Everything Science Grade 12

CHAPTER 4. ELECTROCHEMICAL REACTIONS 4.4

• Metal ions at the bottom of the table are goodat picking up electrons.They are
  easily reduced and are therefore good oxidising agents.


• The reducing ability (i.e. the ability to act as areducing agent) of the metals in
  the table increases as you move up in the table.


• The oxidising ability ofmetals increases as you move down in the table.

Tip

Learning to understand

the question in a prob-

lem.

Before you tackle this

problem, make sure you

understand exactly what

the question is asking.

If magnesium is able to

displace silver from a so-

lution of silver nitrate,

this means that magne-

sium metal will form

magnesium ions and the

silver ions will become

silver metal. In other

words, there will now

be silver metal and a so-

lution of magnesium ni-

trate. This will only hap-

pen if magnesium has

a greater tendency than

silver to form ions. In

other words, what the

question is actually ask-

ing is whether magne-

sium or silver forms ions

more easily.

Example 2: Using the table of Standard Electrode Potentials

QUESTION

The following half-reactions take place in an electrochemical cell:

Cu2++ 2e−� Cu

Ag−+ e−� Ag

Which of these reactions will be the oxidationhalf-reaction in the cell and

which will be the reduction half-reaction?

SOLUTION

Step 1 : Determine the electrode potential for each metal

From the table of standard electrode potentials,the electrode po-

tential for the copper half-reaction is +0.34 V. The electrode po-

tential for the silver half-reaction is +0.80 V.

Step 2 : Use the electrode potential values to determine which metal is

oxidised and which is reduced

Both values are positive,but silver has a higher positive electrode

potential than copper. This means that silver does not form ions

easily, in other words, silver is more likely to be reduced. Cop-

per is more likely to be oxidised and to form ions moreeasily

than silver. Copper is the oxidation half-reactionand silver is the

reduction half-reaction.