Predicting Products in Aqueous Electrolysis Revision Notes | GCSE AQA Higher Chemistry

1. The Electrolysis Puzzle

When you electrolyse molten sodium chloride, you get sodium metal and chlorine gas. But electrolyse sodium chloride solution in water, and something completely different happens - you get hydrogen gas and chlorine gas instead. Why does adding water change everything?

The answer lies in competition. In aqueous solutions, water molecules break down to produce their own ions, and these compete with the original ions to be discharged at the electrodes.

2. What Happens to Water in Electrolysis

Water molecules don't just sit there during electrolysis. They break down to form ions that can be discharged:

This means every aqueous electrolysis has four types of ions present: the original positive and negative ions from the dissolved compound, plus $H^+$ and $OH^-$ ions from water. The question becomes: which ions get discharged first?

3. The Reactivity Series Rules

The reactivity series determines which ions get discharged. Think of it as a queue - some ions are more eager to react than others.

At the cathode (negative electrode): Hydrogen ions ($H^+$) are discharged instead of metal ions that are more reactive than hydrogen. So if you have sodium ions ($Na^+$) and hydrogen ions ($H^+$), the hydrogen gets discharged because sodium is above hydrogen in the reactivity series.

At the anode (positive electrode): The rules are simpler. Hydroxide ions ($OH^-$) from water are usually discharged unless you have halide ions (like $Cl^-$, $Br^-$, $I^-$) present. Halide ions get discharged preferentially.

4. Predicting Products Step by Step

Worked example: Predicting products of aqueous sodium chloride electrolysis

Predict the products when aqueous sodium chloride (NaCl) is electrolysed using inert electrodes.

1. From NaCl: $Na^+$ and $Cl^-$ From water: $H^+$ and $OH^-$ — Always list both the original ions and water ions
2. At cathode: $Na^+$ vs $H^+$ Sodium is above hydrogen in reactivity series Therefore $H^+$ is discharged — More reactive metals stay as ions
3. At anode: $Cl^-$ vs $OH^-$ Chloride is a halide ion Therefore $Cl^-$ is discharged — Halide ions are discharged preferentially
4. Cathode: $2H^+ + 2e^- \rightarrow H_2$ (hydrogen gas) Anode: $2Cl^- \rightarrow Cl_2 + 2e^-$ (chlorine gas) — Plus sodium hydroxide (NaOH) remains in solution

Answer: Hydrogen gas at cathode, chlorine gas at anode, sodium hydroxide solution remains

Notice how different this is from molten NaCl electrolysis, which would give sodium metal and chlorine gas. Water changes everything by providing alternative ions.

5. Common Examples and Patterns

Let's look at the most common aqueous electrolysis examples you'll see in exams:

Worked example: Aqueous copper sulphate electrolysis

What are the products when aqueous copper sulphate (CuSO₄) is electrolysed?

1. From CuSO₄: $Cu^{2+}$ and $SO_4^{2-}$ From water: $H^+$ and $OH^-$ — Four types of ions compete for discharge
2. $Cu^{2+}$ vs $H^+$ Copper is below hydrogen $Cu^{2+}$ discharged — Less reactive metals are discharged preferentially
3. $SO_4^{2-}$ vs $OH^-$ Sulfate is not a halide $OH^-$ discharged — Hydroxide ions discharge when no halides present
4. Cathode: Copper metal deposited Anode: Oxygen gas released — From $4OH^- \rightarrow 2H_2O + O_2 + 4e^-$

Answer: Copper metal at cathode, oxygen gas at anode

Here's the pattern: if the metal is more reactive than hydrogen, you get hydrogen gas at the cathode. If it's less reactive, you get the metal itself.

6. Exam Technique and Common Mistakes

Examiners love testing aqueous electrolysis because students often forget about the water ions. Here's how to get full marks every time:

Common trap: Students often predict sodium metal from sodium chloride solution because they forget about water ions. Always ask yourself: 'What ions does water provide?'

Exam precision note: In Predicting Products in Aqueous Electrolysis - Electrolysis of aqueous solutions - Electrolysis, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.

In aqueous electrolysis, the product at each electrode depends on which ions are present and where they are in the reactivity series. At the cathode, a metal less reactive than hydrogen may be deposited, while at the anode halide ions can form halogens; otherwise oxygen is often produced from hydroxide ions or water.