2.4.2

Electrochemical Series (A2 Only)

Test yourself

The Electrochemical Series

Standard reduction potentials are listed in the electrochemical series.

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.