6.1.1 Extracting metals industrially and by electrolysis

Position relative to carbon decides the extraction method

1. Most metals are found in the Earth's crust as metal compounds, often oxides, mixed with rock.
   1. Extracting a metal means separating it from the other elements in its compound, most often from oxygen.
2. A few very unreactive metals, such as gold and platinum, are found uncombined as the metal itself.
3. In the reactivity series, carbon sits between aluminium and zinc.
   1. Metals less reactive than carbon, such as zinc, iron, tin, lead and copper, can be extracted by heating with carbon.
   2. Metals more reactive than carbon, such as potassium, sodium, calcium, magnesium and aluminium, must be extracted by electrolysis.

Metals below carbon are extracted by heating with carbon

1. When a metal oxide is heated strongly with carbon, the carbon removes the oxygen and leaves the metal: $$metal\; oxide+carbon\to metal+carbon\; dioxide$$
   1. The metal oxide loses oxygen, so it is reduced, while the carbon gains oxygen, so it is oxidised.
2. In a blast furnace, iron ore, coke and limestone are fed in at the top and hot air is blown in near the bottom.
   1. Coke is almost pure carbon, and it burns to heat the furnace and form carbon monoxide: $$2C+O_2\to 2CO$$
3. Carbon monoxide reduces the iron(III) oxide in the ore to molten iron: $${Fe}_2{O}_3+3CO\to 2Fe+3{CO}_2$$
4. Limestone removes sandy impurities by forming slag, which floats on the molten iron and is run off separately.

Copper, a non-ferrous metal, is extracted using carbon

1. Copper is below carbon in the reactivity series, so its compounds can be reduced by carbon.
2. The ore malachite contains copper carbonate, which breaks down on heating to give copper oxide: $${CuCO}_3\to CuO+{CO}_2$$
   1. The copper oxide is then heated with carbon to give copper: $$2CuO+C\to 2Cu+{CO}_2$$
3. Copper made by heating its ore is impure, so it is purified by electrolysis before it is used for electrical wiring.
4. Zinc and lead are other non-ferrous metals below carbon, and zinc can be extracted from zinc oxide in the same way: $$ZnO+C\to Zn+CO$$

Electrolysis extracts metals above carbon but needs more energy

1. Carbon cannot reduce the oxides of metals above carbon, because carbon is less reactive than these metals and cannot take the oxygen from them.
2. These metals are extracted by passing an electric current through a molten compound of the metal.
   1. The compound must be molten so that its ions are free to move to the electrodes.
   2. In an aqueous solution, hydrogen would form at the cathode instead of the reactive metal.
3. Sodium is extracted by electrolysis of molten sodium chloride, which gives sodium at the cathode and chlorine gas at the anode.
4. Electrolysis needs a large amount of electrical energy, so it costs more than reduction with carbon and is used only when carbon cannot do the job.
   1. This is why aluminium smelters are often built near cheap electricity, such as the hydroelectric-powered smelter at Fort William in Scotland.

Aluminium is extracted by electrolysis in molten cryolite

1. The ore bauxite is purified to give aluminium oxide, which is then dissolved in molten cryolite.
   1. Cryolite lowers the operating temperature from over $2000{}^{\circ }C$ to about $950{}^{\circ }C$, which saves a large amount of energy.
2. The cell has carbon electrodes, with the carbon lining of the steel tank acting as the cathode.
3. Positive aluminium ions move to the cathode, where molten aluminium forms and collects at the bottom of the cell.
4. Negative oxide ions move to the anode, where oxygen gas forms.
   1. The oxygen reacts with the hot carbon anodes to form carbon dioxide, so the anodes burn away and must be replaced regularly.
5. The overall change is: $$2{Al}_2{O}_3\to 4Al+3O_2$$

6.1.2 Life cycle assessment and recycling

Life-cycle assessments measure impact at every stage

1. Obtaining raw materials covers mining ores, drilling for crude oil or felling trees, and the energy and land these use.
2. Manufacture covers turning the raw materials into the product and its packaging.
3. Use covers the energy and resources the product needs during its working life, such as the fuel a car burns.
4. Disposal covers what happens at the end of the product's life, whether it goes to landfill, is burned or is recycled.
5. Transport between the stages is also counted, because moving materials and products burns fuel.

