Chemistry, Third edition

106 7 · OXIDATION AND REDUCTION

BOX 7.2

The breathalyser

The first type of breathalyser, still in use

today, was a disposable device

consisting of a plastic tube packed with

yellow crystals. The person was required

to blow through the tube into a 1 dm^3

plastic bag for about 15 s. Alcohol

vapour in the subject’s breath reacted

with the yellow crystals and turned

them green. If the green stain extended

beyond the red line drawn on the tube,

then the subject had failed the test

(Fig. 7.2).

The yellow crystals are sodium

dichromate, Na 2 Cr 2 O 7 and the redox

reaction taking place is the reduction of

the dichromate(VI) ion, Cr 2 O 72 , to the

green Cr^3 ion by alcohol, under acidic

conditions:

Cr 2 O 72 14H6e2Cr^3 7H 2 O

yellow green

Many police forces now use an

Alcolmeter* instead of the disposable

dichromate tubes. The Alcometer is a

small instrument with disposable

mouthpiece tubes. The subject blows into

the instrument and their alcohol level can

be read from a digital display. The

instrument is an electrochemical cell

known as a fuel cell which generates a

voltage in proportion to the alcohol

vapour concentration in the breath.

If a person is ‘over the limit’ for either of

these roadside tests, they are taken to

the police station, where a second

instrument (a simple infrared

spectrometer (see Chapter 20)) is used

to measure accurately the alcohol

concentration.

*Trademarks of Lion Laboratories Ltd, Ty

Verlon Industrial Estate, Barry, South

Wales.

Breathe in

Packing

material

Yellow cr ystals

of Na 2 Cr 2 O 7

Red line

Plastic bag

Fig. 7.2The Alcolyser* breathalyser.

Writing and balancing redox equations

1.Write half-equations for:

(i) the oxidation of Ito IO 3 

(ii)the reduction of NO 3 to NO in acidic solution

(iii)the reduction of Cr 2 O 72 to Cr^3 in acidic solution

(iv)the reduction of H 2 O 2 to H 2 O in acidic solution

(v)the oxidation of S^2 to SO 42 inbasicsolution.

2.Write an overall redox equation for:

(i) The reaction of copper with silver nitrate, in aqueous solution, to produce copper(II) nitrate

and silver.

(ii)The oxidation of copper metal to copper(II) nitrate by concentrated nitric acid (regard the

reduction of nitric acid as the ion NO 3 being reduced to brown NO 2 gas).

(iii)The reduction of I 2 by thiosulfate, S 2 O 32 , to Iin aqueous solution. Thiosulfate is oxidized

to tetrathionate, S 4 O 62 .

(iv)The oxidation of Ito I 2 by H 2 O 2 in acid solution (H 2 O 2 is reduced to H 2 O). (Note that in

H 2 O 2 , the oxidation number of oxygen is 1.)

(v)The oxidation of H 2 O 2 to O 2 by ClO 2 (ClO 2 is reduced to Cl).

Exercise 7G