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Question 1 -- Choosing Metals for a Job
Total: 12 marks
An engineering team in Manchester is designing a new electric bus. They must choose metals for the overhead charging cables, the wiring inside the bus, the body panels and the cooking foil used in the on-board catering trolley.
metal
density in g/cm³
relative electrical conductivity
behaviour in moist air
aluminium
2.70
38
forms a thin unbroken oxide layer
copper
8.96
59
very slowly forms a green coating
iron
7.87
10
rusts, and the rust flakes off
lead
11.3
5
forms a dull grey coating
(a)[4]
(i) Using the data, state which metal should be chosen for the overhead charging cables and give two reasons based on the table. [2]
(ii) State which metal should be chosen for the wiring inside the bus and give one reason. [1]
(iii) Explain why lead is unsuitable for either of these uses. [1]
Model Answer -- 1(a)
(i) Aluminium, because its density of 2.70 g/cm³ is by far the lowest, so a long overhead cable will not sag or break under its own weight [1]
(i) Its conductivity of 38 is high enough to carry the current, and its unbroken oxide layer means it will not corrode away outdoors [1]
(ii) Copper, because it has the highest conductivity in the table and weight matters far less for short lengths inside the vehicle [1]
(iii) Lead has both the highest density and the lowest conductivity of the four, so it would make the bus heavy and would waste energy as heat [1]
⚠ If you missed marks here: When a data table is given you must quote figures from it, not general knowledge; an answer that says only "aluminium is light" does not use the evidence supplied. Notice too that the best conductor is not automatically the best choice — for an overhead cable, mass per metre matters more.
(b)[4]
(i) Aluminium is above zinc in the reactivity series, yet the table shows it survives in moist air. Explain this. [2]
(ii) Explain, using the table, why iron corrodes so much more destructively than aluminium. [2]
Model Answer -- 1(b)
(i) Aluminium reacts immediately with oxygen to form a layer of aluminium oxide on its surface [1]
(i) This layer is thin, unbroken and firmly attached, so it seals the reactive metal underneath and re-forms at once if it is scratched [1]
(ii) The rust formed on iron flakes off, as the table states, so it does not seal the surface [1]
(ii) Fresh iron is therefore continually exposed to water and oxygen and the corrosion carries on until the whole object has been eaten away [1]
⚠ If you missed marks here: Saying "aluminium is unreactive" is wrong and will be penalised; the metal is very reactive but its product protects it. In (ii) the mark is for the contrast — flaking versus adhering — not simply for stating that iron rusts.
(c)[4]
(i) A copper busbar inside the bus has a volume of 250 cm³. Calculate its mass, and calculate the mass of an aluminium busbar of the same volume. [2]
(ii) State the mass saved by using aluminium, and explain one disadvantage of making the change. [2]
Model Answer -- 1(c)
(i) mass of copper = 250 × 8.96 = 2240 g (2.24 kg) [1]
(i) mass of aluminium = 250 × 2.70 = 675 g (0.675 kg) [1]
(ii) The saving is 2240 − 675 = 1565 g, which is about 1.57 kg [1]
(ii) Aluminium conducts less well (38 against 59), so a bar of the same size would have a higher resistance and would waste more energy as heat unless it were made thicker [1]
⚠ If you missed marks here: Mass equals density multiplied by volume; dividing instead is a very common slip and gives an answer with the wrong units. In (ii) a disadvantage must be drawn from the data supplied, so quote the two conductivity figures.
Question 2 -- Metals, Water, Steam and Acid
Total: 12 marks
A teacher in Singapore demonstrates the reactions of potassium, calcium, magnesium, zinc and copper with cold water and with dilute hydrochloric acid.
(a)[4]
(i) Describe three observations made when potassium is added to cold water. [3]
(ii) Explain why the indicator turns purple. [1]
Model Answer -- 2(a)
(i) The metal floats and melts into a shiny ball, showing that it is less dense than water and that the reaction is strongly exothermic [1]
(i) It moves rapidly over the surface and fizzes as hydrogen is released, gradually getting smaller until it disappears [1]
(i) A lilac flame is seen above the metal, because the hydrogen produced ignites [1]
(ii) Potassium hydroxide is formed, which is a strong alkali, so the solution becomes alkaline and the universal indicator turns purple [1]
⚠ If you missed marks here: Three separate observations are required, so "it reacts violently" counts only once however it is reworded. In (ii) name the alkali that is formed rather than just saying "an alkali is made", since the naming is what earns the mark.
