The Electrochemical Series

Page 1

The electrochemical series shows you what's reactive and what's not

An electrochemical series is basically a list of standard electrode potentials for different electrochemical half-cells. They look like this:

Page 2

You can use an electrochemical series to calculate standard cell potentials

You can use standard electrode potential values to calculate the standard cell potential (EMF) when two half cells are joined together. All you have to do is work out which half-equation in going in the oxidation direction and which half equation is going in the reduction direction. Then just substitute the values into this formula:

EMF = VOLTSreduced - VOLTSoxidised

EXAMPLE

Calculate the standard cell potential of a Mg/Fe electrochemical cell using the REDOX equations shown in the series.

FIRST: write the two half-equations down as reductions:

Mg2+ + 2e- <------> Mg V = -2.37

Fe3+ + e- <------> Fe2+ V = +0.77

The Mg/Mg2+ half cell has the more negative electrode potential, so this half reaction will go in the direction of oxidation. The Fe2+/Fe3+ half-cell has the most positive electrode potential, so this half equation will go in the direction of reduction.

So the overall equation is:

2Fe3+ + Mg <------> 2Fe2+ + Mg2+

so, using the EMF equation above, +0.77 - (-2.37) = +3.14 V

Page 3

There a convention for drawing electrochemical cells

There's a short hand way of representing them:

Reduced form | oxidised form || oxidised form | reduced form

Things in different phases are separated by a vertical line. The double vertical line represents the salt bridge

There are a couple of conventions when drawing cells:

The half-cell with the more negative potential goes on the left.

The oxidised forms go in the centre of the cell diagram.

So, the Zn/Cu cell would look like this:

Zn | Zn2+ || Cu2+ | Cu

Where things are in the same phase (e.g. Fe2+ and Fe3+ are both aqueous) they are separated by a comma rather than a vertical line.

Page 4

Use electrode potentials to predict whether a reaction will happen

To figure out if a metal will react with the aqueous ions of another metal, you can use the EMF values.

EXAMPLE

predict whether zinc metal reacts with aqeous copper (II) ions.

First write down the two half-equations as reduction reactions:

Zn2+ + 2e- <------> Zn V = -0.76V

Cu2+ + 2e- <------> Cu V = +0.34V

Then look at the standard electrode potentials

The half-equation with the more negative electrode potential will move to the left: Zn ------> Zn2+ + 2e-

The half-equation with the more postive electrode potential will move to the right: Cu2+ + 2e- ------> Cu

The two half equations combine to give:

Zn + Cu2+ ------> Zn2+ + Cu

This is the feasible direction of the overall reaction. It matches the reaction described in the question, so zinc will react with aqueous copper ions.

For any feasible reaction, EMF is positive. EMF for this reaction is +0.34 - _-0.76) = +1.10V