A life-cycle assessment values each impact to compare products

1. Some impacts can be measured as numbers, such as the energy used in megajoules or the mass of carbon dioxide released.
2. Other impacts, such as the effect of a quarry on the landscape, are hard to measure and depend on value judgements.
   1. Different people weigh these effects differently, so two assessments of the same product can reach different conclusions.
3. Products should be compared when they do the same job, such as one drinks container against another.
4. An assessment may be selective, because a company could leave out stages that make its own product look worse.
   1. A shortened assessment used to advertise a product can therefore give a misleading picture.

Interpreting life-cycle data by comparing overall values

1. Read every row of the table, including raw materials, energy, percentage recycled and cost, before deciding.
2. Convert values given per kilogram into values per product, because containers made of different materials have different masses.
   1. Multiply the value per kilogram by the mass of one product in kilograms.
3. A material with a lower energy use per kilogram can still need more energy per product if each product is heavier.
4. One material is rarely best on every row, so weigh up the factors and state which one decides the choice.

Recycling for a different use needs suitable properties

1. Used PET bottles are collected, sorted by type of plastic, washed and shredded into flakes.
   1. The flakes are melted and pushed through tiny holes to make fibres, which are spun into yarn for fleece clothing.
2. This works because PET keeps the properties a fibre needs, being strong, flexible and able to be melted and reshaped.
3. A fleece does not need the food-grade purity of a drinks bottle, so small traces of contamination do not matter.
4. Making fibres from recycled PET uses less energy and crude oil than making new PET, so the product can be cheaper.
5. Crushed waste glass is reused in a similar way as a substitute for sand in road surfaces and concrete.

Recycling decisions weigh energy, cost, quality and demand

1. Recycling conserves finite resources, such as metal ores and crude oil, and reduces the waste sent to landfill.
2. Recycling aluminium uses only about 5% of the energy needed to extract new aluminium by electrolysis, so it is very worthwhile.
3. Collecting, sorting and transporting waste also use energy and cost money, which can cancel out the savings for low-value materials.
4. Mixed or dirty waste, such as plastic packets with food on them, is hard to separate, so contamination can make recycling uneconomic.
5. Some materials lose quality each time they are recycled, as the fibres in paper get shorter, so they can be recycled only a few times.
6. Recycling needs a market for the recycled material, since collected waste that nobody buys still ends up as waste.

6.1.3 Biological methods of metal extraction

Biological methods make low-grade ores worth extracting

1. The richest copper ores have already been mined, so many remaining ores contain less than 1% copper.
2. Smelting a low-grade ore means digging up, crushing and heating a huge mass of rock for a small mass of metal.
   1. This uses a great deal of energy and produces large amounts of waste rock and polluting gases such as sulfur dioxide.
3. Biological methods use living organisms to concentrate the metal, so the ore does not need to be heated in a furnace.
4. Mine waste and contaminated land that were once worthless can also be treated this way.

Bioleaching uses bacteria to dissolve metal compounds

1. Low-grade ore is piled in heaps and sprayed with dilute sulfuric acid, and bacteria in the heap get their energy by breaking down metal sulfides.
2. This releases metal ions, which dissolve and drain from the bottom of the heap as the leachate.
3. Copper is recovered from the leachate by displacement with scrap iron: $$Fe+{CuSO}_4\to {FeSO}_4+Cu$$
   1. Iron is more reactive than copper, so it displaces copper from its salt solution.
4. Copper can also be recovered from the leachate by electrolysis, which gives purer copper.

Phytoextraction uses plants to absorb metal ions from soil

1. Certain plants are grown on low-grade ore, mine waste or contaminated soil.
2. Their roots absorb metal ions from the soil, and the plants store them in their stems and leaves.
3. The plants are harvested and burned, leaving an ash that is rich in metal compounds.
   1. Burning the plants also releases energy that can be used to generate electricity.
4. The metal is obtained from the ash by dissolving it in acid, then using displacement or electrolysis.
5. Nickel has been extracted this way from plants such as alyssum, which store unusually large amounts of nickel.