(b)[4]
(i) Write balanced symbol equations for the reaction of potassium with cold water and for the reaction of zinc with steam. [2]
(ii) Explain why zinc reacts with steam but not with cold water. [1]
(iii) Predict what would be seen if copper were heated in steam, and explain your answer. [1]
Model Answer -- 2(b)
(i) 2K + 2H₂O → 2KOH + H₂ [1]
(i) Zn + H₂O → ZnO + H₂ — note that steam gives the oxide, not the hydroxide [1]
(ii) Zinc is much lower in the reactivity series than potassium, so it needs the extra energy supplied by hot steam before the reaction will proceed at a measurable rate [1]
(iii) No change would be seen, because copper is below hydrogen in the reactivity series and cannot displace hydrogen from water or steam [1]
⚠ If you missed marks here: Balance the potassium equation by remembering that hydrogen is diatomic, so two formula units of water are needed. The hydroxide-versus-oxide distinction between cold water and steam is tested almost every year, so learn it as a pair.
(c)[4]
Equal masses of magnesium, zinc and iron powder are added to separate portions of excess dilute hydrochloric acid of the same concentration and temperature.
(i) Predict the order in which the three reactions finish, from fastest to slowest, and explain your prediction. [2]
(ii) State, with a reason, whether the total volume of gas collected would be the same in each case. [2]
Model Answer -- 2(c)
(i) Magnesium finishes first, then zinc, then iron [1]
(i) This is the order of the metals in the reactivity series: the higher the metal, the more readily it loses electrons to form positive ions, so the faster it displaces hydrogen from the acid [1]
(ii) The volumes would not be the same, because equal masses of the three metals contain different numbers of moles of atoms — magnesium has the smallest relative atomic mass, so equal masses give the most moles [1]
(ii) Iron and zinc also form 2⁺ ions, so the mole ratio to hydrogen is the same, but the greater amount in moles of magnesium means it releases the largest volume of hydrogen [1]
⚠ If you missed marks here: Rate and yield are different ideas, and this question deliberately tests both. Equal masses do not mean equal moles, so an answer that says "the acid is in excess, so the volumes must be equal" misses the effect of the different relative atomic masses.
Question 3 -- Alloys in Everyday Use
Total: 12 marks
A museum in Delhi displays a bronze statue, a set of brass temple bells and a modern stainless steel handrail. A conservator is asked to explain the chemistry of the three materials.
(a)[3]
(i) Name the two metals in bronze and the two metals in brass. [2]
(ii) State one property, other than appearance, that both alloys have in common with the pure metals they are made from. [1]
Model Answer -- 3(a)
(i) Bronze is a mixture of copper and tin [1]
(i) Brass is a mixture of copper and zinc [1]
(ii) Both alloys still conduct electricity and heat, because the metallic lattice and its sea of delocalised electrons are still present (malleability or a metallic lustre are also acceptable) [1]
⚠ If you missed marks here: Bronze and brass are the pair most often swapped over in examinations; a useful memory aid is that brass and zinc both contain the letter z sound in "brass bells of zinc". In (ii) the reason must refer to the delocalised electrons surviving the mixing.
(b)[5]
(i) Draw or describe, in words, the arrangement of particles in a pure metal and in an alloy. [2]
(ii) Use your descriptions to explain why an alloy is harder and stronger than the pure metal. [2]
(iii) State why an alloy usually has a lower melting point than the pure metals it is made from. [1]
Model Answer -- 3(b)
(i) In a pure metal the positive ions are all the same size and are packed in regular, flat, close-packed layers [1]
(i) In an alloy, ions of a second element with a different size are mixed into the same lattice, so the layers are no longer flat and regular [1]
(ii) In the pure metal the regular layers can slide over one another when a force is applied, so the metal is soft and easily bent [1]
(ii) In the alloy the distorted layers catch on one another and cannot slide, so a much larger force is needed and the material is harder and stronger [1]
(iii) The disturbed lattice is less regularly packed, so less energy is needed to break the structure apart and the melting point falls [1]
⚠ If you missed marks here: Full credit needs both halves of the comparison: what happens in the pure metal and what happens in the alloy. Also make sure you refer to positive ions rather than atoms, since the electrons have been delocalised.
(c)[4]
The bells are to be cast from brass containing 65% copper and 35% zinc by mass.
