In solution, water adds hydrogen and hydroxide ions to the mix
Definition
Inert electrode
An electrode, usually of carbon or platinum, that carries the current but does not react during electrolysis.
Definition
Electrolyte
A molten or dissolved ionic compound that conducts electricity and is broken down during electrolysis.
- An aqueous electrolyte contains the compound's ions plus hydrogen ions and hydroxide ions from the water.
- Because there are more ions to choose from, the product at each electrode is not always the compound's own element.
- These experiments use inert electrodes of carbon, which carry the current but do not react.
Practical
Investigation: electrolysing copper chloride and sodium chloride solutions
- Pour about 50 cm3\text{cm}^3cm3 of copper(II) chloride solution into a small beaker and rest a petri dish lid on top.
- Push two carbon rod electrodes through the holes in the lid so that they dip into the solution but do not touch each other.
- Connect the rods with crocodile leads to the positive and negative terminals of a low voltage power supply.
- Set the supply to about 4 V, switch on, and watch both electrodes closely.
- At the negative cathode a pink-brown coating of copper builds up on the rod.
- At the positive anode bubbles of a pale green gas form, and holding damp blue litmus paper next to it bleaches the paper white, showing chlorine.
- Switch off after about five minutes, rinse the apparatus, fit fresh electrodes, and repeat with sodium chloride solution.
- With sodium chloride, bubbles of hydrogen form at the cathode, chlorine forms at the anode, and the solution left behind is sodium hydroxide.
- Wear eye protection and work in a well ventilated room, because chlorine is toxic.
At the cathode: hydrogen forms unless the metal is less reactive
Definition
Reactivity series
A list of metals arranged in order of reactivity, with carbon and hydrogen included as useful reference points.
- If the metal is more reactive than hydrogen, hydrogen is given off at the cathode.
- If the metal is less reactive than hydrogen, the metal is deposited instead.
- So copper chloride deposits copper, while sodium chloride gives off hydrogen.
- Use the reactivity series to decide which one is discharged.
Common Mistake
- Do not assume the metal is always deposited; a reactive metal such as sodium stays in solution.
- Sodium ions are more stable in solution than sodium metal, so hydrogen is released instead.
At the anode: a halogen forms if a halide is present, otherwise oxygen
Definition
Halide ion
A negatively charged ion formed by a Group 7 element, such as chloride, bromide or iodide.
- If the solution contains a halide ion (chloride, bromide or iodide), the halogen is given off.
- If no halide is present, oxygen is given off, formed from the hydroxide ions.
- So both copper chloride and sodium chloride give chlorine at the anode.
- Chlorine is identified because it bleaches damp litmus paper.
Self review
- Which extra ions does water provide in an aqueous electrolyte?
- What is deposited at the cathode when copper chloride solution is electrolysed?
- Why is hydrogen, not sodium, produced at the cathode with sodium chloride?
- What forms at the anode when a halide is present?
- How can you show that the gas at the anode is chlorine?
