C6.2 Changes at the Electrodes

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: what happens to the ions at each electrode, why water changes the products, and the rules for predicting them. 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

• Describe what happens to ions at the cathode and the anode as reduction and oxidation
• Explain why an aqueous solution contains hydrogen ions and hydroxide ions as well as the ions of the compound
• Use the reactivity of the metal to predict the product at the cathode
• Use the presence or absence of halide ions to predict the product at the anode
• Write half equations for the formation of hydrogen and oxygen (Higher tier)

Key words

• Discharged: An ion is discharged when it gains or loses electrons at an electrode and becomes an atom or molecule.
• Aqueous solution: A substance dissolved in water, shown by the state symbol (aq).
• Halide ion: A negative ion of a Group 7 element: chloride Cl−, bromide Br− or iodide I−.
• Hydroxide ion: OH−. Water provides a small number of these, together with hydrogen ions, H⁺.
• Inert electrode: An electrode that does not react, e.g. graphite or platinum.

Reduction at the cathode, oxidation at the anode

Think about molten lead(II) bromide. A lead ion, Pb²⁺, arrives at the cathode and gains two electrons from the external circuit, becoming a lead atom. Gaining electrons is reduction. A bromide ion, Br−, arrives at the anode and loses one electron, becoming a bromine atom; two atoms join to make a Br₂ molecule. Losing electrons is oxidation. The electrons released at the anode travel through the wires to the cathode.

Key fact: In every electrolysis, positive ions are reduced at the cathode and negative ions are oxidised at the anode. Here oxidation and reduction are about electrons, not oxygen.

What difference does water make?

Water molecules break up very slightly into ions: H₂O ⇌ H⁺ + OH−. So a solution of an ionic compound in water contains four types of ion: the two from the compound, plus H⁺ and OH− from the water. At each electrode only one type of ion is discharged – the one that is discharged more easily.

Worked example: listing the ions

Zinc sulfate solution, ZnSO₄(aq): Zn²⁺ and SO₄²− from the compound; H⁺ and OH− from water.
Positive ions (go to the cathode): Zn²⁺, H⁺. Negative ions (go to the anode): SO₄²−, OH−.

Answer: four ions: Zn²⁺, SO₄²−, H⁺, OH−

The cathode rule

At the cathode, compare the metal with hydrogen in the reactivity series. If the metal is more reactive than hydrogen (for example potassium, sodium, magnesium, zinc), hydrogen gas is produced and the metal ions stay in solution. If the metal is less reactive than hydrogen (for example copper, silver, gold), the metal is deposited on the cathode.

Watch out: Molten compounds and solutions can give different products. Molten potassium iodide gives potassium at the cathode, because K⁺ is the only positive ion. Potassium iodide solution gives hydrogen, because potassium is more reactive than hydrogen.

The anode rule

At the anode, check for halide ions. If chloride, bromide or iodide ions are present, the halogen is produced. If not – for example in sulfate or nitrate solutions – hydroxide ions from water are discharged and oxygen is produced. Sulfate and nitrate ions are not discharged from solutions.

Worked example: predicting the products

Magnesium bromide solution. Magnesium is more reactive than hydrogen, so hydrogen forms at the cathode. Bromide is a halide, so bromine forms at the anode.
Copper(II) nitrate solution. Copper is less reactive than hydrogen, so copper forms at the cathode. Nitrate is not a halide, so oxygen forms at the anode.

Answer: MgBr₂(aq): H₂ and Br₂. Cu(NO₃)₂(aq): Cu and O₂.

Half equations for aqueous solutions (Higher tier)

• hydrogen; Electrode cathode; Half equation 2H⁺ + 2e− → H₂; Change reduction
• copper; Electrode cathode; Half equation Cu²⁺ + 2e− → Cu; Change reduction
• silver; Electrode cathode; Half equation Ag⁺ + e− → Ag; Change reduction
• chlorine; Electrode anode; Half equation 2Cl− → Cl₂ + 2e−; Change oxidation
• oxygen; Electrode anode; Half equation 4OH− → O₂ + 2H₂O + 4e−; Change oxidation

The oxygen half equation is the hardest to remember. Four hydroxide ions each lose one electron. Together they make one oxygen molecule and two water molecules. Check it: 4 O and 4 H on each side, and a charge of −4 on each side.

Exam tip: You can also write anode half equations with the electrons subtracted on the left, e.g. 4OH− − 4e− → O₂ + 2H₂O. Both forms are accepted.

Common mistakes

• Saying sodium or potassium forms at the cathode when a solution is electrolysed. Hydrogen forms instead.
• Thinking sulfate or nitrate ions are discharged. They are not: oxygen forms instead.
• Forgetting the H⁺ and OH− ions from water when listing the ions in a solution.
• Saying oxidation happens at the cathode. Oxidation (loss of electrons) always happens at the anode.

Revision questions

Try these, then check your answers below each one.

Question 1. Lithium chloride solution is electrolysed using inert electrodes. What forms at the cathode? (Choose from: Hydrogen, Lithium, Chlorine, Oxygen.)

Answer: Hydrogen

Working: Lithium is more reactive than hydrogen, so H⁺ ions from water are discharged instead: hydrogen gas forms.

Question 2. Water provides two types of ion in an aqueous solution. One is H⁺. Name the other.

Answer: hydroxide

Working: H₂O ⇌ H⁺ + OH⁻, so the other ion is the hydroxide ion, OH⁻.

Question 3. Use the reactivity series in the diagram. Which of the four solutions would give a metal at the cathode when electrolysed with inert electrodes? (Choose from: Silver nitrate, Zinc sulfate, Magnesium chloride, Sodium nitrate.)

Answer: Silver nitrate

Working: Silver is less reactive than hydrogen, so silver ions are discharged and silver is deposited. Zinc, magnesium and sodium are all above hydrogen, so hydrogen forms instead.

Related lessons

• Previous lesson: C6.1 Introduction to Electrolysis
• Next lesson: C6.3 The Extraction of Aluminium
• In this chapter: C6.4 Electrolysis of Aqueous Solution
• In this chapter: C6 RP Required Practical: Electrolysis

Continue in the full course

This lesson is section C6.2 (AGCSC2302) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.