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Topic 9 · 9.1–9.6

Metals

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Compare the physical properties of metals and non-metals, and describe the reactions of metals with dilute acids, water, steam and oxygen.
  • Explain uses of aluminium and copper, and of alloys (brass, stainless steel), from their properties; identify an alloy from a structure diagram.
  • Explain why alloys are harder than pure metals, in terms of structure EXTENDED.
  • State the reactivity series and deduce an order of reactivity from results; use displacement reactions and the aluminium oxide layer EXTENDED.
  • State the conditions for rusting and barrier methods; explain galvanising and sacrificial protection EXTENDED.
  • Relate extraction to reactivity; describe the blast furnace (equations EXTENDED) and the extraction of aluminium (electrode reactions EXTENDED).

📚The chemistry

Properties and uses

Metals are good thermal and electrical conductors, malleable (can be hammered into shape) and ductile (can be drawn into wires), and most have high melting and boiling points. Non-metals are generally poor conductors, brittle when solid, and have lower melting and boiling points. Chemically, many metals react with dilute acids (salt + hydrogen), with water or steam (hydroxide or oxide + hydrogen) and with oxygen (metal oxide).

aluminium: aircraft (low density); overhead power cables (low density, good conductor); food containers (resists corrosion)
copper: electrical wiring (good conductor, ductile)

Alloys

An alloy is a mixture of a metal with other elements. Brass is copper and zinc; stainless steel is iron with chromium, nickel and carbon, used for cutlery because it is hard and does not rust. Alloys can be harder and stronger than the pure metals, which makes them more useful.

Left: a pure metal, rows of identical atoms in regular layers, with an arrow showing a force making a layer slide. Right: an alloy, the same layers with three larger atoms of a different element among them, which stop the layers sliding.
In a diagram, an alloy is the one with atoms of two different sizes. EXTENDED The different-sized atoms mean the layers can no longer slide over each other, so the alloy is harder.

The reactivity series

The reactivity series from most to least reactive: potassium, sodium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold. Potassium, sodium and calcium react with cold water. Magnesium to iron react with dilute acids giving hydrogen. Copper, silver and gold do not react with dilute acids. Potassium to aluminium are extracted by electrolysis; zinc, iron and copper by reduction with carbon; gold is found uncombined.
Carbon and hydrogen are non-metals placed in the series for comparison: a metal above hydrogen gives H2 with dilute acids, and a metal below carbon can be extracted by heating its oxide with carbon.

Magnesium reacts very slowly with cold water but quickly with steam, giving magnesium oxide and hydrogen:

\[ \mathrm{Mg(s) + H_2O(g) \rightarrow MgO(s) + H_2(g)} \]

EXTENDED Reactivity is the tendency of a metal to form positive ions. A more reactive metal displaces a less reactive one from its aqueous ions: magnesium displaces zinc, iron, copper and silver; copper displaces only silver.

EXTENDED Aluminium seems less reactive than its position suggests because it is covered by a thin, tough layer of aluminium oxide that stops water and acids reaching the metal. The same layer is why aluminium resists corrosion.

Rusting

Iron and steel rust when they are in contact with both water and oxygen. Rust is hydrated iron(III) oxide.

Three test tubes with iron nails. In the first, the nail is partly in water with air above, and it rusts. In the second, the nail is in boiled water under a layer of oil, so there is no oxygen, and it does not rust. In the third, a stoppered tube contains dry air with anhydrous calcium chloride as a drying agent, so there is no water, and the nail does not rust.
Boiling removes dissolved oxygen and the oil stops more dissolving; the drying agent removes water vapour. Only the nail with both rusts.

Barrier methods — painting, greasing, coating with plastic — keep out oxygen and water. EXTENDED Galvanising coats iron with zinc. The zinc is a barrier, and it also gives sacrificial protection: zinc is more reactive than iron, so if the coating is scratched the zinc loses electrons (is oxidised) in preference to the iron. Blocks of zinc or magnesium bolted to ships’ hulls work the same way.

Extracting metals

The more reactive a metal, the harder it is to extract from its ore. Metals below carbon are extracted by heating the oxide with carbon; metals above carbon need electrolysis. Gold is unreactive enough to be found as the element.

Iron is extracted from hematite (mainly iron(III) oxide) in the blast furnace:

  • Coke (carbon) burns in hot air, producing heat and carbon dioxide.
  • The carbon dioxide is reduced by more coke to carbon monoxide.
  • Carbon monoxide reduces the iron(III) oxide to molten iron.
  • Limestone (calcium carbonate) decomposes to calcium oxide, which reacts with sandy impurities (silicon dioxide) to form slag, calcium silicate.
A blast furnace. Coke, hematite and limestone are added at the top; hot air is blown in near the bottom; waste gases leave at the top. Inside, from the bottom up: C + O2 gives CO2 as the coke burns; C + CO2 gives 2CO; Fe2O3 + 3CO gives 2Fe + 3CO2. At the side: CaCO3 gives CaO + CO2, and CaO + SiO2 gives CaSiO3, the slag. Molten slag floats on molten iron at the bottom, and both are tapped off.
Slag is less dense than iron, so it floats on top and is tapped off separately. EXTENDED You need all five symbol equations; Core candidates describe the steps in words.