Biological methods save energy but produce metal slowly

1. Both methods need far less energy than smelting, because the ore is never heated in a furnace.
2. They release less carbon dioxide and sulfur dioxide, and less waste rock has to be dug up and dumped.
3. Phytoextraction can clean up land polluted by old mines while it collects the metal.
4. Both methods are very slow, because bioleaching can take months or years and plants take a whole growing season.
5. Bioleaching produces an acidic leachate containing toxic metal ions that must not escape into rivers or groundwater.
6. Phytoextraction needs large areas of land and gives only a small mass of metal from each harvest.

6.1.4 The Haber process and the production of fertilisers

Crops need nitrogen, phosphorus and potassium compounds

1. Plants take mineral ions from the soil through their roots, and harvesting a crop removes these nutrients from the field.
2. Nitrogen compounds, taken up as nitrate or ammonium ions, are needed to make proteins for growth, especially of leaves.
3. Phosphorus compounds, taken up as phosphate ions, are needed for healthy roots and for respiration.
4. Potassium compounds are needed for healthy flowers and fruit and to help the plant resist disease.
5. An NPK fertiliser contains compounds of all three elements, so it replaces the main nutrients a crop removes.
   1. The compounds must be soluble in water, because roots can absorb only dissolved ions.

The Haber process makes ammonia for fertilisers

1. Nitrogen makes up about 78% of the air, but it is very unreactive, so plants cannot use it directly.
2. The Haber process combines nitrogen with hydrogen to make ammonia: $$N_2+3H_2\rightleftharpoons 2{NH}_3$$
   1. The reaction is reversible, so not all the gases react, and the ammonia is separated out while the unreacted gases are passed through again.
3. It uses an iron catalyst at about $450{}^{\circ }C$ and 200 atmospheres.
4. Ammonia made this way is the starting point for almost all nitrogen fertilisers, such as ammonium nitrate and ammonium sulfate.
5. Before the process began in the early 1900s, nitrogen fertilisers came from limited supplies such as manure and mined nitrates, so the Haber process allowed far more food to be grown.

Industrial fertiliser production uses several linked processes

1. Air supplies the nitrogen, and natural gas and steam supply the hydrogen for the Haber process.
2. Some ammonia is oxidised using air to make nitric acid, which then reacts with more ammonia to make ammonium nitrate: $${NH}_3+{HNO}_3\to {NH}_4{NO}_3$$
3. Sulfuric acid, made from sulfur, reacts with ammonia to make ammonium sulfate: $$2{NH}_3+H_2{SO}_4\to ({NH}_4{)}_2{SO}_4$$
4. Phosphate rock is mined and treated with nitric, sulfuric or phosphoric acid to make soluble phosphate compounds.
5. Potassium compounds are mined, for example at Boulby in North Yorkshire, and need little processing before use.
6. The products are blended in the right proportions to make NPK fertilisers for different crops.
7. The plants on one site are often linked, so that the products and waste heat from one process feed the next.

Making fertiliser salts in the laboratory by neutralisation

1. In the laboratory, ammonium sulfate is made by reacting ammonia solution with dilute sulfuric acid: $$2{NH}_3(aq)+H_2{SO}_4(aq)\to ({NH}_4{)}_2{SO}_4(aq)$$
   1. Ammonia solution is an alkali, so this is a neutralisation reaction that forms a salt.
2. A titration with an indicator finds the volume of acid that reacts completely with a measured volume of ammonia solution.
3. The titration is repeated without indicator, using the same volumes, so the salt is not contaminated.
4. The solution is partly evaporated and left to crystallise, giving crystals of ammonium sulfate.

Comparing industrial and laboratory fertiliser production

1. The laboratory uses batch production, while industry uses a continuous process that runs for months without stopping.
2. The laboratory starts from ready-made ammonia solution and sulfuric acid, while industry makes its ammonia and acids from air, natural gas, water and sulfur.
3. A laboratory makes a few grams in glassware, while a factory makes thousands of tonnes in large steel reactors.
4. The laboratory method needs several steps by hand, including titration, evaporation and crystallisation, which makes it slow.
5. Industry uses automatic control and reuses heat from exothermic reactions, so its cost per tonne is far lower.
6. Both methods make the same salt, because the neutralisation reaction at the heart of each is the same.

