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6.1 Introduction to Electrolysis

GCSE Chemistry · AQA · Chapter 6: Electrolysis

A free GCSE Chemistry lesson for AQA (8462) from OBA School: electrolytes, electrodes, what happens at the cathode and anode, and half equations. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain why ionic compounds conduct electricity only when molten or dissolved
  • Describe electrolysis using the words electrolyte, electrode, cathode and anode
  • Predict the products of electrolysing a molten binary ionic compound and write its equation with state symbols
  • Decide which substances can be electrolysed, including acids formed from covalent compounds
  • Describe oxidation and reduction in terms of electrons
  • (Higher tier) write balanced half equations for the reactions at each electrode

Key words

  • Electrolysis: Splitting up an ionic compound using electricity.
  • Electrolyte: An ionic compound that is molten or dissolved in water, so its ions are free to move and carry a current.
  • Electrode: A conducting rod that carries the current into or out of the electrolyte.
  • Cathode: The negative electrode. Positive ions (cations) move to it.
  • Anode: The positive electrode. Negative ions (anions) move to it.
  • Oxidation / reduction: Oxidation is loss of electrons. Reduction is gain of electrons.
  • Inert electrode: An electrode that conducts but does not react with the electrolyte or products, e.g. graphite or platinum.
  • Half equation: (Higher tier) An equation showing what happens to the ions at one electrode, with electrons written as e−.

Why do ions need to be free to move?

An ionic compound such as sodium chloride is made of positive and negative ions held in a giant lattice by strong electrostatic forces. In the solid, the ions can only vibrate about fixed positions, so the solid does not conduct. When the compound is melted or dissolved in water, the ions are free to move. Moving ions carry charge, so the liquid conducts. This liquid is called the electrolyte.

Key fact: In the wires, the current is carried by electrons. In the electrolyte, it is carried by moving ions. Electrons never travel through the electrolyte.

Which substances can be electrolysed?

To be electrolysed, a substance must contain ions that are free to move. Ionic compounds qualify once they are melted or dissolved. Most covalent substances, such as sugar or ethanol, have no ions, so they cannot be electrolysed. A few covalent substances react with water to form ions. For example, hydrogen chloride gas dissolves to make hydrochloric acid, which contains H⁺(aq) and Cl−(aq) ions, so it can be electrolysed. Metals conduct electricity, but they are not decomposed: the current is carried by delocalised electrons and there is no chemical change.

Melting an ionic compound takes a lot of energy because the ions are held by strong electrostatic forces. Dissolving the compound in water also frees the ions, but then water adds its own ions, so the products can be different (see sections 6.2 and 6.4). From aqueous solutions, only metals less reactive than hydrogen (such as copper and silver) are deposited at the cathode.

The electrolysis cell

Two electrodes dip into the electrolyte and are connected to a direct current (d.c.) supply. Positive ions are attracted to the negative electrode and negative ions to the positive electrode, because opposite charges attract. When ions reach an electrode they gain or lose electrons and become atoms or molecules. These are the products, which form at the electrodes.

  • Cathode; Charge negative; Ions attracted positive ions (cations), e.g. metal ions, H⁺; What the ions do gain electrons; Type of change reduction
  • Anode; Charge positive; Ions attracted negative ions (anions), e.g. Cl−, Br−, O²−; What the ions do lose electrons; Type of change oxidation

Memory aid: OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons). PANiC: Positive Anode, Negative is Cathode.

Electrolysis of molten ionic compounds

A molten binary ionic compound (made of just two elements) contains only one type of positive ion and one type of negative ion. So it always breaks down into its two elements: the metal forms at the cathode and the non-metal forms at the anode.

  • lead(II) bromide, PbBr₂; Product at cathode (−) lead; Product at anode (+) bromine; What you see silvery lead collects below the cathode; brown bromine vapour at the anode
  • zinc chloride, ZnCl₂; Product at cathode (−) zinc; Product at anode (+) chlorine; What you see grey zinc; pale green gas with a choking smell
  • potassium iodide, KI; Product at cathode (−) potassium; Product at anode (+) iodine; What you see purple iodine vapour at the anode
  • lithium oxide, Li₂O; Product at cathode (−) lithium; Product at anode (+) oxygen; What you see bubbles of gas at the anode

Lead bromide is often used to demonstrate electrolysis because its melting point is fairly low for an ionic compound (373 °C). The bromine and lead vapour are toxic, so it is done in a fume cupboard.

