Chemguide: Support for CIE A level Chemistry

Learning outcome 6.2(b)

This statement is about what substances are produced during electrolysis.

Before you go on, you should find and read the statement in your copy of the syllabus.

The electrodes - a reminder

The positive electrode is called the anode; the negative one the cathode. Remember PANC - positive anode, negative cathode.

Because positive ions are attracted to the cathode, they are known as cations. Negative ions, attracted to the anode, are known as anions.

Discharge of ions at the electrodes

What gets discharged at the electrodes during electrolysis depends on whether the electrolyte is molten or in solution. It has to be one or the other because otherwise the ions can't move. Electrolysis can't happen unless the ions can move to the electrodes.

We will start with the simpler case - where the electrolyte is molten.

Molten electrolytes

At the cathode

Positive ions move to the cathode (the negative electrode). When they get there, they pick up electrons from the electrode to form neutral atoms.

For example, electrolysing molten lead(II) bromide gives lead at the cathode:

Or electrolysing molten sodium chloride:

Notice that the gain of electrons at the cathode means that reduction is taking place here.

At the anode

Negative ions are attracted to the positive anode, and are discharged by losing electrons. For example, in the two cases above, you would get chlorine or bromine formed.

The loss of electrons here means that oxidation is taking place at the anode.

Aqueous solutions of electrolytes

The situation is more complicated here because of the presence of the water.

At the cathode

Either the metal is deposited or you get hydrogen produced from the water. Which you get depends on the position of the metal in the electrochemical series and, in some cases, on the concentration of the solution.

  • If a metal is below hydrogen in the electrochemical series (if it has a positive E° value), then you get the metal. Metals like this include copper and silver.

  • If a metal is high in the electrochemical series (if it has a fairly negative E° value), then you get hydrogen. Metals like this include magnesium and sodium.

  • Metals from, say, lead to zinc in the electrochemical series are more complicated. What you get depends on the concentration of the solution. If the solution is concentrated, you will get the metal deposited. If the solution is very dilute, you will get hydrogen. At in-between concentrations, you may get both.

At the anode

As a general rule, if you have a halogen present, you will get the halogen. With all the other common anions (negative ions), you will get oxygen from the water.

But concentration does play a part here. For example, if you have a concentrated solution of sodium chloride, you will get mainly chlorine at the anode. With more and more dilute solutions, you will get less chlorine and more oxygen. Very, very dilute solutions will give mainly oxygen.

A complication occurs if the anode isn't inert. We need to look at this because it will come up again in statement 6.2(d).

If you electrolyse silver nitrate solution using silver electrodes, silver is deposited at the cathode as you would expect.

But at the anode, instead of anything from the solution being discharged, silver from the anode goes into solution as silver ions, leaving the electrons behind on the anode.

The net change is just a transfer of silver from the anode to the cathode.

A similar change happens if you electrolyse copper(II) sulphate solution using copper electrodes. Copper is deposited at the cathode as you would expect, but instead of oxygen being given off at the anode, copper(II) ions go into solution. Again, there is a net transfer of copper from the anode to the cathode.

This is used in the purification of copper, and you can find more about this by reading a part of the page about copper. You don't need the whole page - just the section about the purification. And you don't need to learn this for exam purposes, although you may find questions about it on pre-2016 question papers. It has been removed from the 2016 syllabus.

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© Jim Clark 2011 (last modified April 2014)