Movement and Interactions: Atoms Into Ions and Ions Into Atoms — AQA Combined Science: Synergy
This topic covers the reactivity series, electrolysis and electron transfer (oxidation and reduction).
A reactivity series for metals
Metals react by losing electrons to form positive ions (oxidation). The reactivity series ranks them:
potassium > sodium > lithium > calcium > magnesium > (carbon) > zinc > iron > (hydrogen) > copper > silver > gold
- More reactive metals react more vigorously with water and acids.
- A more reactive metal displaces a less reactive one from its compound.
Metal extraction
Metals less reactive than carbon can be extracted by reduction with carbon (e.g. iron). Metals more reactive than carbon (e.g. aluminium) are extracted by electrolysis.
Electrolysis
Electrolysis uses electricity to break down an ionic compound (the electrolyte), which must be molten or dissolved so the ions can move.
- Cathode (negative): positive ions gain electrons (reduction).
- Anode (positive): negative ions lose electrons (oxidation).
For molten compounds, the metal forms at the cathode and the non-metal at the anode (e.g. molten lead bromide → lead + bromine).
Electrolysis of aqueous solutions
Water also provides H⁺ and OH⁻ ions, so:
- At the cathode: hydrogen forms unless the metal is less reactive than hydrogen (then the metal forms, e.g. copper).
- At the anode: oxygen forms unless a halide (Cl⁻, Br⁻, I⁻) is present (then the halogen forms).
Required practical: electrolysis of aqueous solutions (e.g. copper sulfate) using inert electrodes, identifying the products.
Tests for gases
| Gas | Test | Result |
|---|---|---|
| Hydrogen | lighted splint | squeaky pop |
| Oxygen | glowing splint | relights |
| Carbon dioxide | limewater | turns cloudy |
| Chlorine | damp litmus paper | bleached white |
Electron transfer reactions (Higher Tier)
- Oxidation Is Loss of electrons, Reduction Is Gain (OIL RIG).
- Half equations show what happens at each electrode, e.g. cathode: 2H⁺ + 2e⁻ → H₂; anode: 2Cl⁻ → Cl₂ + 2e⁻.
Exam tips
- Learn the reactivity series and how it decides extraction method.
- Apply the cathode/anode rules for electrolysis of solutions carefully.
- Use OIL RIG for oxidation and reduction, and write balanced half equations.
- Memorise the four gas tests and their results.