What is a standard?
We all compare ourselves to someone. Can I run faster than you? Am I taller than my dad? These are relative comparisons that don’t give a lot of useful data. When we use a standard for our comparisons, everybody can tell how one thing compares to another. One meter is the same distance everywhere in the world, so a 100 meter track in one country is exactly the same distance as a 100 meter track in another country. We now have a universal basis for comparison.
Standard Hydrogen Electrode
The activity series allows us to predict the relative reactivities of different materials when used in oxidation-reduction processes. We also know we can create electric current by a combination of chemical processes. But how do we predict the expected amount of current that will flow through the system? We measure this flow as voltage (an electromotive force or potential difference).
We can then use this system to measure the potentials of other electrodes in the half-cell. A metal and one of its salts (sulfate is often used) is in the second half-cell. We will use zinc as our example (see Figure below).
As we observe the reaction, we notice that the mass of solid zinc decreases during the course of the reaction. This suggests that the reaction occurring in that half-cell is
So, we have the following process occurring in the cell:
and the measured cell voltage is 0.76 volts (abbreviated as v).
We define the standard emf (electromotive force) of the cell as:
We can do the same determination with a copper cell (Figure below).
As we run the reaction, we see that the mass of the copper increases, so we write the half-reaction:
This makes the copper electrode the cathode. We now have the two half-reactions:
Now we want to build a system in which both zinc and copper are involved. We know from the activity series that zinc will be oxidized and cooper reduced, so we can use the values at hand:
- What is the defined potential of the hydrogen electrode?
- What is the chemical composition of this electrode?
- What are the standard conditions for the other half-cell?