Chemistry 3.25(a-d) - Aqueous electrolysis

What changes when water is present?

Electrolysis uses a direct current supply and mobile ions. The negative cathode attracts cations; the positive anode attracts anions. With inert electrodes, the electrode material does not supply the main products.

Molten sodium chloride contains sodium and chloride ions but no water, so it gives sodium at the cathode and chlorine at the anode. In a solution, water supplies additional reacting species. We must therefore consider the solution as well as the dissolved salt when predicting the products.

Products from aqueous solutions

Aqueous means dissolved in water. An aqueous electrolyte contains the ions from the dissolved compound, but water also supplies small amounts of hydrogen ions, $\ce{H+}$, and hydroxide ions, $\ce{OH-}$. There can therefore be more than one possible ion at each electrode, and the product does not always come from the formula of the dissolved compound.

The named Edexcel cases use inert electrodes. Their required products and the evidence for them are:

For these aqueous examples, copper is less reactive than hydrogen and forms as the metal at the cathode; sodium is more reactive than hydrogen, so hydrogen forms instead and sodium ions stay in solution. Chloride can form chlorine at an inert anode in the reasonably concentrated chloride solutions used here. In sulfate solutions, oxygen forms from water or hydroxide-containing species rather than sulfate becoming a sulfur product.

Each row has a particle explanation:

• Copper chloride solution: copper ions and chloride ions are discharged. Copper forms at the cathode and chlorine forms at the anode.
• Sodium chloride solution: in the usual reasonably concentrated laboratory solution, hydrogen from water forms instead of sodium at the cathode; chloride forms chlorine at the anode. Water is why the cathode product differs from molten sodium chloride.
• Sodium sulfate solution: water provides the hydrogen and oxygen products. Sodium ions and sulfate ions remain in the solution rather than becoming sodium or sulfur-containing gases.
• Acidified water: sulfuric acid supplies mobile ions so current can flow well, but the overall substance decomposed is water. Hydrogen forms at the cathode and oxygen at the anode.

The overall change for the two water-decomposition cases is:

$\ce{2H2O(l) -> 2H2(g) + O2(g)}$

This balanced equation also predicts that, under the same conditions, the collected hydrogen volume should be about twice the oxygen volume. The ratio supports the identities, but a gas test is still needed because bubbles alone do not identify a gas.

The central distinction is molten versus aqueous. Molten sodium chloride contains only sodium and chloride ions, so sodium and chlorine form. Sodium chloride solution also contains water, so hydrogen forms instead of sodium at the cathode.

Identifying products safely

A product-identification investigation can connect the visible evidence to the ion model. Its purpose is qualitative: identify what forms at each electrode for the four named solutions, not compare reaction rates.

Apparatus and variables

Use a small electrolysis cell or beaker, two equal graphite electrodes, leads with crocodile clips, a low-voltage DC supply set to about 6 V, and small collection tubes initially filled with electrolyte and inverted above the electrodes.

• Independent variable: the electrolyte used: copper chloride solution, sodium chloride solution, sodium sulfate solution or water acidified with sulfuric acid.
• Dependent variable: the product identified at each electrode from the deposit, gas test and other observations.
• Controlled variables: use the same solution volume, comparable concentrations where appropriate, electrode material and immersed area, electrode separation, supply voltage and electrolysis time. These controls make the observations comparable; the acidified-water composition is necessarily different, so the results must not be used to claim which electrolyte has the fastest rate.