Everything Science Grade 12

CHAPTER 4. ELECTROCHEMICAL REACTIONS 4.2

Example 1: Understanding galvanic cells

QUESTION

For the following cell:

Zn|Zn2+||Ag+|Ag

1. Give the anode andcathode half-reactions.


2. Write the overall equation for the chemical reaction.


3. Give the direction ofthe current in the external circuit.

SOLUTION

Step 1 : Identify the oxidation and reduction reactions

In the standard notationformat, the oxidation reaction is written

on the left and the reduction reaction on the right. So, in this cell,

zinc is oxidised and silver ions are reduced.

Step 2 : Write the two half reactions

Oxidation half-reaction: Reduction half-reaction:

Zn→ Zn2++ 2e− Ag++ e−→ Ag

Step 3 : Combine the half-reactions to get the overall equation.

When you combine thetwo half-reactions, all the reactants must

go on the left side of the equation and the products must go on

the right side of the equation. The overall equation therefore be-

comes:

Zn + Ag++ e−→ Zn2++ 2e−+ Ag

Note that this equationis not balanced. This will be dis-

cussed later in the chapter.

Step 4 : Determine the direction of current flow

A build up of electronsoccurs where oxidationtakes place. This

is at the zinc electrode.Current will therefore flow from the zinc

electrode to the silver electrode.