Electrolysis

In a nutshell

Electrolysis is a process where dissolved or molten ionic substances are broken down into simpler substances as an electric current is passed through them. 

Ionic substances and ions

Ionic substances are made up of charged particles called ions. A molten or dissolved ionic compound allows free ions in solution which are able to move and conduct charge. 

Example 

An example of an ionic substance that can be dissolved or in a molten form is $$NaCl$$.

$$sodium\ chloride\ \rightarrow\ sodium\ ion\ +\ chloride\ ion\\\ \\NaCl(s)\ \rightarrow\ Na^+(aq)\ +\ Cl^-(aq)$$​

Electricity and ions

Electricity is the flow of electrons or ions. For electrolysis to work there must be ions in solution. Substances that have an ability to conduct electricity when dissolved in water are known as electrolytes. 

An electrochemical cell is shown below and it consists of an anode, a cathode, an electrolyte and a power source. 

Note: Covalent compounds cannot act as electrolytes because they do not possess a charge and therefore cannot conduct electricity in solution. 

Oxidation and reduction

Oxidation is the loss of electrons and reduction is the gain of electrons (OIL RIG is a helpful way of remembering this). Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).

Positively charged metal ions (cations) migrate towards the negative electrode (cathode); they are reduced as they receive electrons. 

Negatively charged non-metal ions (anions) migrate towards the positive electrode (anode); they are oxidised as they donate electrons. 

Note: An oxidising agent accepts electrons so it is reduced itself. The opposite is true of a reducing agent which donates electrons so it is oxidised. 

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You can show what is produced using half equations. A half equation shows the reaction that happens at each electrode during electrolysis. It shows the gain and loss of electrons. 

Example

Magnesium can be oxidised to produce magnesium ions and electrons. As magnesium forms $$2+$$ ions, it must lose two electrons.

$$magnesium\rightarrow magnesium ion + 2 \ electrons\\ \ \\Mg\rightarrow Mg^{2+} + 2e^-$$​​

Setting up electrochemical cells

To be able to carry out an electrolysis experiment, you need to know how to set up your electrochemical cell. Your solution can be a solution or a molten substance. 

Solutions

Molten

Electrolysis of molten substances

Molten ionic compounds can conduct electricity as their constituent ions can move freely. When electrolysing molten substances, the positive metal ions will be reduced at the cathode to produce metal atoms. The negative ions will be oxidised at the anode to form atoms. 

Example 1

Electrolysis of molten lead bromide. In this compound, lead is a positive metal ion so it will form lead metal at the cathode. Whereas, the bromide ions will be oxidised to form bromine at the anode.

Reduction reaction at the cathode:

$$lead\ ion\ +\ 2 \space electrons\ \rightarrow \ lead \\\ \\ Pb^{2+}\ +\ 2e^-\ \rightarrow\ Pb$$

Oxidation reaction at the anode:   

$$bromide\ ion\ \rightarrow\ bromine\ +\ 2\ electrons \\\ \\ 2Br^-\ \rightarrow\ Br_2\ +\ 2e^-$$​​

Example 2

Electrolysis of molten potassium iodide. 

Reduction reaction at the cathode:

$$potassium\ ion\ +\ electron\ \rightarrow \ potassium \\\ \\ K^{+}\ +\ e^-\ \rightarrow\ K$$

Oxidation reaction at the anode:   

$$iodide\ ion\ \rightarrow\ iodine\ +\ 2\ electrons \\\ \\ 2I^-\ \rightarrow\ I_2\ +\ 2e^-$$​​

Electrolysis of aqueous solutions

Aqueous solutions contain water as well as the ions present in the ionic compound. This means that the solution also contains $$H^+$$ and $$OH^-$$ ions. ​

In an aqueous solution, $$H_2$$ is produced at the cathode if the metal is more reactive than hydrogen. If the metal is less reactive than hydrogen, then metal will be produced at the cathode. 

If the solution contains hydroxide and halide ions, then halogens will be produced at the anode. If not, then oxygen will be produced at the anode. 

Example 1

Electrolysis of aqueous sodium chloride $$(NaCl)$$. The solution contains a halide ion so chlorine will be produced at the anode. Sodium is more reactive than hydrogen so at the cathode, hydrogen is produced. 

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Reduction reaction at the cathode:

$$hydrogen\ ions\ +\ 2 \space electrons\ \rightarrow \ hydrogen\ gas \\\ \\ 2H^+\ +\ 2e^-\ \rightarrow\ H_2$$

Oxidation reaction at the anode:   

$$chloride \ \rightarrow\ chlorine\ +\ 2\ electrons \\\ \\ 2Cl^-(aq)\ \rightarrow\ \ Cl_2(g)\ +\ 2e^-$$​​

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Example 2

Electrolysis of aqueous sodium sulfate $$(Na_2SO_4)$$. 

Reduction reaction at the cathode:

$$hydrogen\ ions\ +\ 2 \space electrons\ \rightarrow \ hydrogen\ gas \\\ \\ 2H^+\ +\ 2e^-\ \rightarrow\ H_2$$

Oxidation reaction at the anode:   

$$hydroxide\ ions \ \rightarrow\ oxygen\ +\ water+4\ electrons \\\ \\ 4OH^-\rightarrow O_2+2H_2O+4e^-$$​​

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