4.4 CHAPTER 4. ELECTROCHEMICAL REACTIONS
Measuring electrode potential ESCBU
If we put the two half cells together, a potential difference is set up in twoplaces in the
Zn-Cu cell:
- There is a potential difference between the metal and the solution surrounding it
because one is more negative than the other. - There is a potential difference between the Zn and Cu electrodes because one is
more negative than theother.
It is the potential difference (recorded as a voltage)between the two electrodes that
causes electrons, and therefore current, to flow from the more negative electrode to the
less negative electrode.
The problem though isthat we cannot measurethe potential difference(voltage) be-
tween a metal and its surrounding solution in the cell. To do this, we would need to
connect a voltmeter toboth the metal and thesolution, which is not possible. This
means we cannot measure the exact electrode potential (E◦V) of a particular metal.
The electrode potentialdescribes the ability of ametal to give up electrons. And if the
exact electrode potential of each of the electrodes involved can’t be measured, then it
is difficult to calculate the potential difference between them. But whatwe can do is
to try to describe the electrode potential of a metal relative to another substance. We
need to use a standard reference electrode for this.
The standard hydrogenelectrode ESCBV
Before we look at the standard hydrogen electrode, it may be useful to have some more
understanding of the ideas behind a ’reference electrode’.
Understanding the ideas behind a reference electrode Adapted from http://www.chemguide.co.uk
Let’s say that you have adevice that you can useto measure heights fromsome distance
away. You want to usethis to find out how talla particular person is. Unfortunately,
you can’t see their feetbecause they are standing in long grass. Although you can’t
measure their absoluteheight, what you can do is to measure their height relative to
the post next to them.Let’s say that person Afor example is 15 cm shorter than the
height of the post. You could repeat this for a number of other people (B and C). Person
B is 30 cm shorter thanthe post and person C is10 cm taller than the post.