Everything Science Grade 12

4.4 CHAPTER 4. ELECTROCHEMICAL REACTIONS

4.4 Standard ElectrodePotentials

ESCBR

If a voltmeter is connected in the circuit of anelectrochemical cell, areading is ob-

tained. In other words,there is a potential difference between the two half cells. In

this section, we are going to look at this in moredetail to try to understand more about

the electrode potentials of each of the electrodes in the cell. We are going to break this

section down so that you build up your understanding gradually. Makesure that you

understand each subsection fully before movingon, otherwise it might get confusing!

The different reactivitiesof metals ESCBS

All metals have different reactivities. When metals react, they give away electrons

and form positive ions.But some metals do thismore easily than others. Look at the

following two half reactions:

Zn→ Zn2++ 2e−

Cu→ Cu2++ 2e−

Of these two metals, zinc is more reactive and ismore likely to give awayelectrons to

form Zn2+ions in solution, than iscopper.

Equilibrium reactions inhalf cells ESCBT

Let’s think back to the Zn-Cu electrochemical cell. This cell is made up of two half cells

and the reactions that take place at each of the electrodes are as follows:

Zn→ Zn2++ 2e−

Cu2++ 2e−→ Cu

At the zinc electrode, the zinc metal loses electrons and forms Zn2+ions. The electrons

are concentrated on thezinc metal while the Zn2+ions are in solution. But some of

the ions will be attractedback to the negatively charged metal, will gain their electrons

again and will form zinc metal. A dynamic equilibrium is set up between thezinc

metal and the Zn2+ions in solution when the rate at which ions are leaving the metal

is equal to the rate at which they are joining it again. The situationlooks something

like the diagram in figure 4.1.

The equilibrium reactionis represented like this: