Electrochemical Series (A2 Only)
Chemistry: AQA A Level, topic 2.4.2
The Electrochemical Series
Standard reduction potentials are listed in the electrochemical series.
The electrochemical series table
The electrochemical series
- We list lots of half-equations with their standard reduction potentials in a table called the electrochemical series.
- The most negative potentials (the hardest reductions to do) are at the top.
- The standard hydrogen electrode potential is in the middle at 0 V.
- The easiest reductions are at the bottom.
Calculating cell potentials
- We can use standard reduction potentials to calculate the standard cell potential. This will allow us to predict the outcome of reactions.
- The process for calculating the standard cell potential is:
- Pick your two half equations.
- Decide which is happening as an oxidation and which is happening as a reduction.
- Subtract the standard reduction potential for the oxidation from the potential for the reduction.
- Eθcell = Eθreduced − Eθoxidised
E.g. Magnesium and bromine
- The standard reduction potential for Mg2+ + 2e- ⇌ Mg(s) is Eθ = −2.37 V.
- The standard reduction potential for Br2 + 2e- ⇌ 2Br-(aq) is Eθ = 1.07 V.
- The magnesium potential is more negative, so will go in the oxidation direction.
- The bromine potential is more positive, so the bromine will be reduced.
E.g. Magnesium and bromine cont.
- The equation for the reaction between magnesium and bromine is:
- Mg(s) + Br2(aq) → MgBr2(s)
- Eθcell = 1.07 V − (−2.37 V) = 3.44 V
Drawing Cells and Predicting Reactions
There is a standard shorthand way of drawing electrochemical cells in chemistry.
Shorthand way of drawing cells
- There’s a standard shorthand way of writing electrochemical cells.
- The more negative potential goes on the left.
- The more positive potential goes on the right.
- The oxidised forms go in the centre.
- The different couples are split by a double vertical line that indicates a salt bridge.
Predicting cell reactions
- We can use standard cell potentials to predict whether or not a reaction is feasible.
- If the standard cell potential is positive, the reaction is feasible.
- If the standard cell potential is negative, the reaction is NOT feasible.
Test yourself on Electrochemical Series (A2 Only)
What is the correct equation for the standard cell potential?
Which word best describes a reaction with a positive cell potential?
On what side of the double line does the most negative redox potential sit?