The electrodes are usually inert (graphite or platinum), so they do not react with the electrolyte or with the products. Products are either deposited on an electrode (a metal) or given off as a gas or vapour (a non-metal such as chlorine, bromine or oxygen).

Worked example: an overall equation with state symbols

Molten potassium bromide, KBr, is electrolysed. Write the word equation and a balanced symbol equation with state symbols.
Products: potassium at the cathode, bromine at the anode. potassium bromide → potassium + bromine
Bromine is diatomic, Br₂, so two KBr are needed: 2KBr → 2K + Br₂ (2 K and 2 Br on each side).
State symbols: the electrolyte is molten (l). The cell is far hotter than the melting point of potassium (63 °C), so potassium forms as a liquid (l). Bromine boils at 59 °C, so it forms as a gas (g).

Answer: 2KBr(l) → 2K(l) + Br₂(g)

Exam tip: Use (l) for a molten electrolyte and (aq) for a dissolved one. For the metal product, (s) or (l) are both accepted – it depends on whether the cell is hotter than the melting point of the metal.

Half equations (Higher tier)

A half equation shows the change at one electrode. Electrons are written e−. The total charge on each side must be the same.

  • At the cathode positive ions gain electrons, so electrons go on the left: Pb²⁺ + 2e− → Pb
  • At the anode negative ions lose electrons, so electrons go on the right: 2Br− → Br₂ + 2e−
  • Halogens and oxygen form diatomic molecules, so start with two ions: 2Cl− → Cl₂ + 2e− and 2O²− → O₂ + 4e−

Worked example: molten magnesium chloride, MgCl₂

Ions present: Mg²⁺ and Cl−.
Cathode: each Mg²⁺ needs 2 electrons to become an atom: Mg²⁺ + 2e− → Mg (charge: +2 − 2 = 0 on the left, 0 on the right).
Anode: chlorine is Cl₂, so 2Cl− → Cl₂ + 2e− (charge: −2 on the left, 0 − 2 = −2 on the right).
Magnesium ions are reduced (gain electrons). Chloride ions are oxidised (lose electrons).

Answer: Mg²⁺ + 2e− → Mg and 2Cl− → Cl₂ + 2e−

Exam tip: You may also see the anode half equation written as 2Cl− − 2e− → Cl₂. Both are accepted by AQA.

Common mistakes

  • Saying electrons flow through the electrolyte. Ions move in the electrolyte; electrons move in the wires.
  • Mixing up the electrodes. The cathode is negative and attracts positive ions.
  • Saying solid ionic compounds do not conduct because they have “no ions”. They do have ions, but the ions cannot move.
  • Writing Br or Cl as the product. Halogens are diatomic: Br₂, Cl₂.
  • Putting the electrons on the wrong side of a half equation. Check that the charges balance.
  • Thinking a metal wire is electrolysed. Metals conduct by delocalised electrons and are not decomposed.

Revision questions

Try these, then check your answers below each one.

Question 1. Why does solid sodium chloride not conduct electricity? (Choose from: Its ions are held in fixed positions and cannot move, It contains no ions, Its electrons are delocalised, It is made of small molecules.)

Answer: Its ions are held in fixed positions and cannot move

Working: In the solid the ions are held in a giant lattice and can only vibrate. When molten or dissolved, the ions are free to move and carry the current.

Question 2. Molten potassium iodide is electrolysed. Name the product formed at the cathode.

Answer: potassium

Working: K⁺ ions move to the cathode and gain electrons, forming potassium metal. Iodine forms at the anode.

Question 3. True or false? During electrolysis, electrons flow through the electrolyte.

Answer: False

Working: False. Electrons flow in the external wires. In the electrolyte the current is carried by moving ions.

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