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GCSE Chemistry Revision

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GCSE Chemistry revision

Using electrolysis to extract metals

Electrolysis

AQA 4.4.3.3 Foundation & Higher
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Why Electrolysis is Needed for Reactive Metals

  • Metals exist as oxides in nature, and to obtain pure metal, the oxygen must be removed in a process called reduction.
  • Carbon reduction is a cheap method that displaces metals from their oxides, but it only works for metals less reactive than carbon, such as zinc, iron, and copper.
  • For metals more reactive than carbon, such as aluminium, electrolysis must be used instead, which is far more expensive due to its high energy demands.

What is Electrolysis?

  • Electrolysis is the process of separating an ionic compound into its pure elements by passing an electric current through an electrolyte.
  • For electrolysis to work, the ions within the compound must be free to move, meaning the ionic compound must be either dissolved in water or in a molten (liquid) state.
  • Aluminium oxide is a solid at room temperature, so its ions are fixed in place and it cannot act as an electrolyte until it is melted.

Preparing Aluminium Oxide for Electrolysis

  • Aluminium is found in an ore called bauxite, which must first be purified to extract the aluminium oxide.
  • Pure aluminium oxide has a very high melting point of over 2000°C, making it extremely energy-intensive to melt on its own.
  • To reduce the melting point, aluminium oxide is mixed with a mineral called cryolite before being melted, though the process still requires very high temperatures and large amounts of energy.

Setting Up the Electrolysis Equipment

  • The molten mixture of aluminium oxide and cryolite acts as the electrolyte and is held in a container.
  • Two carbon electrodes are used as conductors: the positive electrode is called the anode, and the negative electrode is called the cathode.
  • The electrodes are connected via a wire and a power source, such as a battery, allowing electrons to flow around a complete circuit.

Reactions at the Electrodes

  • In molten aluminium oxide, the ions present are aluminium 3+ ions (Al³⁺) and oxide 2− ions (O²⁻).
  • Negatively charged oxide ions are attracted to the positive anode, where they lose electrons and are discharged as oxygen atoms, which pair up to form oxygen molecules (O₂) that escape as gas.
  • Positively charged aluminium ions are attracted to the negative cathode, where they gain electrons and are discharged as molten aluminium metal, which sinks to the bottom of the container.

Half Equations for Electrolysis

  • At the anode, oxide ions are oxidised: 2O²⁻ → O₂ + 4e⁻ At the cathode, aluminium ions are reduced: Al³⁺ + 3e⁻ → Al The overall equation for the electrolysis of aluminium oxide is: 2Al₂O₃(l) → 4Al(l) + 3O₂(g)

Oxidation and Reduction: OIL RIG

  • Oxidation Is Loss of electrons and Reduction Is Gain of electrons — remembered using the mnemonic OIL RIG.
  • At the anode, oxygen is oxidised because the oxide ions lose electrons.
  • At the cathode, aluminium is reduced because the aluminium ions gain electrons.

Why Electrolysis of Aluminium is So Expensive

  • The need to melt aluminium oxide, even when mixed with cryolite, still demands extremely high temperatures and therefore vast amounts of electrical energy.
  • The continuous supply of electricity required to maintain the electrolysis process adds significantly to the overall cost of production.
  • Despite the high cost, electrolysis is the only viable method for extracting metals that are more reactive than carbon, making it essential for aluminium production.