6.1.5 Alloys and their uses

Adding elements to a metal makes an alloy

1. In a pure metal, the atoms are all the same size and sit in regular layers that slide over each other easily, so many pure metals are soft.
2. In an alloy, atoms of a different size distort the layers, so the layers cannot slide as easily and the alloy is harder.
3. Changing the composition of an alloy changes its properties, so the proportions are chosen to suit each use.
4. Alloys can also be stronger, more resistant to corrosion or lower in melting point than the pure metal.

Carbon content decides the properties of steel

1. Steel is mainly iron mixed with a small percentage of carbon.
2. Low-carbon steel, with about 0.1% to 0.3% carbon, is soft and easily shaped, so it is used for car bodies and food cans.
3. High-carbon steel, with about 0.6% to 1.5% carbon, is much harder but more brittle, so it is used for cutting tools and chisels.
4. Stainless steel is iron mixed with chromium and often nickel, and a common type contains about 18% chromium and 8% nickel.
   1. The chromium makes it very resistant to corrosion, so it is used for cutlery, saucepans and surgical instruments.

Copper alloys are harder than copper and resist corrosion

1. Brass is copper mixed with zinc, often about 70% copper and 30% zinc.
   1. It is harder than copper, does not corrode easily and has a gold colour, so it is used for door handles, musical instruments and the pins of plugs.
2. Bronze is copper mixed with tin, usually about 88% copper and 12% tin.
   1. It is hard, strong and resists corrosion even in sea water, so it is used for statues, bells and ship propellers.
3. Cupronickel, copper mixed with nickel, is hard-wearing and silver-coloured, so it has been used for coins.

Uses of aluminium, tin and gold alloys

1. Duralumin is aluminium with about 4% copper and small amounts of magnesium and manganese.
   1. It has a low density but is much stronger than pure aluminium, so it is used for aircraft bodies.
2. Solder is mainly tin with small amounts of copper and silver, and it melts at a low temperature.
   1. This lets it melt and join electrical components without damaging them.
3. Pure gold, called 24-carat gold, is too soft for jewellery, so it is mixed with copper or silver.
   1. 18-carat gold contains 18 parts of gold in every 24 parts of alloy, which is 75% gold.

6.1.6 Corrosion and its prevention

Corrosion is a metal reacting with its surroundings

1. Most metals corrode by reacting with oxygen, often helped by water, to form metal oxides or other compounds.
   1. The metal gains oxygen, so it is oxidised.
2. The more reactive a metal, the faster it tends to corrode, while gold and platinum hardly corrode at all.
3. Aluminium is more reactive than iron but corrodes very little, because a thin layer of aluminium oxide forms on its surface and seals the metal underneath.
4. Corrosion weakens metal structures, so bridges, cars and ships need regular protection and repair.

Iron rusts only with both oxygen and water

1. Iron reacts with oxygen and water to form hydrated iron(III) oxide, which is rust: $$iron+oxygen+water\to hydrated\; iron(III)\; oxide$$
2. Iron in dry air does not rust, and neither does iron in water that contains no dissolved oxygen.
3. Salt water makes iron rust faster, which is why ships at sea and cars driven on gritted winter roads rust quickly.
4. Acidic conditions, such as acid rain, also speed up rusting.
5. Rust is weak and flakes off, exposing fresh iron underneath, so rusting carries on until the iron is used up.

A physical barrier stops oxygen and water reaching iron

1. Paint protects large structures such as bridges, ships and car bodies.
2. Oil or grease protects moving parts, such as bicycle chains, where paint would rub off.
3. A plastic coating protects items such as garden furniture, fences and dish racks.
4. Tin plating protects the steel of food cans, because tin is unreactive and not toxic.
5. A barrier protects the iron only while it is unbroken, so a scratch lets oxygen and water reach the iron and rusting starts.

In sacrificial protection a more reactive metal corrodes instead

1. A block of a metal more reactive than iron, such as zinc or magnesium, is attached to the iron or steel.
   1. The more reactive metal forms its positive ions more readily than iron, so it reacts with oxygen and water instead of the iron.
2. The iron does not rust while the sacrificial metal remains, so the blocks are replaced once they have corroded away.
3. Magnesium blocks protect underground steel pipes, and zinc blocks protect the steel hulls of ships.
4. Sacrificial protection still works where the iron is exposed, which a barrier alone cannot do.