(i) Calculate the mass of copper and the mass of zinc needed to cast a bell of mass 12.0 kg. [2]
(ii) The handrail is stainless steel. Name the main metal in stainless steel, name one element added to it, and state the property this element gives to the alloy. [2]
Model Answer -- 3(c)
(i) mass of copper = 65/100 × 12.0 = 7.80 kg [1]
(i) mass of zinc = 35/100 × 12.0 = 4.20 kg, and the two masses add up to 12.0 kg as a check [1]
(ii) The main metal is iron, and chromium is added [1]
(ii) The chromium gives the alloy its resistance to rusting, because it forms a tough unbroken oxide film on the surface; nickel and carbon add hardness and strength [1]
⚠ If you missed marks here: Always check that the two component masses add up to the total; this catches arithmetic slips instantly. In (ii) the property must be linked to the named element, not just listed on its own.
Question 4 -- Corrosion and Its Prevention
Total: 12 marks
A student in Liverpool sets two iron nails in agar jelly containing an indicator that turns deep blue wherever iron(II) ions are released into the jelly. One nail is wrapped with a strip of zinc and the other with a strip of copper.
(a)[5]
(i) State which nail is corroding and explain how you can tell. [1]
(ii) Explain fully, in terms of the reactivity series and electron transfer, why the two nails behave so differently. [3]
(iii) Name the process by which the zinc protects the nail. [1]
Model Answer -- 4(a)
(i) The nail wrapped with copper is corroding, shown by the deep blue colour, which indicates that Fe²⁺ ions have been released into the jelly [1]
(ii) Zinc is above iron in the reactivity series, so zinc has the greater tendency to form positive ions and is oxidised in preference to the iron [1]
(ii) The electrons released by the zinc flow into the iron, keeping it supplied with electrons so that it cannot form Fe²⁺ ions, and no blue colour appears [1]
(ii) Copper is below iron, so it has less tendency to form ions; the iron therefore gives up its electrons to the copper and corrodes even faster than it would on its own [1]
(iii) Sacrificial protection [1]
⚠ If you missed marks here: The three marks in (ii) require the reactivity comparison, the direction of electron flow, and the contrasting effect of the less reactive metal. An answer that deals only with the zinc can score at most two of them, because the copper result is half of the evidence.
(b)[4]
Steel food cans are plated with tin, whereas steel buckets are galvanised with zinc.
(i) Explain why a scratched tin-plated can rusts faster than an unplated steel can would. [2]
(ii) Explain why a scratched galvanised bucket does not rust. [2]
Model Answer -- 4(b)
(i) Tin is below iron in the reactivity series, so once the coating is broken the iron is in contact with a less reactive metal [1]
(i) The iron therefore loses electrons even more readily than it would alone, so rusting is concentrated at the scratch and proceeds quickly [1]
(ii) Zinc is above iron, so the zinc corrodes instead and continues to feed electrons to the exposed steel [1]
(ii) The protection therefore continues even where the coating is missing, and it lasts until all the zinc has been used up [1]
⚠ If you missed marks here: This is a contrast question, so the two answers must be genuinely different: tin protects only as a barrier, zinc protects as a barrier and sacrificially. Stating merely that "tin is not as good" without the reactivity argument scores nothing.
(c)[3]
(i) State the two substances needed for iron to rust and give the chemical name of rust. [2]
(ii) Suggest, with a reason, why steel structures corrode faster beside the sea than inland. [1]
Model Answer -- 4(c)
(i) Water and oxygen must both be present [1]
(i) Rust is hydrated iron(III) oxide [1]
(ii) Sea spray leaves dissolved salt on the surface, and salt water conducts far better than pure water, so the transfer of electrons involved in rusting happens more quickly [1]
⚠ If you missed marks here: Salt is a catalyst for the process only in the loose sense that it speeds it up; the accepted explanation is that it makes the water a better conductor. Do not claim that salt is one of the substances needed for rusting, because rusting happens perfectly well in fresh water.
Question 5 -- Choosing an Extraction Method
Total: 10 marks
A mining company in Western Australia has deposits of four ores: bauxite (aluminium oxide), zinc blende (converted to zinc oxide), hematite (iron(III) oxide) and a rock containing free gold.
