Most tested C12.4

Predicting Products of Electrolysis

Electrolysis uses electricity to decompose ionic compounds. For the ESAT, you must predict the substances formed at the positive and negative electrodes for both molten compounds and aqueous solutions, where water itself can react.

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

  • In molten binary compounds, the metal forms at the negative cathode and the non-metal forms at the positive anode.
  • In aqueous solutions, water provides H+ and OH- ions which compete with the solute's ions at the electrodes.
  • At the cathode (negative), the less reactive positive ion is discharged. If the metal ion is more reactive than hydrogen (e.g., Na+, K+, Mg2+), hydrogen gas (H2) is produced from the reduction of water. If less reactive (e.g., Cu2+, Ag+), the metal is deposited.
  • At the anode (positive), if concentrated halide ions (Cl-, Br-, I-) are present, they are oxidized to form the halogen (Cl2, Br2, I2). In all other cases (e.g., dilute solutions, or presence of SO42-, NO3-), oxygen gas (O2) is produced from the oxidation of water or hydroxide ions.
  • Inert electrodes, like graphite or platinum, do not participate in the reaction. Active electrodes, like copper, can be oxidized at the anode instead of the anions.

Formulae

2H2O(l) + 2e- → H2(g) + 2OH-(aq)

For the reduction at the cathode in aqueous solutions when the metal ion is more reactive than hydrogen.

2H2O(l) → O2(g) + 4H+(aq) + 4e-

For the oxidation at the anode in aqueous solutions when concentrated halides are not present.

Definitions

Electrolysis
The process of splitting up a molten or dissolved ionic compound (an electrolyte) using a direct electric current.
Anode
The POSITIVE electrode where anions lose electrons in an oxidation reaction.
Cathode
The NEGATIVE electrode where cations gain electrons in a reduction reaction.

Worked example

Predict the products formed at the anode and the cathode during the electrolysis of a concentrated aqueous solution of magnesium bromide (MgBr2) using inert graphite electrodes.

  1. 1

    Identify all ions present in the electrolyte:

    Mg2+, Br-, H+, and OH- (from the dissociation of water).

  2. 2

    Consider the cathode (negative electrode).

    Positive ions Mg2+ and H+ are attracted.

  3. 3

    Apply the reactivity series rule:

    Magnesium is much more reactive than hydrogen.

    Therefore, H+ ions (effectively from water) will be reduced, producing hydrogen gas.

  4. 4

    Consider the anode (positive electrode).

    Negative ions Br- and OH- are attracted.

  5. 5

    Apply the anode rule:

    The solution contains a high concentration of a halide ion (Br-).

    Therefore, bromide ions will be preferentially oxidized, forming bromine.

  6. 6

    State the final products at each electrode.

Answer: Product at cathode: Hydrogen gas (H2). Product at anode: Bromine (Br2).

Common mistakes

  • ×Confusing molten and aqueous rules: Forgetting that in aqueous solutions, water provides H+ and OH- ions that compete. Molten MgBr2 would produce magnesium metal, but aqueous MgBr2 produces hydrogen gas.
  • ×Ignoring the 'concentrated' condition: Forgetting that halide ions must be concentrated to be oxidized at the anode. In a dilute solution of MgBr2, oxygen gas would be produced instead of bromine.
  • ×Reactivity series mistakes: Incorrectly judging the reactivity of a metal compared to hydrogen, leading to the wrong product at the cathode.
  • ×Mixing up electrodes: Forgetting which electrode is positive/negative and which process (oxidation/reduction) occurs at each. Use the mnemonic PANIC: Positive Anode, Negative Is Cathode.

No-calculator tips

  • ✓Use mnemonics: 'PANIC' (Positive Anode, Negative Is Cathode) and 'OIL RIG' (Oxidation Is Loss, Reduction Is Gain) to keep the basics straight.
  • ✓Anode Decision Rule: Is there a concentrated halide (Cl, Br, I)? If YES → Halogen forms. If NO (or F-, or SO42-) → Oxygen forms.
  • ✓Cathode Decision Rule: Is the metal very reactive (Group 1, 2, Al)? If YES → Hydrogen forms. If NO (Cu, Ag, Au) → Metal forms.

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

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