Page 318 - Physical chemistry understanding our chemical world

INTRODUCTION TO CELLS: TERMINOLOGY AND BACKGROUND 285

Worked Example 7.2 How much silver is generated by reductively
Silver has the symbol
passing 1F of charge through a silver-based watch battery?
‘Ag’ because its Latin
name is argentium,
We will answer this question in terms Faraday’s first law,which was
itself derived from the
first formulated in 1834 (see Table 7.1). Greek for money, argu-
+
Each electron is required to effect the reduction reaction Ag (aq) + rion.The Spanish col-
−
e → Ag , so one electron generates one atom of Ag, and 1F of
(s) onized parts of South
charge (i.e. 1 mol of electrons) generates 1 mol of Ag atoms. The America in the 16th
metal forms at the expense of 1 mol of Ag + ions. Similarly, 1 mol century. They named
+
of electrons, if passed oxidatively, would generate 1 mol of Ag ion it Argentina (dog-Latin
from Ag metal. for ‘silver land’) when
We see a direct proportionality between the charge passed and the they discovered its vast
amount of material formed during electrolysis, as predicted by Fara- reserves of silver.
day’s first law.

SAQ 7.2 10 −10 F of charge are inserted into a hair pore through a fine nee-
dle electrode. Each electron generates one molecule of a chemical to poison
the hair root. How much of the chemical is formed?

Worked Example 7.3 A Daniell cell (see p. 345) is constructed, and 1F of charge is
passed through a solution containing copper(II) ions. What mass of copper is formed?
Assume that the charge is only consumed during the reduction reaction, and is performed
with 100 percent efficiency.

We will answer this question by introducing Faraday’s second law (see Table 7.1).
The reduction of one copper(II) ion requires two electrons according to the reaction

Cu 2+ (aq) + 2e −−−→ Cu (s) (7.7)
−
We need 2F of charge to generate 1 mol of Cu (s) ,and 1F of charge will form 0.5 mol
of copper.
1 mol of copper has a mass of 64 g, so 1F generates (64 ÷ 2) g of copper. The mass
of copper generated by passing 1F is 32 g.

SAQ 7.3 How much aluminium metal is formed by passing 2F of charge
through a solution of Al 3+ ions? [Hint: assume the reaction at the electrode
−
is Al 3+ (aq) + 3e → Al (s) .]

How is coloured (‘anodized’) aluminium produced?
Currents generate chemicals: dynamic electrochemistry

Saucepans and other household implements made of aluminium often have a brightly
coloured, shiny coating. This outer layer comprises aluminium oxide incorporating a
small amount of dye.