(a)[4]
(i) State the method that should be used to obtain each of the four metals, and give the reason for each choice in terms of the reactivity series. [3]
(ii) Explain why gold needs no chemical reduction at all. [1]
Model Answer -- 5(a)
(i) Aluminium must be obtained by electrolysis of the molten oxide, because aluminium is above carbon in the reactivity series and carbon cannot reduce its oxide [1]
(i) Zinc and iron are both below carbon, so their oxides can be reduced by heating with carbon or carbon monoxide in a furnace, which is much cheaper [1]
(i) Gold is simply separated physically from the rock, for example by crushing and panning, because it occurs as the free metal [1]
(ii) Gold is at the very bottom of the reactivity series and has almost no tendency to form positive ions, so it does not combine with oxygen and is found uncombined [1]
⚠ If you missed marks here: The dividing line in the series is carbon, not hydrogen, when you are deciding between furnace reduction and electrolysis. Note also that cost is part of the reasoning: electrolysis would work for every metal but is used only where nothing cheaper will.
(b)[3]
Zinc oxide is reduced by carbon according to the equation 2ZnO + C → 2Zn + CO₂.
(i) State which substance is oxidised and which is reduced, and give a reason in each case. [2]
(ii) Calculate the mass of zinc that could be obtained from 16.2 tonnes of pure zinc oxide. (Aᵣ: Zn = 65, O = 16) [1]
Model Answer -- 5(b)
(i) The carbon is oxidised, because it gains oxygen to become carbon dioxide [1]
(i) The zinc oxide is reduced, because it loses its oxygen and the zinc ions gain electrons to become zinc atoms [1]
(ii) Mᵣ of ZnO = 81, of which 65 is zinc, so mass = 65/81 × 16.2 = 13.0 tonnes [1]
⚠ If you missed marks here: The balancing number 2 in front of ZnO and Zn does not change the mass ratio, because it appears on both sides; dividing your answer by two is a common and costly error. Remember that oxidation and reduction always happen together in the same reaction.
(c)[3]
(i) State the two main raw materials, other than the ore, that are fed into a blast furnace, and give the purpose of each. [2]
(ii) Write the equation for the thermal decomposition that takes place in the furnace. [1]
Model Answer -- 5(c)
(i) Coke, which burns in the hot air blast to supply heat and which also produces the carbon monoxide that reduces the ore [1]
(i) Limestone, which decomposes to calcium oxide and removes the sandy silicon dioxide impurity as molten slag [1]
(ii) CaCO₃ → CaO + CO₂ [1]
⚠ If you missed marks here: Hot air is blown in but is not usually counted as one of the solid raw materials in the charge, so name coke and limestone. Each material needs its purpose stated for the mark; a bare list scores nothing.
Question 6 -- The Electrolysis of Aluminium Oxide
Total: 12 marks
A smelter in Iceland extracts aluminium electrolytically. The plant is sited beside a geothermal power station because of the enormous quantity of electricity the process requires.
(a)[4]
(i) State the name of the ore from which the aluminium oxide is obtained. [1]
(ii) Explain why the aluminium oxide must be molten or dissolved before it can be electrolysed. [2]
(iii) State the role of the cryolite. [1]
Model Answer -- 6(a)
(i) Bauxite [1]
(ii) In the solid the ions are held in fixed positions in the giant ionic lattice, so they cannot move towards the electrodes [1]
(ii) Once molten or dissolved, the ions are free to move, so they can carry the current and be discharged at the electrodes [1]
(iii) The cryolite acts as a solvent for the aluminium oxide, allowing the cell to be run at about 950 °C instead of over 2000 °C and so saving a great deal of energy [1]
⚠ If you missed marks here: Do not confuse bauxite with hematite, which is the ore of iron. The two marks in (ii) are for the contrast between fixed ions in the solid and mobile ions in the melt; an answer that mentions only one state of matter cannot score both.
(b)[4]
(i) Write the ionic half-equation for the reaction at the cathode and state whether it is oxidation or reduction. [2]
(ii) Write the ionic half-equation for the reaction at the anode. [1]
(iii) Identify the gas seen bubbling from the carbon blocks and explain why the blocks slowly get smaller. [1]
Model Answer -- 6(b)
(i) Al³⁺ + 3e⁻ → Al [1]
(i) This is reduction, because the aluminium ions gain electrons [1]
(ii) 2O²⁻ → O₂ + 4e⁻ [1]
(iii) The gas is oxygen; at the high temperature it reacts with the hot carbon of the anodes to form carbon dioxide, so the blocks burn away and must be replaced [1]
⚠ If you missed marks here: Check that each half-equation balances for charge as well as for atoms: 3⁺ plus three negative electrons gives zero, and two 2⁻ ions release four electrons. A frequent error in (iii) is to name carbon dioxide as the gas seen at the electrode; the gas discharged there is oxygen, and the carbon dioxide is formed afterwards.
