Electrolysis of Aqueous Solutions

Paper 4 sets one aqueous-electrolysis prediction question in most series, and the examiner report names the same wrong answer each time: sodium metal from aqueous sodium chloride. Water changes everything. It adds H+ and OH− ions, so each electrode becomes a competition with a fixed rule deciding the winner. Learn the two rules and every electrolyte in the syllabus becomes predictable.

Why water complicates the cell

Water partially ionises into H+ and OH− ions. An aqueous solution of, say, sodium chloride therefore contains four ions: Na+, Cl−, H+ and OH−. Two positive ions compete at the cathode; two negative ions compete at the anode. The molten metal-cathode, non-metal-anode shortcut from Electrolysis no longer applies on its own.

The cathode rule: reactivity decides

Hydrogen is produced at the cathode unless the metal is less reactive than hydrogen.

Metal ion in solution	Position vs hydrogen	Cathode product
Cu2+, Ag+	Below hydrogen	The metal (copper, silver)
Na+, K+, Mg2+, Zn2+	Above hydrogen	Hydrogen gas

So copper(II) sulfate solution deposits copper, but sodium chloride solution gives hydrogen. The Na+ ions stay in solution. The reactivity series doing the deciding is the same list tested in Metals.

Cathode half-equations (electrons on the left: the ion gains them):

• Cu2+ + 2e− → Cu
• 2H+ + 2e− → H2

The anode rule: halide concentration decides

With inert electrodes, oxygen is produced from OH− ions unless a halide ion (Cl−, Br−, I−) is present in high concentration, in which case the halogen is discharged.

• Concentrated NaCl(aq): chlorine at the anode.
• Dilute NaCl(aq): mainly oxygen at the anode.
• Dilute sulfuric acid, copper(II) sulfate (carbon electrodes): oxygen, since no halide is present.

Anode half-equations (electrons on the right: the ion loses them):

• 2Cl− → Cl2 + 2e−
• 4OH− → O2 + 2H2O + 4e−

The hydroxide half-equation is the hardest single equation in the topic. Learn it whole; it cannot be improvised under exam pressure.

One more marking point hides in concentrated sodium chloride: after hydrogen and chlorine leave, Na+ and OH− remain, so the solution becomes sodium hydroxide. “Name the substance left in solution” is a regular 1-mark add-on.

Copper(II) sulfate: the electrode-swap question

This electrolyte is examined more than any other because changing the electrodes changes the chemistry.

	Carbon (inert) electrodes	Copper electrodes
Cathode	Copper deposited	Copper deposited
Anode	Oxygen given off	Anode dissolves: Cu → Cu2+ + 2e−
Blue colour	Fades (Cu2+ removed)	Unchanged (Cu2+ replaced)
Use	None	Refining copper

With copper electrodes, the mass lost by the anode equals the mass gained by the cathode. A question that quotes electrode masses before and after is testing exactly this.

Worked exam question

Dilute aqueous copper(II) sulfate is electrolysed using carbon electrodes. (a) Name the product at each electrode. [2] (b) Write the half-equation for the reaction at the anode. [2] (c) State and explain what happens to the colour of the solution. [2]

Model answer: (a) Cathode: copper (1). Anode: oxygen (1). (b) 4OH− → O2 + 2H2O + 4e− (2: one for species, one for balancing). (c) The blue colour fades (1) because Cu2+ ions are removed from solution and deposited as copper at the cathode (1).

Mark-by-mark: (a) tests both rules at once: copper is below hydrogen, so the metal deposits; sulfate is not a halide, so OH− discharges as oxygen. (b) loses a mark for unbalanced charges or electrons on the left. (c) is two separate points: the observation (fades) and the ion-level reason (Cu2+ removed). “The copper is used up” earns the observation logic but not the second mark. Name the ion.

The mistakes that cost marks

1. Producing sodium metal from aqueous solutions. Sodium is more reactive than hydrogen; it stays in solution as ions. Only molten NaCl gives sodium.
2. Ignoring the word “concentrated” or “dilute”. That single word switches the anode product between chlorine and oxygen.
3. Writing SO4^2− or NO3^− discharging at the anode. Sulfate and nitrate ions are never discharged in 0620. The choice is always halide vs hydroxide.
4. Misbalancing the OH− half-equation. Four hydroxide ions give one O2, two H2O and four electrons; check atoms and charge before moving on.

How examiners want it phrased

Student wording	Mark-scheme wording
”Sodium can’t escape the water"	"Sodium is more reactive than hydrogen, so hydrogen is discharged at the cathode"
"Chlorine wins because there’s lots of it"	"In concentrated solution, chloride ions are discharged in preference to hydroxide ions"
"The blue gets weaker as copper leaves"	"Cu2+ ions are removed from solution and deposited at the cathode, so the blue colour fades"
"The anode gets smaller"	"The copper anode dissolves: Cu → Cu2+ + 2e−”

Every prediction answer should show its working: name the two competing ions, state the rule, give the winner. That three-step structure is also what holds a 6-mark electrolysis answer together. See the 6-mark extended response technique guide for the level-marked version. The full topic map sits on the Electrochemistry pillar, and if you want the decision routine tested live on past-paper questions, book a free 1-hour trial lesson with one of our Chemistry specialists.

Test yourself

All three are Supplement level. Name the competing ions, apply the rule, then click to check.

Q1 (3 marks). Concentrated aqueous potassium chloride is electrolysed using inert electrodes. Name the product at each electrode and the substance left in solution at the end.

Show answer

• Cathode: hydrogen. Potassium is more reactive than hydrogen, so H+ ions are discharged [1] • Anode: chlorine. Chloride ions are discharged in preference to hydroxide ions in concentrated solution [1] • Potassium hydroxide remains in solution (K+ and OH− ions are left) [1]

Q2 (2 marks). Explain why electrolysis of aqueous copper(II) sulfate deposits copper at the cathode, but electrolysis of aqueous magnesium sulfate gives hydrogen instead.

Show answer

• Copper is less reactive than hydrogen, so Cu2+ ions are discharged and copper is deposited [1] • Magnesium is more reactive than hydrogen, so hydrogen is produced and the Mg2+ ions stay in solution [1]

Q3 (3 marks). Aqueous copper(II) sulfate is electrolysed using copper electrodes. State what happens at the anode, write the half-equation for the anode reaction, and explain why the blue colour of the solution does not fade.

Show answer

• The (copper) anode dissolves / loses mass [1] • Cu → Cu2+ + 2e− [1] • Cu2+ ions are replaced (by the dissolving anode) as fast as they are removed at the cathode, so the concentration (and the blue colour) stays constant [1]