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Question 1 -- Metals, Bonding and Properties
Total: 12 marks
A class in Chennai is given samples of magnesium ribbon, sulfur powder and a length of copper wire, and is asked to compare the properties of metals with those of non-metals.
(a)[4]
Use the diagram to describe the structure and bonding in a metal.
(i) Name the two kinds of particle shown and state the charge on each. [2]
(ii) Explain, in terms of these particles, why a metal conducts electricity when it is solid. [2]
Model Answer -- 1(a)
(i) The large particles are positive metal ions, formed when each atom loses its outer-shell electrons [1]
(i) The small particles are the delocalised electrons, each carrying a single negative charge, and they belong to the whole lattice rather than to any one ion [1]
(ii) The delocalised electrons are free to move throughout the whole structure, so they can drift towards the positive terminal when a voltage is applied [1]
(ii) The ions stay in their fixed lattice positions, so the metal keeps its shape and does not need to be melted before it will conduct [1]
⚠ If you missed marks here: The commonest error is calling the large particles "atoms". They have lost electrons, so they must be described as positive ions. The second frequent loss is saying only that "electrons move" — you must add that they are delocalised, that is free to move through the whole lattice, otherwise the answer does not distinguish a metal from any other substance.
(b)[4]
Magnesium melts at 650 °C and can be rolled into thin sheets. Sulfur melts at 115 °C and crumbles to a powder when it is hammered.
(i) Explain, in terms of structure and bonding, why magnesium has the higher melting point. [2]
(ii) Explain why magnesium can be rolled into sheets but sulfur cannot. [2]
Model Answer -- 1(b)
(i) Magnesium has a giant metallic lattice, and melting it means overcoming the strong electrostatic attraction between the positive ions and the sea of delocalised electrons throughout the structure [1]
(i) Sulfur is a simple molecular solid, so melting it only requires the weak intermolecular forces between S₈ molecules to be overcome; no strong bonds are broken [1]
(ii) In magnesium the layers of ions can slide over one another into new positions and the delocalised electrons continue to hold the lattice together, so the metal changes shape without breaking [1]
(ii) Sulfur has no such layers of ions; the blow simply separates the molecules from one another, so the solid shatters and is described as brittle [1]
⚠ If you missed marks here: Writing that "covalent bonds break when sulfur melts" is a serious error — the S₈ rings survive melting intact, and only the forces between them are overcome. In (ii) you must say that the electrons keep holding the layers together after they have moved; an answer that stops at "the layers slide" does not explain why the metal does not simply fall apart.
(c)[4]
The class then reacts magnesium with dilute hydrochloric acid, and separately heats magnesium in oxygen.
(i) Write the balanced symbol equation for the reaction of magnesium with dilute hydrochloric acid. [1]
(ii) Describe one observation that would be made during this reaction, and state how you would test the gas produced. [2]
(iii) State whether the oxide formed when magnesium burns is acidic or basic, and name the type of compound formed when it reacts with dilute nitric acid. [1]
(ii) Rapid effervescence, the ribbon gets smaller and disappears, and the tube becomes warm [1]
(ii) Collect the gas and hold a lighted splint at the mouth of the tube; hydrogen burns with a squeaky pop [1]
(iii) Magnesium oxide is a basic oxide, and with dilute nitric acid it forms a salt, magnesium nitrate, together with water [1]
⚠ If you missed marks here: Writing MgCl rather than MgCl₂ loses the equation mark, because magnesium forms a 2⁺ ion. For the gas test the word "pop" alone is not enough — you must state that a lighted splint is used, since a glowing splint tests for oxygen instead.
Question 2 -- Alloys and Their Uses
Total: 12 marks
A manufacturer in Sheffield produces cutlery from stainless steel and door fittings from brass. Both materials are alloys.
(a)[4]
(i) Define the term alloy. [1]
(ii) State which of the two structures shown represents an alloy, and give the reason for your choice. [1]
(iii) Explain, in terms of the structure, why the alloy is harder than the pure metal. [2]
Model Answer -- 2(a)
(i) An alloy is a mixture of a metal with one or more other elements [1]
(ii) Structure B, because it contains particles of two different sizes mixed together in one lattice, whereas structure A contains only one kind [1]
(iii) The different sized ions distort the regular close-packed layers, so the layers no longer lie flat against one another [1]
(iii) The layers can therefore no longer slide over one another when a force is applied, so a much greater force is needed to deform the alloy and it is harder and stronger [1]
⚠ If you missed marks here: Calling an alloy a "compound" loses the definition mark, because the proportions of the elements can be varied continuously. In (iii) an answer of "the atoms are different sizes" scores only the first mark; you must go on to say that the layers can no longer slide, which is the property that makes the material hard.