Aluminium’s main ore is bauxite, and aluminium is extracted by electrolysis. EXTENDED Purified aluminium oxide is dissolved in molten cryolite, which lowers the operating temperature (saving energy) and improves the conductivity.

An electrolysis cell: a steel tank with a carbon lining that is the negative cathode, containing aluminium oxide dissolved in molten cryolite, with molten aluminium collecting at the bottom. Three carbon blocks dip into the electrolyte as positive anodes, with oxygen bubbles forming on them. Half-equations: at the cathode, Al3+ + 3e- gives Al; at the anode, 2O2- gives O2 + 4e-.
The anodes are replaced regularly because the oxygen formed on them reacts with the hot carbon to make carbon dioxide. EXTENDED
\[ \text{cathode: } \mathrm{Al^{3+} + 3e^{-} \rightarrow Al} \]
\[ \text{anode: } \mathrm{2O^{2-} \rightarrow O_2 + 4e^{-}} \]

✏️Worked example

Three metals, W, X and Y, are tested. W fizzes rapidly in dilute hydrochloric acid, Y fizzes slowly, and X does not react. X placed in aqueous silver nitrate becomes coated with a grey solid. (a) Put W, X, Y and silver in order of reactivity, most reactive first. [2] (b) Which of the metals could be copper? Explain. [1] (c) EXTENDED Zinc is added to aqueous iron(II) sulfate. Write the ionic equation and explain the reaction in terms of ions. [3]

(a) W, Y, X, silver. W and Y react with acid (W faster); X does not, so it is below hydrogen; X displaces silver, so X is above silver.

(b) X: copper is below hydrogen (no reaction with dilute acid) but above silver (it displaces silver from silver nitrate).

(c) \( \mathrm{Zn(s) + Fe^{2+}(aq) \rightarrow Zn^{2+}(aq) + Fe(s)} \). Zinc has a greater tendency to form positive ions than iron, so zinc atoms lose electrons to become Zn2+ and the Fe2+ ions gain them to become iron atoms.

Check it. Every conclusion in (a) comes from a result: acid tests place metals relative to hydrogen, and displacement places X relative to silver. Charge balances in (c): +2 on each side ✓.
“X is unreactive.” Not reacting with acid only shows X is below hydrogen. The silver nitrate result shows it still reacts with the salt of a less reactive metal.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Which two properties make aluminium suitable for overhead power cables? A: high density and good conductor. B: low density and good conductor. C: low density and high melting point. D: resists corrosion and is malleable.
B. The cables must carry current and be light enough to hang between pylons.
2. (Theory.) Name the metals in brass, and give one reason why stainless steel is used for cutlery. [2]
Copper and zinc. Stainless steel is hard and resistant to rusting.
3. (Theory.) State the two substances needed for iron to rust, and explain how painting a bicycle frame prevents rusting. [3]
Water and oxygen. The paint is a barrier that keeps water and oxygen away from the iron.
4. (Theory.) Write the word equation for the reaction of calcium with cold water, and predict whether copper reacts with steam. [2]
calcium + water → calcium hydroxide + hydrogen. No: copper is below hydrogen in the reactivity series.
5. (Theory.) Explain why aluminium cannot be extracted from its oxide by heating with carbon. [1]
Aluminium is more reactive than carbon, so carbon cannot remove the oxygen from aluminium oxide.
6. (Theory.) EXTENDED A steel bucket is galvanised. The zinc coating is scratched, exposing the steel, but the steel does not rust. Explain, in terms of electrons. [3]
Zinc is more reactive than iron, so it loses electrons (is oxidised) more readily. The zinc corrodes instead of the iron: sacrificial protection.
7. (Theory.) EXTENDED In the blast furnace, write the equations for (a) the formation of carbon monoxide from carbon dioxide, (b) the formation of slag. [2]
(a) \( \mathrm{C + CO_2 \rightarrow 2CO} \). (b) \( \mathrm{CaO + SiO_2 \rightarrow CaSiO_3} \).
8. (Theory.) EXTENDED Aluminium is high in the reactivity series, yet it is used for food containers. Explain. [2]
Aluminium is covered by a thin, unreactive layer of aluminium oxide that stops water, air and acids in food reaching the metal, so it resists corrosion.
9. (Theory.) EXTENDED State the role of cryolite in the extraction of aluminium, and write the half-equation at the cathode. [2]
It lowers the melting point of the electrolyte (so less energy is needed) and improves conductivity. \( \mathrm{Al^{3+} + 3e^{-} \rightarrow Al} \).

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • Royal Society of Chemistry — the rusting nail experiment with controls
  • BBC Bitesize — the blast furnace and aluminium extraction, animated