Doc Brown's GCSE/IGCSE Science-Chemistry Revision Questions
ANSWERS to the questions on the Reactivity Series of Metals
1a its a fast reaction, indicator turns purple (alkali formed), metal melts to a silvery ball whizzing over the surface, colourless gas (hydrogen, can be ignited by this very exothermic reaction, test - pops with lit splint)
1b word equation: sodium + water ==> sodium hydroxide + hydrogen
and in symbols: 2Na(s) + 2H2O(l) ==> 2NaOH(aq) + H2(g)
2a hydrogen gas - always formed if a metal reacts with water - pops with a lit splint
2b magnesium + water ==> magnesium hydroxide + hydrogen
Mg(s) + 2H2O(l) ==> Mg(OH)2(aq) + H2(g)
3a burns brightly in air/oxygen to form the white powder magnesium oxide
2Mg(s) + O2(g) ==> 2MgO(s) ; 3b copper(II) oxide CuO
4 readily dissolves and reacts, sodium oxide + water ==> sodium hydroxide, very alkaline, indicator purple, pH 13-14
5a fizzing of gas, colourless gas, all dissolves in fast reaction, temperature rises (exothermic reaction)
5b zinc + sulphuric acid ==> zinc sulphate + hydrogen; Zn + H2SO4 ==> ZnSO4 + H2
5c hydrogen gas - usually formed if a metal reacts with acid - pops with a lit splint
5d 'mini' reactivity series (fastest) Mg > Zn > Fe > Cu (slowest)
6a RUSTING needs BOTH oxygen and water to be in contact with the iron.
6b red-brown solid of hydrated iron(III) oxide Fe2O3, or more correctly, Fe2O3.xH2O , where x is a variable amount of water.
Iron + water + oxygen => an iron oxide is an oxidation because iron has gained/combined with oxygen and the formation of the iron(III) ion in the oxide involves electron loss Fe ==> Fe3+ + 3e- (e- details for higher only).
6c … below
6d Magnesium (or zinc) blocks will corrode first, being more reactive than iron and leaves the steel/iron intact. This is known as sacrificial corrosion. Higher only … However the reactions involve oxidation/reduction via electron transfer. The magnesium must lose electrons, which move to another part of the metal structure i.e. the iron and ensure the iron doesn't get oxidised by electron loss. If they are electrically insulated from each other by paint, they will both oxidise by electron loss, though the Mg or Zn will corrode faster BUT to no protective effect!
7a zinc dissolves, dark crystals on zinc/brown bits formed at bottom of test tube (copper), blue colour fades (copper removed from solution), heats up (exothermic)
7b Zn(s) + CuSO4(aq) ==> ZnSO4(aq) + Cu(s)
7c the more reactive zinc has replaced or displaced the less reactive copper in the sulphate salt solution, the rule is - for a displacement reaction to occur, the added metal must be higher in the reactivity series.
7d Zn(s) + Cu2+ (aq) ==> Zn2+(aq) + Cu(s) (the SO42- ion is not involved)
the zinc atom Zn loses 2 electrons - oxidation/is oxidised
the copper ion Cu2+ gains 2 electrons - reduction/is oxidised
the two electrons lost by the zinc atom are the two electrons gained by the copper(II) ion
8a (i) the green copper(II) carbonate is decomposed into the black powder, copper(II) oxide, and the colourless gas carbon dioxide
copper(II) carbonate ==> copper(II) oxide + carbon dioxide
CuCO3(s) ==> CuO(s) + CO2(g)
8a (ii) the black copper(II) oxide powder is reduced by the carbon to form bits of copper and colourless carbon dioxide gas
copper(II) oxide + carbon ==> copper + carbon dioxide (another displacement reaction)
2CuO(s) + C(s) ==> 2Cu(s) + CO2(g)
8b (i) thermal decomposition means breaking down a larger molecule into smaller ones with heat
8b (ii) reduction means oxygen loss, the CuO has lost its oxygen to leave the Cu
(higher only: the Cu2+ ion in the copper(II) oxide gains two electrons from the carbon, i.e. the copper ion is reduced by electron gain)
9a the magnesium strip burns brightly, a very exothermic reaction, this triggers the real 'thermit' reaction which goes off with lots of sparks and the mixture glows red. At the end a white powder (aluminium oxide) and a grey blob (iron) remain.
9b aluminium + iron(III) oxide ==> aluminium oxide + iron
Fe2O3 + 2Al ==> Al2O3 + 2Fe
9c Mg + CuO ==> MgO + Cu
they will react on heating together because magnesium is higher in the reactivity series and can therefore displace the less reactive copper from its oxide
9d (i) between copper and iron. Iron reacts with acid to displace hydrogen but copper will not react with acid in the same way.
9d (ii) Iron + sulphuric acid ==> iron(II) sulphate + hydrogen
Fe(s) + H2SO4(aq) ==> FeSO4(aq) + H2(g)
9d (iii) Fe(s) + 2H+(aq) ==> Fe2+(aq) + H2(g)
9d (iv) gold is very low in the reactivity series and does not readily combine with other elements.
10a 3Fe + 2O2 ==> Fe3O4
10b 2Mg + O2 ==> 2MgO
10c Fe2(SO4)3 + 3Zn ==> 3ZnSO4 + 2Fe
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