Electrolysis and Electrolytes
What Is Electrolysis?
Electrolysis is the process of using an electric current to break down an ionic compound. The compound must be in a state where its ions are free to move — either molten (heated until liquid) or dissolved in water. Solid ionic compounds cannot be electrolysed because the ions are held in fixed positions within the lattice and cannot carry the current.
The ionic compound used in electrolysis is called the electrolyte. It is placed in a container called an electrolytic cell, and two electrodes are connected to a power supply. The electrode connected to the positive terminal of the supply is the anode, and the electrode connected to the negative terminal is the cathode.
Anode = positive electrode (oxidation occurs). Cathode = negative electrode (reduction occurs). Positive ions (cations) move to the cathode; negative ions (anions) move to the anode.
What Happens at the Electrodes
At the cathode, positive ions (cations) arrive and gain electrons. This is reduction. Metal ions are reduced to metal atoms; hydrogen ions are reduced to hydrogen gas. At the anode, negative ions (anions) arrive and lose electrons. This is oxidation. The products depend on what ions are present.
Half equations show what happens at each electrode separately. For example, in the electrolysis of molten sodium chloride:
$$\text{At cathode: } Na^+(l) + e^- \rightarrow Na(l) \text{ (reduction)}$$
$$\text{At anode: } 2Cl^-(l) \rightarrow Cl_2(g) + 2e^- \text{ (oxidation)}$$
So electrolysis of molten sodium chloride produces sodium metal at the cathode and chlorine gas at the anode. For molten binary compounds (two elements only), prediction is straightforward: the metal or hydrogen forms at the cathode; the non-metal forms at the anode.
When an ionic compound is dissolved in water, the water itself also contains \(H^+\) and \(OH^-\) ions. This creates competition at each electrode. At the cathode, either the metal ion or hydrogen ion from water is reduced, depending on relative reactivity. If the metal is less reactive than hydrogen (e.g. copper), copper is deposited at the cathode. If the metal is more reactive than hydrogen (e.g. sodium), hydrogen gas is produced instead because the water’s \(H^+\) ions are discharged more easily than sodium ions. At the anode, either oxygen is produced (from \(OH^-\) ions in water) or the halogen is produced if a concentrated solution of halide ions is present. For example, electrolysis of aqueous copper sulfate with inert carbon electrodes gives copper at the cathode and oxygen at the anode; electrolysis of concentrated sodium chloride solution gives hydrogen at the cathode and chlorine at the anode.
Extracting Aluminium by Electrolysis
Aluminium is the most abundant metal in the Earth’s crust but is too reactive to be extracted using carbon reduction. Instead, it is extracted by electrolysis of molten aluminium oxide (from the ore bauxite). The aluminium oxide must be heated to a very high temperature to melt it, which uses a large amount of energy and makes the process expensive.
In the cell, aluminium ions are reduced at the cathode to form liquid aluminium metal, which sinks to the bottom and is tapped off. At the anode (made of carbon), oxygen ions are oxidised to form oxygen molecules. The oxygen reacts with the carbon electrodes at this high temperature, forming carbon dioxide and gradually burning away the electrodes — so the carbon anodes must be replaced regularly. This adds to the overall cost of production.
Question: What are the products of electrolysing aqueous copper(II) sulfate solution with inert (carbon) electrodes?
Step 1 — Identify all ions present: Cu²⁺, SO₄²⁻ from the salt; H⁺ and OH⁻ from water.
Step 2 — Cathode (reduction): Cu²⁺ or H⁺ will be reduced. Copper is below hydrogen in the reactivity series, so Cu²⁺ ions are discharged in preference to H⁺. Product: copper metal deposited on the cathode.
Step 3 — Anode (oxidation): SO₄²⁻ or OH⁻ will be oxidised. No concentrated halide ions present, so OH⁻ is oxidised. Product: oxygen gas released at the anode.
Half equation at cathode (Higher tier): \(Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)\)
Half equation at anode (Higher tier): \(4OH^-(aq) \rightarrow 2H_2O(l) + O_2(g) + 4e^-\)
In the electrolysis of aqueous solutions: metals less reactive than hydrogen are deposited at the cathode; metals more reactive than hydrogen produce hydrogen gas instead. At the anode, oxygen is produced unless a concentrated halide solution is used, in which case the halogen (e.g. chlorine) is produced.