⚡ Metal more reactive than hydrogen
The metal ion stays in solution. Hydrogen gas is produced. Sodium, potassium, calcium and magnesium all behave like this.
Electrolysis » Electrolysis of Aqueous Solutions
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.4.3.4
- Why electrolysing a solution is different from electrolysing a melt
- How to decide what forms at the cathode
- How to decide what forms at the anode
- How to predict the products from a solution of one ionic compound
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Unlock This CourseWhy solutions are different
In the last two lessons the ionic compounds were melted. A melt contains only the ions of the compound, so the choice was easy: the metal formed at the cathode and the non-metal at the anode.
A solution is different. It contains the ions of the dissolved compound and water. Water molecules break down a tiny bit to make hydrogen ions (H+) and hydroxide ions (OH-). So there are now two positive ions heading for the cathode and two negative ions heading for the anode. Only one of each pair is discharged.
The ions that are discharged depend on the relative reactivity of the elements involved. We use inert electrodes, so the electrodes themselves do not react.
Remember, halide ions are the negative ions of the halogens, such as chloride, bromide and iodide.
At the cathode (negative electrode)
In salt water, sodium is more reactive than hydrogen, so bubbles of hydrogen gas form at the cathode
Positive ions go to the cathode. The two choices are the metal ion and the hydrogen ion from water.
🔷 Metal less reactive than hydrogen
The metal ion is discharged. The metal is produced. Copper and silver behave like this.
The rule is simple: the less reactive element is the one that forms. A very reactive metal likes being an ion, so it stays in solution and hydrogen forms instead.
At the anode (positive electrode)
Chloride ions are a halide, so chlorine forms at the anode - it's why salt water electrolysis makes bleach
Negative ions go to the anode. The two choices are the ion from the compound and the hydroxide ion from water.
- If the solution contains halide ions (chloride, bromide or iodide), the halogen is produced: chlorine, bromine or iodine.
- If there are no halide ions, oxygen is produced. This happens with sulfate and nitrate solutions.
The hydroxide ions from water are the source of the oxygen. The other ions, such as sulfate, stay in solution.
Quick rules
Cathode: hydrogen if the metal is more reactive than hydrogen, otherwise the metal.
Anode: a halogen if there is a halide ion, otherwise oxygen.
Predicting the products
You should be able to predict the products for a solution containing a single ionic compound. Follow three steps:
- Name the metal ion and the negative ion in the compound.
- Cathode: is the metal more reactive than hydrogen? If yes, hydrogen. If no, the metal.
- Anode: is there a halide ion? If yes, the halogen. If no, oxygen.
Worked example 1: sodium chloride solution
Sodium is more reactive than hydrogen, so hydrogen forms at the cathode. Chloride is a halide ion, so chlorine forms at the anode.
Worked example 2: copper sulfate solution
Copper is less reactive than hydrogen, so copper forms at the cathode. Sulfate is not a halide, so oxygen forms at the anode.
Worked example 3: potassium nitrate solution
Potassium is more reactive than hydrogen, so hydrogen forms at the cathode. Nitrate is not a halide, so oxygen forms at the anode.
Worked example 4: copper bromide solution
Copper is less reactive than hydrogen, so copper forms at the cathode. Bromide is a halide ion, so bromine forms at the anode.
Melt or solution?
🔥 Molten compound
Only two types of ion. The metal always forms at the cathode and the non-metal at the anode.
💧 Aqueous solution
Four types of ion, because water adds H+ and OH-. Reactivity decides what forms.
Common mistakes
1. Saying sodium forms at the cathode in a solution. It does not, because sodium is more reactive than hydrogen.
2. Saying a sulfate gives a halogen. It never does, because sulfate is not a halide, so oxygen forms.
3. Forgetting that water is there. The H+ and OH- ions are the reason the products can change.
Exam-style question
A student electrolyses silver nitrate solution using inert electrodes. Name the product at each electrode and explain your answers. (4 marks)
Model answer
At the cathode, silver is produced because silver is less reactive than hydrogen. At the anode, oxygen is produced because the solution does not contain halide ions. The oxygen comes from the hydroxide ions in water.
Exam tip
Always give the reason as well as the product. Say "more reactive than hydrogen" or "no halide ions" to earn the explanation marks.