(b)[4]
(i) Name the two metals that are mixed to make brass. [1]
(ii) Name the main metal in stainless steel and name one other element that is added to it. [1]
(iii) Give two properties of stainless steel that make it suitable for cutlery, and explain why each property is needed. [2]
Model Answer -- 2(b)
(i) Copper and zinc [1]
(ii) Mainly iron, with chromium added (nickel or carbon are also accepted) [1]
(iii) It is hard, so a knife blade keeps a sharp cutting edge and the surface is not scratched by repeated use [1]
(iii) It resists rusting, so the cutlery can be washed in water again and again without corroding or staining the food [1]
⚠ If you missed marks here: Brass and bronze are easily confused: brass contains zinc, bronze contains tin. In (iii) simply listing two properties is not enough for both marks, because the question asks you to explain why each one is needed for this particular use.
(c)[4]
A batch of brass is to contain 68% copper and 32% zinc by mass.
(i) Calculate the mass of zinc needed to make 250 kg of this brass. [2]
(ii) A sample of the finished brass is found to conduct electricity less well than pure copper. Explain this observation in terms of the structure. [2]
Model Answer -- 2(c)
(i) mass of zinc = 32/100 × 250 kg [1]
(i) = 80 kg of zinc (and therefore 170 kg of copper) [1]
(ii) Brass still contains delocalised electrons, so it does conduct, but the lattice now contains ions of two different sizes and is no longer perfectly regular [1]
(ii) The irregular lattice scatters the drifting electrons more, so they move through the metal less freely and the resistance is higher than in pure copper [1]
⚠ If you missed marks here: In (i) show the percentage step even if you can do it in your head, because the method mark is available separately from the answer. In (ii) do not say that the zinc "removes the delocalised electrons" — zinc adds electrons to the sea; the conductivity falls only because the electrons are scattered by an irregular lattice.
Question 3 -- Deducing a Reactivity Series
Total: 12 marks
A technician in a school in Nairobi labels four metals A, B, C and D so that a class cannot recognise them. Each metal is tested with cold water, with steam and with dilute hydrochloric acid. The results are shown in the table.
metal
cold water
steam
dilute hydrochloric acid
A
no change
no change
no change
B
no change
reacts when strongly heated
steady stream of bubbles
C
steady bubbles, solution turns cloudy
reacts very readily
violent reaction
D
no change
very slow reaction when red hot
a few slow bubbles
(a)[3]
(i) Place the four metals in order, starting with the most reactive. [1]
(ii) Give the evidence from the table that identifies the most reactive metal. [1]
(iii) Give the evidence from the table that identifies the least reactive metal. [1]
Model Answer -- 3(a)
(i) C, B, D, A — most reactive first [1]
(ii) C is the only metal that reacts with cold water, and it also reacts violently with dilute acid, so it must be highest in the series [1]
(iii) A gives no reaction with water, with steam or with acid, so it lies below hydrogen and is the least reactive of the four [1]
⚠ If you missed marks here: B and D are separated only by the vigour of their reactions, not by whether they react at all, so read the descriptive words carefully — "steady stream" beats "a few slow bubbles". Quoting the acid column alone is not enough evidence for the top of the series; the cold water result is the decisive one.
(b)[5]
Metal C is calcium, metal B is magnesium, metal D is iron and metal A is copper.
(i) Write a balanced symbol equation for the reaction of calcium with cold water. [2]
(ii) Write a balanced symbol equation for the reaction of magnesium with steam. [2]
(iii) State why no equation can be written for copper with dilute hydrochloric acid. [1]
(ii) Correct formulae, with the oxide rather than the hydroxide because steam is used: Mg + H₂O → MgO + H₂ [1]
(ii) The equation as written is already balanced, with one of each element on both sides [1]
(iii) Copper lies below hydrogen in the reactivity series, so it cannot displace hydrogen from the acid and no reaction takes place [1]
⚠ If you missed marks here: The key distinction is that cold water gives the hydroxide while steam gives the oxide — writing Mg(OH)₂ for the steam reaction loses the formula mark. Remember also that Ca(OH)₂ needs the bracket, because without it CaOH₂ would mean something quite different.
