Everything Science Grade 12

CHAPTER 4. ELECTROCHEMICAL REACTIONS 4.4

A

B C

You could summarise your findings as follows:

Person Height relative to post (cm)

A -15

B -30

C +10

Although you don’t knowany of their absolute heights, you can rank themin order, and
do some very simple sums to work out exactlyhow much taller one isthan another.
For example, person C is 25 cm taller than A and 40 cm taller than B.

As mentioned earlier, itis difficult to measure the absolute electrode potential of apar-
ticular substance, but we can use a reference electrode (similar to the ’post’ in the Tip
box example) that we use to calculate relative electrode potentials forthese substances.
The reference electrodethat is used is the standard hydrogen electrode (figure 4.3).

DEFINITION: Standard hydrogen electrode

The standard hydrogen electrode is a redox electrode which forms the

basis of the scale of oxidation-reduction potentials. The actual elec-

trode potential of the hydrogen electrode is estimated to be 4.44±

0.02 V at 25^0 C, but its standard electrode potential is said tobe zero

at all temperatures so that it can be used as for comparison with other

electrodes. The hydrogen electrode is based on the following redox

half cell:

2H+(aq) + 2e−→ H 2 (g)