Electrochemical Series
Chemistry: Cambridge International A Level, topic 1.6.2
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
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?