Modern inorganic chemistry

ACIDS AND BASES: OXIDATION AND REDUCTION 93

ELECTRON TRANSFER

Using the electron transfer definition, many more reactions can be

identified as redox (reduction-oxidation) reactions. An example is

the displacement of a metal from its salt by a more reactive metal.

Consider the reaction between zinc and a solution of copper(II)

sulphate, which can be represented by the equation

CuSO 4 + Zn -> ZnSO 4 + Cu

This can be written as two simple ionic 'half equations

• 2e~ -»Cu(s)

2. Zn(s)
   "(aq) + Zn(s) -> Zn^2 + (aq) 4- Cu(s)
   In raetion 1 the copper ions are being reduced; zinc is responsible
   and is therefore the reducing agent. In reaction 2, which occurs
   simultaneously, the zinc is being oxidised and the copper(II) ions are
   responsible and must therefore be the oxidising agent. Electron
   transfer in this case can easily be established using the apparatus
   shown in Figure 4.1. When M is a valve voltmeter taking no current

Zinc

Salt bridge

Copper

Zinc (n)sulphate

solution

Copper(n) sulphate

solution

Figure 4.1. Apparatus to show electron transfer between copper and zinc

it gives an indication of the differing energy of the two systems
(p. 97); when M is an ammeter, electron flow is from the zinc (the
negative), which is being oxidised, to the copper (the positive) and
hence to the copper(II) ions, which are being discharged and
therefore reduced. The salt bridge is filled with an electrolyte,
usually potassium chloride solution, to complete the circuit. This
cell is more commonly encountered as the Daniell cell in the form