(c)[4]
(i) Explain, in terms of electrons, why calcium is more reactive than iron. [2]
(ii) Aluminium lies above zinc in the reactivity series, yet a strip of aluminium foil placed in dilute hydrochloric acid gives no bubbles for the first minute. Explain this observation and suggest what would then be seen. [2]
Model Answer -- 3(c)
(i) The reactivity of a metal depends on how readily it forms a positive ion, that is on how easily it loses electrons [1]
(i) Calcium loses its two outer electrons more readily than iron does, so calcium is oxidised more easily and is higher in the series [1]
(ii) The aluminium is covered by a thin, tough layer of aluminium oxide, which must be attacked and removed by the acid before the metal underneath can be reached [1]
(ii) Once the oxide has gone, bubbles are produced rapidly, faster than with zinc, because aluminium is the more reactive metal [1]
⚠ If you missed marks here: Never write that a metal "gains electrons" to become more reactive — metals always lose electrons. In (ii) an answer that says "aluminium is unreactive" contradicts its position in the series; the point is that it only appears unreactive because of its oxide coat.
Question 4 -- Displacement Reactions
Total: 12 marks
A student in Kuala Lumpur places a clean strip of zinc in a beaker of blue copper(II) sulfate solution and leaves it for twenty minutes.
(a)[4]
(i) Describe two observations the student would make. [2]
(ii) Write the balanced symbol equation for the reaction. [1]
(iii) Write the ionic equation for the reaction, omitting the spectator ions, and name the ion that is a spectator. [1]
Model Answer -- 4(a)
(i) The blue colour of the solution fades and becomes colourless (or very pale) [1]
(i) A pink-brown solid coats the zinc strip, the strip becomes thinner, and the mixture warms up [1]
(ii) Zn + CuSO₄ → ZnSO₄ + Cu [1]
(iii) Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s); the sulfate ion is the spectator ion [1]
⚠ If you missed marks here: "The solution changes colour" is too vague for a mark — state that the blue fades. In the ionic equation the state symbols matter, because they show which species is the solid metal and which is in solution; leaving the sulfate ion in means the equation is not an ionic equation at all.
(b)[3]
(i) Write the half-equation for the change undergone by the zinc, and state whether it is oxidation or reduction. [2]
(ii) Explain why no reaction occurs when a copper strip is placed in zinc sulfate solution. [1]
Model Answer -- 4(b)
(i) Zn → Zn²⁺ + 2e⁻ [1]
(i) This is oxidation, because the zinc loses electrons [1]
(ii) Copper is below zinc in the reactivity series, so copper has less tendency to form ions than zinc and cannot give electrons to zinc ions [1]
⚠ If you missed marks here: Electrons must appear on the right-hand side when a metal is oxidised; putting them on the left describes a metal ion being deposited instead. Use OIL RIG to check: oxidation is loss of electrons.
(c)[5]
The student uses 100 cm³ of 0.150 mol/dm³ copper(II) sulfate solution and adds an excess of zinc powder.
(i) Calculate the amount, in moles, of copper(II) sulfate used. [2]
(ii) Calculate the maximum mass of copper that could be deposited. (Aᵣ: Cu = 64) [2]
(iii) The student recovers only 0.82 g of dry copper. Suggest one reason for this. [1]
Model Answer -- 4(c)
(i) volume in dm³ = 100 / 1000 = 0.100 dm³ [1]
(i) amount = 0.100 × 0.150 = 0.0150 mol [1]
(ii) The equation shows a 1 : 1 ratio, so 0.0150 mol of copper is formed [1]
(ii) mass = 0.0150 × 64 = 0.96 g [1]
(iii) Any one of: some finely divided copper was lost when the mixture was filtered or washed; some copper remained stuck to the excess zinc; the copper was not completely dry or some was left in the beaker [1]
⚠ If you missed marks here: Forgetting to divide the volume by 1000 makes the answer a thousand times too large, and it is the single commonest slip in this style of calculation. In (iii) "experimental error" on its own scores nothing; you must name a specific loss of product.
