C6.4 Electrolysis of Aqueous Solution

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: sodium chloride solution, copper(II) sulfate solution and dilute sulfuric acid: predicting, explaining and using the products. 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

• Predict and explain the products of electrolysing sodium chloride solution, copper(II) sulfate solution and dilute sulfuric acid
• Describe what is left in the solution after electrolysis
• Compare the electrolysis of molten and aqueous sodium chloride
• Use the 2 : 1 ratio of hydrogen to oxygen when water is split up
• Write half equations for the formation of hydrogen, oxygen, chlorine and copper (Higher tier)

Key words

• Brine: A concentrated solution of sodium chloride in water.
• Inert electrodes: Electrodes that do not react, e.g. graphite or platinum.
• Hydroxide ion: OH−. Water provides a small number of these, plus H⁺ ions.
• Alkaline: Having a pH above 7, because of an excess of hydroxide ions.
• Discharged: Turned from an ion into an atom or molecule at an electrode.

Recap: two rules

An aqueous solution of an ionic compound contains the ions of the compound plus H⁺ and OH− ions from water. With inert electrodes:

• Cathode (−): the metal forms if it is less reactive than hydrogen; otherwise hydrogen forms.
• Anode (+): a halogen forms if halide ions (Cl−, Br−, I−) are present; otherwise oxygen forms.

Sodium chloride solution (brine)

Sodium is more reactive than hydrogen, so hydrogen bubbles off at the cathode. Chloride ions are halide ions, so chlorine bubbles off at the anode. The Na⁺ and OH− ions are left behind, so the solution turns into sodium hydroxide solution, which is alkaline. All three products are useful, so the electrolysis of brine is a major industry:

• chlorine: killing bacteria in drinking water and swimming pools; making bleach and plastics such as PVC
• hydrogen: making ammonia and margarine; a clean fuel
• sodium hydroxide: making soap, paper and bleach

• molten, NaCl(l); Cathode (−) sodium; Anode (+) chlorine; Why only Na⁺ and Cl− ions are present, so sodium ions must be discharged
• solution, NaCl(aq); Cathode (−) hydrogen; Anode (+) chlorine; Why water adds H⁺ and OH−; sodium is more reactive than hydrogen, so H⁺ is discharged; sodium hydroxide is left

Higher tier: Cathode: 2H⁺ + 2e− → H₂ (reduction). Anode: 2Cl− → Cl₂ + 2e− (oxidation).

If universal indicator is added, the solution near the cathode turns purple because hydroxide ions build up there. The volume of chlorine collected is often slightly less than the volume of hydrogen, because some chlorine dissolves in the water.

Copper(II) sulfate solution

Copper is less reactive than hydrogen, so copper is deposited on the cathode as a pink-brown coating. Sulfate is not a halide, so hydroxide ions are discharged and oxygen bubbles off at the anode. As blue copper ions are removed, the blue colour fades. Hydrogen ions and sulfate ions are left, so the solution becomes acidic (sulfuric acid).

Higher tier: Cathode: Cu²⁺ + 2e− → Cu. Anode: 4OH− → O₂ + 2H₂O + 4e−.

Splitting water: hydrogen and oxygen

Dilute sulfuric acid and sodium sulfate solution both give hydrogen at the cathode and oxygen at the anode. Overall, water is split up: 2H₂O(l) → 2H₂(g) + O₂(g). The equation shows that the volume of hydrogen is twice the volume of oxygen.

Worked example: using the graph and the 2 : 1 ratio

From the graph, after 6 minutes 15 cm³ of hydrogen has formed. Volume of oxygen = 15 ÷ 2 = 7.5 cm³.
Rate of hydrogen production = 25 cm³ ÷ 10 min = 2.5 cm³/min.

Answer: 7.5 cm³ of oxygen; hydrogen forms at 2.5 cm³/min

Gas tests: Hydrogen: a lighted splint gives a squeaky pop. Oxygen: relights a glowing splint. Chlorine: bleaches damp litmus paper.

Summary: products with inert electrodes

• sodium chloride; Ions present Na⁺, Cl−, H⁺, OH−; Cathode hydrogen; Anode chlorine; Left in solution sodium hydroxide
• copper(II) sulfate; Ions present Cu²⁺, SO₄²−, H⁺, OH−; Cathode copper; Anode oxygen; Left in solution sulfuric acid
• dilute sulfuric acid; Ions present H⁺, SO₄²−, OH−; Cathode hydrogen; Anode oxygen; Left in solution sulfuric acid (more concentrated)
• potassium iodide; Ions present K⁺, I−, H⁺, OH−; Cathode hydrogen; Anode iodine; Left in solution potassium hydroxide
• silver nitrate; Ions present Ag⁺, NO₃−, H⁺, OH−; Cathode silver; Anode oxygen; Left in solution nitric acid

Common mistakes

• Saying sodium forms at the cathode in sodium chloride solution. Hydrogen forms.
• Saying sulfur or sulfur dioxide forms at the anode. Oxygen forms.
• Forgetting what is left: sodium chloride solution becomes sodium hydroxide solution.
• Getting the 2 : 1 ratio the wrong way round. There is twice as much hydrogen as oxygen.

Revision questions

Try these, then check your answers below each one.

Question 1. Sodium chloride solution is electrolysed with inert electrodes. Which products form? (Choose from: Hydrogen at the cathode and chlorine at the anode, Sodium at the cathode and chlorine at the anode, Hydrogen at the cathode and oxygen at the anode, Chlorine at the cathode and hydrogen at the anode.)

Answer: Hydrogen at the cathode and chlorine at the anode

Working: Sodium is more reactive than hydrogen, so hydrogen forms at the cathode. Chloride ions are discharged at the anode, giving chlorine.

Question 2. Name the alkaline substance left in solution after brine has been electrolysed.

Answer: sodium hydroxide

Working: Na⁺ and OH⁻ ions are left, so the solution becomes sodium hydroxide.

Question 3. Copper(II) sulfate solution is electrolysed with graphite electrodes. What happens to the blue colour of the solution? (Choose from: It fades, because copper ions are removed, It gets darker, because more copper ions form, It stays the same, because copper ions are replaced, It turns green, because chlorine forms.)

Answer: It fades, because copper ions are removed

Working: Copper ions are discharged and deposited as copper on the cathode, so the concentration of blue Cu²⁺ ions falls and the colour fades.

Related lessons

• Previous lesson: C6.3 The Extraction of Aluminium
• Next lesson: C6 RP Required Practical: Electrolysis
• In this chapter: C6.1 Introduction to Electrolysis
• In this chapter: C6.2 Changes at the Electrodes

Continue in the full course

This lesson is section C6.4 (AGCSC2304) 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.