(c)[4]
(i) Calculate the mass of aluminium that could be obtained from 510 tonnes of pure aluminium oxide, Al₂O₃. (Aᵣ: Al = 27, O = 16) [2]
(ii) The plant actually produces 243 tonnes from this charge. Calculate the percentage yield. [1]
(iii) Give one reason, in terms of energy, why aluminium drink cans are recycled rather than thrown away. [1]
Model Answer -- 6(c)
(i) Mᵣ of Al₂O₃ = (2 × 27) + (3 × 16) = 102, of which 54 units are aluminium [1]
(i) mass of aluminium = 54/102 × 510 = 270 tonnes [1]
(ii) percentage yield = 243/270 × 100 = 90.0% [1]
(iii) Melting down scrap aluminium uses only a small fraction of the energy needed to electrolyse the oxide, because no strong ionic attractions have to be broken by a large electric current [1]
⚠ If you missed marks here: Percentage yield is always the actual mass divided by the theoretical mass, never the other way round; if your answer is above 100% you have inverted the fraction. In (i), 27 rather than 54 in the numerator halves the answer.
Question 7 -- Placing an Unknown Metal
Total: 10 marks
A research student is given a sample of a metal labelled only as M, together with solutions of the sulfates of magnesium, zinc, iron, copper and silver.
solution
observation when a clean strip of M is left in it
magnesium sulfate
no change
zinc sulfate
no change
iron(II) sulfate
a dark grey deposit forms and the pale green colour fades
copper(II) sulfate
a pink-brown deposit forms and the blue colour fades
silver nitrate
silvery crystals form on the strip
(a)[3]
(i) Deduce the position of M in the reactivity series, naming the two metals it lies between. [1]
(ii) Explain how the results lead to this conclusion. [2]
Model Answer -- 7(a)
(i) M lies between zinc and iron [1]
(ii) M displaces iron, copper and silver from their solutions, so M must be above all three of those metals in the series [1]
(ii) M does not displace magnesium or zinc, so M must lie below both of them; the only position satisfying every result is immediately below zinc and above iron [1]
⚠ If you missed marks here: Both directions of the argument are needed: what M does react with fixes the lower boundary and what it fails to react with fixes the upper one. Quoting only the positive results narrows the position far too little to score both marks.
(b)[4]
M forms ions with a charge of 2⁺.
(i) Write the ionic equation for the reaction of M with copper(II) sulfate solution. [1]
(ii) Write the two half-equations for this reaction and state which species is oxidised. [2]
(iii) Predict what would be seen if M were added to dilute hydrochloric acid, and explain your prediction. [1]
Model Answer -- 7(b)
(i) M(s) + Cu²⁺(aq) → M²⁺(aq) + Cu(s) [1]
(ii) M → M²⁺ + 2e⁻ and Cu²⁺ + 2e⁻ → Cu [1]
(ii) M is oxidised, because it loses electrons; the copper ions are reduced [1]
(iii) Bubbles of hydrogen would be seen and the metal would dissolve, because M lies above hydrogen in the reactivity series and can therefore displace hydrogen from the acid [1]
⚠ If you missed marks here: In the ionic equation the sulfate ion is a spectator and must be left out. For (iii) the deduction depends on having placed M above iron in part (a), and iron itself reacts with dilute acid, so M must do so too.
(c)[3]
(i) Suggest one reason why the strip of M should be cleaned with sandpaper before each test. [1]
(ii) Suggest two ways in which the student could make the comparison between the five solutions a fair test. [2]
Model Answer -- 7(c)
(i) Cleaning removes any oxide or tarnish, which would otherwise have to be attacked first and could delay or hide a genuine reaction [1]
(ii) Use strips of M of the same size and surface area, and use the same volume of each solution [1]
(ii) Use the same concentration for every solution, keep all the tubes at the same temperature, and leave each for the same length of time [1]
⚠ If you missed marks here: "Do it carefully" or "repeat the experiment" are not control variables and score nothing here. Name quantities that are kept the same, such as surface area, volume, concentration, temperature and time.
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