Question 5 -- Rusting and Its Prevention
Total: 10 marks
A student in Kochi sets up three sealed boiling tubes, each containing one identical iron nail, and leaves them on the windowsill for a week.
(a)[4]
(i) State the two substances that must both be present for iron to rust. [1]
(ii) State, with a reason, in which tube or tubes the nail rusts. [2]
(iii) Give the chemical name of rust. [1]
Model Answer -- 5(a)
(i) Water and oxygen (from the air) [1]
(ii) Only the nail in tube 3 rusts, because it is the only one in contact with both water and air [1]
(ii) In tube 1 boiling has driven out the dissolved air and the oil layer stops more entering, so there is water but no oxygen; in tube 2 the drying agent removes water vapour, so there is oxygen but no water [1]
(iii) Hydrated iron(III) oxide [1]
⚠ If you missed marks here: A common error is to say that "air" is needed rather than oxygen, which is the reactive component. In (iii) the answer must include both the word hydrated and the oxidation state (III); "iron oxide" alone is not specific enough.
(b)[3]
(i) Name two barrier methods used to prevent iron from rusting. [1]
(ii) Explain how a barrier method works. [1]
(iii) State one disadvantage of relying on a barrier method alone. [1]
Model Answer -- 5(b)
(i) Any two of: painting, greasing or oiling, coating with plastic, or electroplating with an unreactive metal [1]
(ii) The coating keeps water and oxygen away from the surface of the iron, so the two substances needed for rusting cannot reach the metal [1]
(iii) If the coating is scratched or chipped, water and air reach the bare iron at once and rusting begins and spreads underneath the coating [1]
⚠ If you missed marks here: Do not offer "galvanising" as a plain barrier method here without comment; it is a barrier but it also provides sacrificial protection, which is exactly why it does not suffer the disadvantage asked for in (iii).
(c)[3]
A steel bucket is galvanised by dipping it in molten zinc.
(i) Explain why the zinc coating still protects the steel even after it has been deeply scratched. [2]
(ii) Write the half-equation for the reaction of the zinc that provides this protection. [1]
Model Answer -- 5(c)
(i) Zinc is above iron in the reactivity series, so it has the greater tendency to form positive ions and is oxidised in preference to the iron [1]
(i) The electrons released by the zinc pass into the steel, so the iron atoms are kept supplied with electrons and cannot form Fe²⁺ ions; this is called sacrificial protection [1]
(ii) Zn → Zn²⁺ + 2e⁻ [1]
⚠ If you missed marks here: The direction of electron flow is the mark that is most often lost: electrons travel from the zinc into the iron, never the other way round. Saying only that "zinc is more reactive" earns the first mark but not the second, which requires the electron argument.
Question 6 -- The Blast Furnace
Total: 12 marks
Iron is extracted from hematite at an integrated steel plant in Jamshedpur. The furnace is charged with iron ore, coke and limestone, and hot air is blown in near the base.
(a)[5]
Write balanced symbol equations for each of the following stages in the furnace.
(i) The burning of coke in the hot air blast. [1]
(ii) The reduction of carbon dioxide by more hot coke. [1]
(iii) The reduction of iron(III) oxide by carbon monoxide. [2]
(iv) The thermal decomposition of limestone. [1]
Model Answer -- 6(a)
(i) C + O₂ → CO₂ [1]
(ii) C + CO₂ → 2CO [1]
(iii) Correct formulae, with carbon monoxide as the reducing agent: Fe₂O₃ + CO → Fe + CO₂ [1]
⚠ If you missed marks here: The main reducing agent is carbon monoxide, not carbon, so an equation using C to reduce the ore does not gain the formula mark. Check the balancing of (iii) by counting oxygen atoms: three from the ore plus three from the CO gives six, which is exactly what three molecules of CO₂ require.
