Less common C12.1

Key Terms in Electrolysis

This topic introduces the fundamental vocabulary for electrolysis, the process of using direct electrical current to break down ionic compounds. Understanding these core terms is essential for describing what happens in an electrolytic cell and predicting the products.

Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.

Key points

  • Electrolysis requires an electrolyte, which is a substance containing free-moving ions, either in a molten state or dissolved in a solution.
  • Two electrodes, which are solid electrical conductors, are submerged in the electrolyte to complete the circuit.
  • The anode is the POSITIVE electrode. It attracts negatively charged ions (anions).
  • The cathode is the NEGATIVE electrode. It attracts positively charged ions (cations).
  • The polarity of the electrodes is fixed because a direct current (DC) supply is used.

Diagram

Electrolysis cellElectrolysis of molten electrolyte with two inert electrodes. cathode (-)anode (+)molten electrolyte
An electrolysis cell consists of two electrodes (anode and cathode) immersed in an electrolyte containing free ions. The anode is positive and attracts anions; the cathode is negative and attracts cations.

Definitions

Electrolyte
A liquid or solution that conducts electricity due to the presence of mobile ions. This can be a molten ionic compound or an aqueous solution of an ionic compound.
Electrode
A solid conductor, typically made of metal or graphite, through which an electric current enters or leaves the electrolyte.
Cathode
The negative electrode in an electrolytic cell. It is connected to the negative terminal of the power supply and attracts positive ions (cations).
Anode
The positive electrode in an electrolytic cell. It is connected to the positive terminal of the power supply and attracts negative ions (anions).

Worked example

An electrolytic cell is set up to decompose molten lead(II) bromide (PbBr2). Which of the following statements about the apparatus is correct?

  1. 1

    Step 1:

    Identify the components.

    The electrolyte is molten PbBr2, which contains mobile Pb2+ (cation) and Br^- (anion) ions.

  2. 2

    Step 2:

    Recall the definition of the electrodes.

    The anode is the positive electrode and the cathode is the negative electrode.

  3. 3

    Step 3:

    Apply the principle of electrostatic attraction.

    Positive cations (Pb2+) will be attracted to the negative electrode (the cathode).

    Negative anions (Br^-) will be attracted to the positive electrode (the anode).

  4. 4

    Step 4:

    Evaluate the options based on these principles.

    The statement that correctly links the negative Br^- ion to the positive anode is the correct one.

Answer: The Br- ions are attracted to the positive electrode, which is called the anode.

Common mistakes

  • ×Confusing the anode and cathode. Remember that in electrolysis, the anode is positive and the cathode is negative.
  • ×Mixing up anions and cations. Anions are negative ions attracted to the anode; Cations are positive ions attracted to the cathode.
  • ×Forgetting that the electrolyte must contain mobile ions. Solid ionic compounds do not conduct electricity and cannot be electrolysed.

No-calculator tips

  • ✓Use the mnemonic 'PANIC': Positive Anode, Negative Is Cathode. This is a reliable way to remember the electrode charges in electrolysis.
  • ✓Remember that 'Anions' go to the 'Anode', and 'Cations' go to the 'Cathode'.
  • ✓Think of opposites attract: POSITIVE ions (cations) go to the NEGATIVE electrode (cathode).

Read this topic in the official UAT-UK ESAT guide →

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