Chemical Changes
Reactivity, extraction, acids, bases, and electrolysis.
In this topic
Reactivity Series and Metal Extraction
The reactivity series ranks metals from most reactive to least reactive: K > Na > Ca > Mg > Al > C > Zn > Fe > H > Cu > Ag > Au
A more reactive metal will displace a less reactive metal from its compound.
Extraction of metals depends on reactivity: - Metals MORE reactive than carbon: extracted by electrolysis (e.g., aluminium) - Metals LESS reactive than carbon: extracted by reduction with carbon (e.g., iron in a blast furnace) - Very unreactive metals (gold, silver): found native/free in the Earth
Key Points
- Displacement: more reactive metal pushes out a less reactive one
- Oxidation = loss of electrons (OIL)
- Reduction = gain of electrons (RIG)
- Electrolysis is expensive due to the electricity needed
Example Questions
2Explain why aluminium is extracted by electrolysis rather than reduction with carbon.
Aluminium is more reactive than carbon, so carbon cannot reduce aluminium oxide. Electrolysis must be used instead, which uses electrical energy to decompose the aluminium oxide into aluminium and oxygen.
[2 marks]
3Predict what happens when iron is added to copper sulfate solution. Write a word equation.
Iron is more reactive than copper, so it displaces copper. The solution changes from blue to green/colourless, and brown copper metal appears. Iron + copper sulfate → iron sulfate + copper
[3 marks]
Acids, Bases, and Salts
Acids produce H⁺ ions in solution. Bases neutralise acids. An alkali is a soluble base that produces OH⁻ ions.
pH scale: 0-6 acidic, 7 neutral, 8-14 alkaline.
Key reactions: - Acid + Metal → Salt + Hydrogen - Acid + Metal Oxide → Salt + Water - Acid + Metal Hydroxide → Salt + Water - Acid + Metal Carbonate → Salt + Water + Carbon Dioxide
Common acids: HCl (hydrochloric), H₂SO₄ (sulfuric), HNO₃ (nitric) Naming salts: HCl → chlorides, H₂SO₄ → sulfates, HNO₃ → nitrates
Key Points
- Neutralisation: H⁺ + OH⁻ → H₂O
- Strong acids fully dissociate (ionise) in water
- Weak acids partially dissociate
- Indicators change colour to show pH
Example Questions
2Write a balanced symbol equation for the reaction between sodium hydroxide and sulfuric acid.
2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
[2 marks]
4Describe how you would make pure, dry crystals of copper sulfate from copper oxide and sulfuric acid.
1) Warm sulfuric acid. 2) Add excess copper oxide and stir. 3) Filter to remove excess copper oxide. 4) Heat the filtrate gently to evaporate some water. 5) Leave to crystallise. 6) Pat dry with filter paper.
[4 marks]