Electrolysis & Extraction of Metals
What Is Electrolysis?
Electrolysis is the process of using electrical energy from a direct current (d.c.) supply to decompose (break down) an ionic compound. The ionic compound must be either molten or dissolved in water so that its ions are free to move. In this state, the compound is called an electrolyte.
During electrolysis, two electrodes are placed in the electrolyte and connected to a d.c. power supply. Positively charged ions (cations) are attracted to the negatively charged electrode (the cathode). Negatively charged ions (anions) are attracted to the positively charged electrode (the anode).
Cations (positive ions) migrate to the cathode (negative electrode). Anions (negative ions) migrate to the anode (positive electrode). Remember: cations go to the cathode and anions go to the anode.
Electrolysis of Specific Substances
When electrolysing molten ionic compounds (e.g. molten lead bromide), the products are straightforward: the metal forms at the cathode and the non-metal forms at the anode. Lead bromide gives lead at the cathode and bromine at the anode.
Electrolysis of solutions is more complex because water also contains ions (\(H^+\) and \(OH^-\)). The product at each electrode depends on the position of the element in the reactivity series:
At the cathode: If the metal is more reactive than hydrogen, hydrogen gas is produced. If the metal is less reactive than hydrogen, the metal is deposited.
At the anode: If a halide ion is present, the halogen is produced. If no halide is present, oxygen is produced from the water.
Copper chloride solution: Copper at the cathode (Cu is less reactive than H), chlorine at the anode (halide present). Sodium chloride solution: Hydrogen at the cathode (Na is more reactive than H), chlorine at the anode (halide present). Sodium sulfate solution: Hydrogen at the cathode, oxygen at the anode (no halide). Acidified water (dilute sulfuric acid): Hydrogen at the cathode, oxygen at the anode.
Half Equations & Oxidation-Reduction
Oxidation is the loss of electrons. Reduction is the gain of electrons. Remember OILRIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
At the cathode, reduction occurs — cations gain electrons. At the anode, oxidation occurs — anions lose electrons. Half equations show what happens at each electrode separately:
For molten lead bromide:
Cathode (reduction): \(Pb^{2+} + 2e^- \rightarrow Pb\)
Anode (oxidation): \(2Br^- \rightarrow Br_2 + 2e^-\)
Reduction always occurs at the cathode (cations gain electrons). Oxidation always occurs at the anode (anions lose electrons). OILRIG: Oxidation Is Loss, Reduction Is Gain.
When copper sulfate solution is electrolysed using copper electrodes (not inert), the copper anode dissolves and pure copper is deposited on the cathode. This is used industrially to purify copper. The impure copper is the anode, the pure copper is the cathode, and the electrolyte is copper sulfate solution. The anode loses mass and the cathode gains mass.
Cathode: \(Cu^{2+} + 2e^- \rightarrow Cu\). Anode: \(Cu \rightarrow Cu^{2+} + 2e^-\).
The Reactivity Series
Metals can be ranked by their reactivity with water, dilute acids, and salt solutions. The reactivity series (most to least reactive) is: potassium, sodium, calcium, magnesium, aluminium, (carbon), zinc, iron, (hydrogen), copper, silver, gold. More reactive metals react more vigorously with water and acids, and have a greater tendency to form positive ions (cations).
In a displacement reaction, a more reactive metal can displace a less reactive metal from a solution of its salt. For example, iron displaces copper from copper sulfate solution because iron is more reactive than copper:
$$Fe + CuSO_4 \rightarrow FeSO_4 + Cu$$
Extracting Metals from Ores
Most metals are found in the Earth’s crust as ores — compounds that must be chemically processed to obtain the pure metal. Unreactive metals such as gold are found as uncombined elements because they do not readily react with other substances.
Extracting a metal from its ore involves reduction — removing the oxygen. The method of extraction depends on the metal’s position in the reactivity series:
Metals below carbon (e.g. iron, zinc, copper) can be extracted by heating the ore with carbon. Carbon is more reactive, so it displaces the metal. For example: \(2Fe_2O_3 + 3C \rightarrow 4Fe + 3CO_2\).
Metals above carbon (e.g. aluminium, sodium) are too reactive to be extracted by carbon. They must be extracted by electrolysis of the molten ore, which is more expensive because it requires large amounts of electrical energy.
Metals less reactive than carbon are extracted by heating with carbon (reduction). Metals more reactive than carbon must be extracted by electrolysis. Electrolysis is more expensive due to the energy costs.
Recycling metals saves energy (less mining and extraction needed), preserves finite ore reserves, and reduces landfill. A life-cycle assessment evaluates the environmental impact of a product from obtaining raw materials through manufacturing, use, and disposal. These assessments help compare the true environmental cost of products and materials.
Bioleaching uses bacteria to extract metals from low-grade ores. The bacteria produce acidic solutions that dissolve metal compounds. Phytoextraction uses plants that absorb metal ions from the soil through their roots. The plants are then harvested and burned, and the metal is extracted from the ash. Both methods are slower than traditional mining but are less environmentally damaging and can be used on ores with very low metal concentrations.