This section deals with calculating the emf of a chemical cell, based on the the individual half cells' standard redox potentials. This isn't actually very difficult if you follow the rules exactly as given.
Electrode potentials are always written in the same particular conventional form, shown below: -
M+ + e- ® M
It doen't matter whether or not you know that the equation above is written the "wrong" way round to normal, or that it is going to be reversed in the cell, you start from the standard redox potential and apply the rules. Then you always get the right answer. The rules for manipulating these equations, predicting the direction of reaction and the cell emf, or the probablity of the reaction going to completion or being an equilibrium which can be altered by changing the conditions are really very simple indeed.
Here's a few examples to show the rules in action.
Ex 1 - Calculate the cell emf for the following pair of half cells, and draw the resulting cell diagram. Is the resulting reaction likely to occur?
|Cu2+ + 2e- ® Cu(s)||+0.34 V||
|Zn2+ + 2e- ® Zn(s)||-0.78 V|
Rule 1 - We are using the standard half cells
Rule 2 -Reverse the Zn cell, use the Cu cell as written
|Zn(s) ® Zn2+ + 2e-||
|Cu2+ + 2e- ® Cu(s)|
Notice that we can clearly see that the Zn will be oxidised, and the Cu2+ reduced if this reaction occurs (which we will discover in a few seconds time.)
Rule 3 - Zn|Zn2+||Cu2+|Cu and that's the cell diagram written out. We miss out the electrons in these diagrams to keep them simple.
Rule 4 - E(CELL) = E(RHS) - E(LHS) with the signs as written in the original half cells
So we get E(CELL) = (0.34) - (-0.78) = 0.34 + 0.78 = 1.12 V.
By putting the numbers in the brackets, we make it easier to work out the signs to use. This is always a good idea when numbers have signs. Don't do bits in your head that might go wrong, write them down and automatically get them right. Some of my friends think I'm rather good at maths, but I don't think I'm anything special, I just don't break the rules. So I don't make mistakes.
Rule 5 - The cell emf is positive and it is greater than 0.4 V - this reaction will go as written in the cell diagram we have drawn.