Chemical Changes — AQA Combined Science: Trilogy
This unit covers the reactivity of metals, reactions of acids, salts, the pH scale and electrolysis. Lots of reactions to learn here — focus on the patterns.
Reactivity of metals
Metals react by losing electrons to form positive ions — this is oxidation. The reactivity series ranks metals by how readily they do this:
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 metal from a compound (displacement reaction).
Oxidation and reduction
- Oxidation is the loss of electrons (or gain of oxygen).
- Reduction is the gain of electrons (or loss of oxygen).
- Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
Extraction of metals
A metal less reactive than carbon can be extracted from its ore by reduction with carbon (e.g. iron oxide + carbon → iron + carbon dioxide).
Metals more reactive than carbon (e.g. aluminium) must be extracted by electrolysis, which is more expensive because it uses lots of energy.
Unreactive metals like gold are found native (as the metal itself).
Reactions of acids
Acids react in predictable ways. The general patterns to learn:
- acid + metal → salt + hydrogen
- acid + base (metal oxide/hydroxide) → salt + water (neutralisation)
- acid + metal carbonate → salt + water + carbon dioxide
The salt produced depends on the acid:
- Hydrochloric acid → chlorides
- Sulfuric acid → sulfates
- Nitric acid → nitrates
You can test for hydrogen with a lighted splint (squeaky pop) and for carbon dioxide with limewater (turns cloudy).
Required practical: preparing a pure, dry sample of a soluble salt from an insoluble base (e.g. copper oxide + sulfuric acid → copper sulfate), then filtering, crystallising and drying.
The pH scale and neutralisation
The pH scale (0–14) measures how acidic or alkaline a solution is, measured with universal indicator or a pH probe.
- pH < 7 acidic; pH = 7 neutral; pH > 7 alkaline.
- Acids produce H⁺ ions in solution.
- Alkalis produce OH⁻ ions in solution.
Neutralisation: H⁺ + OH⁻ → H₂O. An acid reacts with a base to form a neutral salt and water.
Strong and weak acids (Higher Tier)
- A strong acid (e.g. hydrochloric, sulfuric, nitric) is fully ionised in water.
- A weak acid (e.g. ethanoic, citric, carbonic) is only partially ionised.
- As pH decreases by one unit, the H⁺ concentration increases by a factor of 10.
- Concentration is about how much acid is dissolved; strength is about how much ionises — these are different ideas.
Electrolysis
Electrolysis uses electricity to break down an ionic compound (the electrolyte) into elements. It works because the ions are free to move when molten or dissolved.
- Cathode (negative electrode): positive ions gain electrons (reduction).
- Anode (positive electrode): negative ions lose electrons (oxidation).
Molten ionic compounds
The metal forms at the cathode and the non-metal at the anode. e.g. molten lead bromide → lead (cathode) + bromine (anode).
Aqueous solutions
Water also provides H⁺ and OH⁻ ions, so the rules are:
- At the cathode: hydrogen is produced unless the metal is less reactive than hydrogen (then the metal is produced, e.g. copper).
- At the anode: oxygen is produced unless a halide (Cl⁻, Br⁻, I⁻) is present (then the halogen is produced).
Half equations show what happens at each electrode, e.g. at the cathode: 2H⁺ + 2e⁻ → H₂.
Extracting aluminium
Aluminium oxide is electrolysed (mixed with cryolite to lower the melting point and save energy). Aluminium forms at the cathode; oxygen at the anode reacts with the carbon electrodes, so they must be replaced regularly.
Required practical: electrolysis of aqueous solutions (e.g. copper sulfate) using inert electrodes, identifying the products.
Exam tips
- Learn the reactivity series and the three general acid reaction patterns.
- Match the acid to its salt (chloride/sulfate/nitrate).
- Use OIL RIG for oxidation and reduction.
- For electrolysis of solutions, apply the cathode and anode rules carefully.
- Distinguish strong/weak (degree of ionisation) from concentrated/dilute (amount dissolved).