Electrolysis of Solutions and Getting Reactive Metals Out of Ores
Predict products for common aqueous salt solutions using inert electrodes. Explain how reactivity affects industrial metal extraction, how molten cryolite is used in aluminium production, and why recycling aluminium saves energy.
What a learner can do afterwards
- Predicts electrode products for the common aqueous salt solutions taught here, using inert electrodes and the stated concentration conditions
- Explains why aluminium is extracted by electrolysis while iron is reduced with carbon
- Explains why cryolite is added to the aluminium oxide and what it saves
- Explains why recycling aluminium uses far less energy than extracting new aluminium from ore
1 · Read
A melt has mobile ions. An aqueous solution also contains water, which can react at the electrodes. For the salt solutions here, assume inert electrodes and a direct-current supply. At the cathode, a metal less reactive than hydrogen is deposited. If the metal is more reactive than hydrogen, hydrogen forms from water instead. Copper is below hydrogen in reactivity; sodium and aluminium are above it.
At an inert anode, sulfate solutions give oxygen from water. Concentrated chloride, bromide or iodide solutions give chlorine, bromine or iodine. These rules do not cover every solution or electrode. In a separated brine cell, hydrogen forms at the cathode, chlorine at the anode and sodium hydroxide solution is produced. Sodium ions remain dissolved. Keeping the products apart prevents them reacting with each other.
Iron is below carbon in the reactivity series, so carbon-based reduction can extract it from its oxide. In a blast furnace, carbon monoxide made from coke does most of this reduction. Aluminium is above carbon, so carbon reduction is not the usual industrial extraction method. Industry uses electrolysis of purified aluminium oxide dissolved in molten cryolite.
Aluminium is above carbon in reactivity. Iron is below carbon.
Recycling aluminium reuses metal that has already been extracted. Scrap still needs collection, sorting, cleaning and melting. It does not need fresh ore to be mined and purified, or another extraction by electrolysis. Avoiding these stages explains why recycling aluminium uses far less energy than making new aluminium from ore.
Pure aluminium oxide needs a very high temperature to melt. Factories instead dissolve it in molten cryolite. The mixture stays liquid at a lower temperature than pure aluminium oxide, reducing the energy needed to keep it molten. The dissolved ions can move to the electrodes. Electrolysis still needs a large electrical energy input; cryolite does not replace that step.
For the cases here, water competes at inert electrodes. Aluminium extraction uses a molten cryolite mixture. Recycling avoids extracting fresh aluminium from ore.
This lesson, question bank and flashcards were written for this dataset.
The readings and flashcards were written for this dataset. OpenStax Chemistry section 17.7 was consulted for scientific facts, without adapting its prose. The linked video, Introduction to electrolysis, is by Khan Academy.
More to read
Conditions come before products
Naming a dissolved salt does not give every condition needed to predict its electrolysis products. Check the electrodes, concentration and whether water is present.
For the cases taught here, use inert electrodes. Copper is below hydrogen in reactivity, so copper ions form copper at the cathode. Sodium is above hydrogen, so sodium chloride solution gives hydrogen from water instead of sodium metal.
At an inert anode, sulfate solutions give oxygen from water. Concentrated chloride, bromide and iodide solutions give their matching halogens. These rules cover the named cases, not every possible solution.
A molten electrolyte has no water competing in these reactions. This is why molten sodium chloride and sodium chloride solution give different cathode products.
Comparing gases from water
Electrolysis can turn water into hydrogen and oxygen. Pure water conducts poorly. In a supervised laboratory demonstration, dilute sulfuric acid can supply ions to improve conduction with suitable inert electrodes.
The overall equation is 2H2O -> 2H2 + O2. Hydrogen forms at the cathode and oxygen at the anode. The equation gives two hydrogen molecules for each oxygen molecule. Gases collected without losses at the same temperature and pressure therefore have an expected volume ratio of 2:1.
The added substance matters. Chloride can lead to chlorine at the anode instead of oxygen. Gas collection and testing require laboratory supervision, and the gases must be kept apart.
Sodium chloride with and without water
Molten sodium chloride contains moving Na+ and Cl- ions. With inert electrodes, sodium forms at the cathode and chlorine at the anode. No water is present.
Brine contains sodium chloride dissolved in water. At its cathode, water produces hydrogen rather than sodium metal. Concentrated chloride produces chlorine at the anode. Sodium ions remain dissolved, while the cathode reaction produces hydroxide ions.
An industrial cell keeps the products apart so they do not react with each other. Hydrogen, chlorine and sodium hydroxide solution can then be collected.
To compare these electrolytes, begin with the presence or absence of water. Then check the concentration and cell conditions.
Making aluminium and reusing it
New aluminium production begins with ore and purified aluminium oxide. Aluminium is above carbon in reactivity, so industry does not use the usual carbon-based reduction used for iron. It uses electrolysis.
The electrolyte is aluminium oxide dissolved in molten cryolite. This mixture remains liquid below the melting point of pure aluminium oxide. Keeping it molten therefore needs less energy than melting the pure oxide would. Electrolysis still requires a large electrical energy input.
Recycling begins with aluminium that is already metal. Collection, sorting, cleaning and melting all use energy, but fresh ore does not need to be purified and reduced again. Avoiding those extraction stages explains why recycling uses far less energy.
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