA AND COLOUR

purple glow. The emission stops as soon as the light is turned off. Other common

fluorescent molecules include caffeine, benzene and ‘fluorescein’. Fluorescent mate-

rials are used to label valuables (‘security tagging’) – the written information can

only be seen in UV light.

Chemiluminescence

When fuels are burned in a flame, both heat andlight are produced. There are some

chemical reactions which produce light but no heat. These reactions are said to be

chemiluminescent, and the light is emitted by electronically excited molecules or

atoms produced during the reaction mechanism. Examples of chemiluminescence

in nature are to be found in the fire-fly, glow-worms and in some types of fish found

in the (otherwise dark) ocean depths. The familiar glow of air when subjected to

high voltage discharges (including lightning) is also due to chemiluminescence.

Chemiluminescent ‘fun sticks’ contain reactants in separate tubes. On bending the

stick, the tubes are broken and the reactants mix, producing the chemiluminescent

emission. The observed colour of the emission depends upon the type of dye in the reac-

tion mixture. The same principle is used in ‘light sticks’ designed to be used in emergen-

cies by cavers and walkers, some of which continue to emit light for several hours.

Absorption spectra and colour

Colour of compounds

The colour of a solution is controlled by the light that is transmittedthrough the solution.

Think of the manganate(VII) ion. Its solution, viewed under white light, is violet.

From its spectrum, Fig. 20.9, we see that the most intense absorptions in the visible

wavelengths lie between 520 and 550 nm. This is yellow-green light. The remaining

colours of white light pass through the solution, and it happens that the human eye

interprets this combination of transmitted wavelengths (and their relative intensities)

as violet in colour (Fig. 20.10).

Similarly, a coloured solid absorbs only some wavelengths of light. The observed

colour of the solid is due to the wavelengths of light that are unabsorbed, and which

arereflectedinto the eyes of the observer.

It follows that the colour of a compound can be predictedfrom its absorption spec-

trum in the visible region.

20.6

377

Fig. 20.9Electronic spectrum of the

manganate(VII) ion between 400 and

600 nm (perspex 1.0 cm cell).

Chemiluminescence

The rate of

chemiluminescent emission

is controlled by the speed

at which the excited

electronic states are

produced by the chemical

reaction. With this in mind,

what would you expect to

happen to the intensity of

emission from a funstick if

the stick was placed in (i)

iced water, (ii)warm water?

Exercise 20G