Topic 9
3 subtopics · Cambridge IGCSE Chemistry 0620
Key Idea
The reactivity series ranks metals from most to least reactive. It determines how metals react with water, acids, and oxygen, and how they can be extracted.
Explanation
Reactivity series (most to least reactive): K > Na > Ca > Mg > Al > Zn > Fe > Sn > Pb > H > Cu > Ag > Au Reactions with water: - K, Na, Ca: react vigorously with cold water, giving metal hydroxide + H2. - Mg: reacts very slowly with cold water; reacts with steam to give MgO + H2. - Al, Zn, Fe: react with steam only. - Cu, Ag, Au: no reaction with water. Reactions with dilute acids: - Metals above H react with dilute HCl/H2SO4 to give salt + H2. - Metals below H (Cu, Ag, Au) do NOT react with dilute acids. Displacement reactions: - A more reactive metal displaces a less reactive metal from its salt solution. - Fe + CuSO4(aq) to FeSO4(aq) + Cu (iron displaces copper).
Practice Questions
Which metal would react with cold water to produce a metal hydroxide and hydrogen gas?
Key Idea
The method of extraction depends on the metal's position in the reactivity series. Reactive metals are extracted by electrolysis; less reactive metals by reduction with carbon.
Explanation
Methods of extraction: 1. Electrolysis (very reactive metals: K, Na, Ca, Mg, Al): - Used because carbon cannot reduce their oxides. - Example: Aluminium from Al2O3 (bauxite) dissolved in molten cryolite. 2. Reduction with carbon/carbon monoxide (moderately reactive: Zn, Fe, Sn, Pb): - Carbon is a cheaper reducing agent. - Example: Iron from Fe2O3 in a blast furnace. - Fe2O3 + 3CO to 2Fe + 3CO2 3. Found native or heating alone (unreactive: Cu, Ag, Au). Blast furnace (iron extraction): - Raw materials: iron ore (haematite, Fe2O3), coke (C), limestone (CaCO3), hot air. - Coke burns: C + O2 to CO2, then CO2 + C to 2CO. - CO reduces iron ore: Fe2O3 + 3CO to 2Fe + 3CO2. - Limestone removes impurities (slag): CaCO3 to CaO + CO2; CaO + SiO2 to CaSiO3 (slag).
Practice Questions
Explain why aluminium is extracted by electrolysis rather than by reduction with carbon. [2 marks]
Key Idea
Rusting requires both oxygen AND water. Methods to prevent rusting include painting, galvanising, and sacrificial protection. Alloys are mixtures of metals with improved properties.
Explanation
Rusting of iron: - Requires both oxygen AND water (neither alone is sufficient). - Iron + oxygen + water to hydrated iron(III) oxide (rust). Preventing rusting: - Physical barriers: painting, oiling, greasing, plastic coating, tin plating. - Galvanising: coating with zinc. Zinc acts as sacrificial metal (more reactive than Fe). - Sacrificial protection: attach a more reactive metal (Mg or Zn) to iron. - Stainless steel: alloy of Fe with Cr and Ni, resistant to corrosion. Alloys: - Alloy = mixture of a metal with one or more other elements. - Alloys are harder and stronger than pure metals because atoms of different sizes disrupt the regular lattice, making it harder for layers to slide. - Examples: Steel (Fe + C), Stainless steel (Fe + Cr + Ni), Brass (Cu + Zn), Bronze (Cu + Sn).
Practice Questions
Explain why alloys are harder than pure metals. [2 marks]