Aqueous Electrolysis: Products and Practical Technique

Why water changes the products

Electrolysis uses an electric current to decompose an electrolyte. When an ionic compound dissolves in water, its ions become free to move and carry charge through the solution. Electrons carry charge through the connecting wires; moving ions carry charge through the electrolyte.

An aqueous solution has water as its solvent. Unlike a molten binary ionic compound, it contains more than just the ions from the salt. Water also supplies small amounts of hydrogen ions, $H^+$, and hydroxide ions, $OH^-$. These can take part in the electrode reactions, so there are competing possibilities at each electrode.

The negative cathode attracts positive ions, called cations. The positive anode attracts negative ions, called anions. Attraction alone does not tell us which product forms: we must decide which ions are discharged. The following predictions assume inert electrodes, such as graphite, which provide conducting surfaces without reacting chemically themselves.

The cathode: metal or hydrogen?

Metal ions and hydrogen ions are attracted to the cathode, where reduction takes place by gain of electrons. The product depends on the metal’s position relative to hydrogen in the reactivity series.

• If the metal is more reactive than hydrogen, hydrogen gas forms and the metal ions remain in solution. Sodium, magnesium and aluminium are examples.
• If the metal is less reactive than hydrogen, the metal is deposited on the cathode. Copper and silver are examples.

For example, aqueous sodium chloride produces hydrogen rather than sodium at the cathode. Copper(II) sulfate solution produces copper rather than hydrogen. The presence of water therefore makes a crucial difference to the prediction.

The anode: chlorine or oxygen?

Negative ions compete to lose electrons at the anode. Hydroxide ions from water can be discharged to produce oxygen. Chloride ions can instead be discharged to produce chlorine.

In concentrated sodium chloride solution, commonly called brine, chlorine forms at the anode. Concentration matters: in sufficiently dilute chloride solutions, oxygen can form instead. Therefore, the presence of chloride ions does not mean chlorine must form under every condition.

When no halide ions are present, oxygen normally forms at an inert anode. For example, sulfate ions in copper(II) sulfate solution remain in solution; hydroxide ions are discharged to produce oxygen. Sulfate ions are not converted into sulfur at the electrode.

Connecting predictions to observations

Consider these two aqueous electrolytes with graphite electrodes:

With brine, bubbles appear at both electrodes. Sodium ions remain in solution, and hydroxide ions accumulate as hydrogen is produced, leaving an alkaline sodium hydroxide solution.

With copper(II) sulfate, a pink-brown copper deposit builds up on the cathode, while oxygen bubbles form at the anode. The blue solution becomes paler as copper ions are removed. These observations connect the visible changes to the competing electrode reactions.

Setting up aqueous electrolysis

A school investigation uses an electrolysis cell or beaker, the salt solution, two graphite electrodes, a direct-current power supply or battery, connecting wires and crocodile clips. Small test tubes can collect gases, supported securely with a stand and clamps.

1. Add the salt solution to the cell and place the two graphite electrodes in it, keeping them apart so that they do not touch.
2. Connect one electrode to the negative terminal and the other to the positive terminal of the power supply. Identify the cathode and anode before switching on.
3. Where a gas is expected, fill a small test tube with the electrolyte and invert it over the electrode. Gas entering the tube displaces the liquid.
4. Switch on the current. Record bubbles, deposits and colour changes separately for each electrode.
5. Once enough gas has been collected for testing, switch off the supply. For copper(II) sulfate, collect gas at the anode and look for a copper deposit at the cathode; no gas collection is expected at that cathode.
6. Test the collected gases and record the results. Before investigating another solution, clean and check the electrodes for deposited metal so that previous products do not affect the next investigation.

Identifying products safely

Wear eye protection and avoid skin contact with the solutions. Copper(II) sulfate solution is harmful, and the sodium hydroxide formed from brine can be corrosive. Chlorine is toxic, so brine electrolysis should be carried out in a fume cupboard or a suitably well-ventilated area under teacher supervision. Never identify a gas by deliberately smelling it.

Hydrogen is flammable: keep flames away from the working apparatus. After switching off the supply, test only a small collected sample using the teacher’s instructions.

• Hydrogen: a lighted splint produces a squeaky pop.
• Oxygen: a glowing splint relights.
• Chlorine: damp blue litmus paper turns red and then is bleached white.

The gas tests provide evidence for the predicted products. A visible deposit, rather than bubbles, is the evidence to look for when a metal forms at the cathode.

Predicting aqueous products

Include ions from both the salt and water: $H^+$ and $OH^-$.

Cathode (−): cations gain electrons.

• Metal more reactive than hydrogen → hydrogen.
• Metal less reactive than hydrogen → metal deposit.

Anode (+), with inert electrodes:

• Concentrated chloride solution → chlorine.
• No halide ions → oxygen from hydroxide ions.
• Dilute chloride solutions can produce oxygen instead of chlorine.

Essential examples

Practical recall

Salt solution + separated graphite electrodes + DC supply. Collect gases in electrolyte-filled inverted tubes; switch off before testing.

• Hydrogen: squeaky pop with a lighted splint.
• Oxygen: relights a glowing splint.
• Chlorine: bleaches damp blue litmus.
• Copper: pink-brown cathode deposit; blue solution fades.

Wear eye protection. Use suitable ventilation for chlorine; keep flames away from the working apparatus.