(b)[4]
(i) State the role of the coke in the furnace, giving two separate functions. [2]
(ii) Explain how the limestone removes the sandy impurity, silicon dioxide, and write the equation for the reaction. [2]
Model Answer -- 6(b)
(i) The coke burns in the hot air blast, and this exothermic reaction supplies the very high temperature the furnace needs [1]
(i) The coke also reacts with the carbon dioxide produced to make carbon monoxide, which is the reducing agent that removes oxygen from the ore [1]
(ii) The limestone decomposes to calcium oxide, a basic oxide, which reacts with the acidic oxide silicon dioxide to give molten calcium silicate, the slag [1]
(ii) CaO + SiO₂ → CaSiO₃ [1]
⚠ If you missed marks here: Two distinct functions are needed in (i); writing "it provides heat" twice in different words earns only one mark. In (ii) it is the calcium oxide, not the limestone itself, that reacts with the silicon dioxide, so mention the decomposition step first.
(c)[3]
(i) Explain why the molten slag and the molten iron can be run off separately. [1]
(ii) Calculate the maximum mass of iron that could be obtained from 800 tonnes of pure Fe₂O₃. (Aᵣ: Fe = 56, O = 16) [2]
Model Answer -- 6(c)
(i) The slag is less dense than the molten iron and the two liquids do not mix, so the slag floats on top and is tapped off through a higher opening [1]
(ii) Mᵣ of Fe₂O₃ = (2 × 56) + (3 × 16) = 160, and the iron in it accounts for 112 of those units [1]
(ii) mass of iron = 112/160 × 800 = 560 tonnes [1]
⚠ If you missed marks here: Using 56 instead of 112 forgets that each formula unit contains two iron atoms and halves the answer. There is no need to convert tonnes to grams: as long as the same unit is used throughout, the ratio method works directly.
Question 7 -- Aluminium: Extraction and Uses
Total: 10 marks
Aluminium is extracted from purified bauxite, which is essentially aluminium oxide, in a cell operating at about 950 °C.
(a)[3]
(i) Explain why aluminium cannot be extracted by heating its ore with carbon. [1]
(ii) State the name of the substance in which the aluminium oxide is dissolved, and explain why this is done. [2]
Model Answer -- 7(a)
(i) Aluminium is above carbon in the reactivity series, so it holds its oxygen more strongly than carbon does and carbon cannot reduce its oxide [1]
(ii) The oxide is dissolved in molten cryolite [1]
(ii) Aluminium oxide alone melts at over 2000 °C, but the solution in cryolite is molten at about 950 °C, which greatly reduces the energy needed and therefore the cost [1]
⚠ If you missed marks here: Cryolite is a solvent, not a reactant, so any answer suggesting that it reduces the oxide or supplies aluminium is wrong. The explanation mark needs the idea of a lower operating temperature and hence lower energy cost, not simply "it makes it melt".
(b)[4]
(i) Write the ionic half-equation for the reaction at the negative electrode. [1]
(ii) Write the ionic half-equation for the reaction at the positive electrodes. [1]
(iii) Explain why the carbon anodes have to be replaced regularly. [2]
Model Answer -- 7(b)
(i) Al³⁺ + 3e⁻ → Al [1]
(ii) 2O²⁻ → O₂ + 4e⁻ [1]
(iii) Oxygen is produced at the anodes, and at the high operating temperature this oxygen reacts with the hot carbon of the electrodes [1]
(iii) The carbon is converted to carbon dioxide and so burns away, which means the anodes gradually get smaller and must be renewed [1]
⚠ If you missed marks here: Electrons must be on the left for the cathode reaction and on the right for the anode reaction; reversing them is the commonest error. In (iii) saying only "they burn away" earns one mark — the second requires you to say that the oxygen released at the anode is what attacks the carbon.
(c)[3]
(i) Aluminium is used for overhead electrical cables and for cooking foil. State the property of aluminium that is most important in each case. [2]
(ii) Calculate the maximum mass of aluminium that can be obtained from 20.4 tonnes of pure aluminium oxide, Al₂O₃. (Aᵣ: Al = 27, O = 16) [1]
Model Answer -- 7(c)
(i) For overhead cables the key property is low density combined with good electrical conductivity, so the cable does not sag between the pylons [1]
(i) For cooking foil the key property is resistance to corrosion, because the protective oxide layer stops the metal reacting with the food or the air [1]
(ii) Mᵣ of Al₂O₃ = 102, of which 54 is aluminium, so mass = 54/102 × 20.4 = 10.8 tonnes [1]
⚠ If you missed marks here: Do not give the same property for both uses; the examiner is testing whether you can match a different property to each application. In the calculation remember that Al₂O₃ contains two aluminium atoms, so the numerator is 54 and not 27.
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