Products at each inert electrode during electrolysis
Definition
Electrolysis
The breaking down of an ionic compound, when molten or in solution, by passing a direct electric current through it.
Definition
Inert electrode
An electrode that conducts electricity but does not react with the electrolyte or the products, such as graphite or platinum.
- The cathode is the negative electrode, so positive ions are attracted to it.
- Metal ions and hydrogen ions are positive, so a metal or hydrogen forms at the cathode.
- The anode is the positive electrode, so negative ions are attracted to it.
- Non-metal ions such as chloride and oxide ions are negative, so a non-metal forms at the anode.
- Graphite and platinum make good inert electrodes, because they conduct but do not react with the electrolyte or the products.
Key Idea
Opposite charges attract, so positive ions go to the negative cathode and negative ions go to the positive anode.
Molten binary ionic compounds split into their elements
Definition
Binary ionic compound
An ionic compound made from two elements only, such as sodium chloride or lead bromide.
- Melting frees the ions to move, and a molten compound contains no water, so its own ions are the only ones present.
- Molten sodium chloride contains only Na+\text{Na}^+Na+ and Cl−\text{Cl}^-Cl− ions, so it gives sodium at the cathode and chlorine at the anode.
- Molten lead bromide, PbBr2\text{PbBr}_2PbBr2, gives lead at the cathode and bromine at the anode.
- The lead collects as a silvery liquid under the cathode, and the bromine is given off as a red-brown vapour.
- Molten zinc chloride gives zinc and chlorine, and molten potassium iodide gives potassium and iodine.
- Write the non-metal products as diatomic molecules, such as Cl2\text{Cl}_2Cl2, Br2\text{Br}_2Br2 and I2\text{I}_2I2.
Example
- Predict the product at each electrode when molten calcium bromide, CaBr2\text{CaBr}_2CaBr2, is electrolysed.
- The only ions present are Ca2+\text{Ca}^{2+}Ca2+ and Br−\text{Br}^-Br−, because the compound is molten and contains no water.
- The positive calcium ions form calcium at the cathode.
- The negative bromide ions form bromine, Br2\text{Br}_2Br2, at the anode.
Aqueous solutions contain competing ions from water
Definition
Aqueous solution
A solution in which the solvent is water, shown by the state symbol (aq).
Definition
Discharge
The change of an ion into an uncharged atom or molecule when it reaches an electrode.
- A small fraction of water molecules split into hydrogen ions and hydroxide ions, so every aqueous solution contains some of each.
- Aqueous sodium chloride therefore contains four kinds of ion: Na+\text{Na}^+Na+, H+\text{H}^+H+, Cl−\text{Cl}^-Cl− and OH−\text{OH}^-OH−.
- At the cathode, the metal ions and hydrogen ions compete, and only one of them forms a product.
- At the anode, the non-metal ions and hydroxide ions compete in the same way.
- The winner depends on the reactivity of the metal and on whether a halide ion is present.
- A molten compound has no ions from water to compete with its own, which is why molten and aqueous sodium chloride give different products at the cathode.
Practical
Electrolysis of copper(II) chloride solution
- Aim: find the products of electrolysing copper(II) chloride solution.
- Apparatus: a Petri dish with a lid, two graphite electrodes, a low-voltage direct current supply, connecting leads with crocodile clips, a dropping pipette, copper(II) chloride solution, blue litmus paper and distilled water.
Method
- Push the two graphite electrodes through holes in the base of the Petri dish, a short distance apart and not touching.
- Connect the electrodes to the low-voltage supply.
- Add about ten drops of copper(II) chloride solution, so that it covers the gap between the electrodes.
- Place a small piece of damp blue litmus paper in the dish near the positive electrode, then put the lid on.
- Switch on at about 3 V3\,\text{V}3V for up to two minutes and watch both electrodes.
- Switch off and look closely at the negative electrode and at the litmus paper.
Accuracy and safety
- A brown coating of copper forms on the negative electrode, because copper is less reactive than hydrogen.
- Bubbles form at the positive electrode and the litmus turns red and then white, showing that chlorine forms from the chloride ions.
- Keep the electrodes apart, because touching electrodes cause a short circuit.
- Keep the lid on and the room well ventilated, and run the supply for no longer than needed, because chlorine is toxic.
Hydrogen or less reactive metals form at the cathode
- Metal ions of elements more reactive than hydrogen, such as sodium, potassium, magnesium and zinc, stay in solution, and hydrogen gas bubbles off instead.
- Aqueous sodium chloride therefore gives hydrogen, not sodium, at the cathode.
- Metals less reactive than hydrogen, such as copper and silver, are deposited on the cathode.
- Aqueous copper(II) sulfate therefore gives a pink-brown coating of copper on the cathode.
- The reactivity series decides the product, with hydrogen placed between lead and copper.
Common Mistake
Do not predict sodium or potassium at the cathode in an aqueous solution, because these metals would react with the water as soon as they formed.
A halogen or oxygen forms at the anode
Definition
Halide ion
The negative ion formed when a halogen atom gains one electron, such as chloride, bromide or iodide.
- Chloride, bromide and iodide ions are discharged first, giving chlorine, bromine or iodine.
- Aqueous sodium chloride gives chlorine, a pale green gas, at the anode.
- If no halide ion is present, hydroxide ions are discharged and oxygen gas forms.
- Sulfate and nitrate ions are not discharged from solution, so copper(II) sulfate and sodium nitrate solutions give oxygen.
- Putting both electrodes together, aqueous sodium chloride gives hydrogen and chlorine, while aqueous copper(II) sulfate gives copper and oxygen.
Exam technique
- When a question asks for the products from an aqueous solution, list every ion present, including H+\text{H}^+H+ and OH−\text{OH}^-OH−, before choosing the product at each electrode.
- Name the element that forms, such as chlorine, and not the ion it came from, such as chloride.
Self review
- Which electrode is the cathode, and what forms there with inert electrodes?
- What are the products when molten lead bromide is electrolysed?
- Which four ions are present in aqueous sodium chloride?
- Why does aqueous sodium chloride give hydrogen rather than sodium at the cathode?
- What forms at the anode during the electrolysis of aqueous copper(II) sulfate with inert electrodes?
