Everything Science Grade 12

CHAPTER 4. ELECTROCHEMICAL REACTIONS 4.4

In the zinc half-reaction, the equilibrium lies far to the left and in the hydrogen

half-reaction, the equilibrium lies far to the right. A simplified representation of

the cell is shown in figure 4.4.

V

H electrode

(less negative)

Zn electrode with electrons

Figure 4.4: When zinc is connected to the standard hydrogen electrode,relatively few
electrons build up on the platinum (hydrogen) electrode. There are lotsof electrons on
the zinc electrode.

The voltmeter measuresthe potential differencebetween the charge onthese

electrodes. In this case,the voltmeter would read 0.76 and would showthat Zn

is the negative electrode(i.e. it has a relatively higher number of electrons).

2. Copper

Copper has a lower tendency than hydrogen toform ions, so if the standard

hydrogen electrode is connected to the copperhalf cell, the hydrogenwill be

relatively more negative. The equilibria on the two electrodes are as follows:

Cu2+(aq) + 2e−� Cu (s)

2 H+(aq) + 2e−� H 2 (g)

In the copper half-reaction, the equilibrium liesfar to the right and in the hydro-

gen half-reaction, the equilibrium lies far to theleft. A simplified representation

of the cell is shown in figure 4.5.