Galvanising uses zinc as a barrier and sacrificial metal

1. Galvanised steel is coated with a layer of zinc, usually by dipping it in molten zinc.
2. While the coating is unbroken, the zinc acts as a physical barrier, keeping oxygen and water away from the steel.
3. If the coating is scratched, the zinc still protects the steel by sacrificial protection, because zinc is more reactive than iron.
   1. The zinc corrodes instead of the iron, so the exposed steel does not rust.
4. Galvanised steel is used for buckets, nails, fences and motorway crash barriers.

6.1.7 Comparing and selecting materials

Ceramics, polymers, composites and metals have typical properties

1. Glass is a ceramic made by melting sand with other substances, and it is hard, transparent and brittle.
2. Clay ceramics, such as bricks, tiles and china, are made by shaping wet clay and heating it in a kiln, and they are hard, brittle and strong when squashed.
3. Polymers, such as poly(ethene), have low densities, are often flexible and are good electrical and thermal insulators.
4. Composites combine two materials, with fibres or particles of one set in a matrix of the other, to get properties that neither has alone.
5. Metals are strong, dense, malleable and good conductors of heat and electricity, but many corrode.

Quantitative data show how much materials differ

1. Density data show that steel, at about $7800{kg/m}^3$, is roughly eight times as dense as poly(ethene), at about $950{kg/m}^3$.
2. A carbon-fibre composite can have a tensile strength of about $600MPa$, compared with about $400MPa$ for steel and about $30MPa$ for poly(ethene).
3. Clay ceramics are strong when compressed but weak when stretched, with a tensile strength often below $10MPa$.
4. Melting points range from about $110{}^{\circ }C$ for poly(ethene) to about $1500{}^{\circ }C$ for steel.
5. Aluminium conducts heat about 200 times better than glass, which is why saucepans are made of metal but have polymer handles.

Choosing a material by matching properties to use

1. List the properties the use needs, such as strength, density, heat resistance or resistance to corrosion.
2. Then find the material whose values meet every requirement, not the one that is best at only one of them.
3. Window glass is chosen because it is transparent and hard to scratch, even though it is brittle.
4. Bricks suit walls because a wall pushes down on them, and clay ceramics are strong when compressed.
5. Poly(ethene) suits carrier bags and bottles because it has a low density, is flexible and does not react with water.
6. Steel suits bridges and building frames because it has a high tensile strength, even though it is dense and needs protecting from rust.

Composites combine materials to offset each other's weaknesses

1. Reinforced concrete sets steel rods inside concrete, because concrete is strong when compressed and steel is strong when stretched.
2. Fibreglass sets glass fibres in a polymer resin, giving a strong, low-density material for boat hulls and canoes.
3. Carbon-fibre composite sets carbon fibres in a polymer resin, giving very high strength with low density for racing bikes and aircraft parts.
4. Composites are often expensive to make and hard to recycle, because their materials cannot easily be separated.
5. Wood is a natural composite of cellulose fibres held in a matrix of lignin.

6.1.8 Industrial conditions, rate and position of equilibrium

Yield and rate often need different conditions

1. In a reversible reaction, the position of equilibrium decides how much product is present once equilibrium is reached.
2. The rate of reaction decides how quickly the product forms and how soon equilibrium is reached.
3. A change in conditions can improve one while making the other worse, so industry chooses compromise conditions.
4. A company wants the most product per hour at the lowest cost, rather than the highest possible yield.
   1. A high yield reached very slowly gives less product each day than a lower yield reached quickly.
5. The same trade-off applies to making ethanol from ethene and steam, an exothermic reversible reaction run at about $300{}^{\circ }C$ and 60 to 70 atmospheres with a phosphoric acid catalyst.

Low temperature favours ammonia but slows the reaction

1. The forward reaction is exothermic: $$N_2(g)+3H_2(g)\rightleftharpoons 2{NH}_3(g)$$
2. Lowering the temperature shifts the position of equilibrium to the right, towards the exothermic direction, so the yield of ammonia increases.
   1. At 200 atmospheres, the equilibrium yield is about 36% at $400{}^{\circ }C$ but only about 18% at $500{}^{\circ }C$.
3. At a low temperature, fewer particles have energy greater than the activation energy, so the rate is too slow to be economical.
4. About $450{}^{\circ }C$ is used as a compromise, giving a lower yield but a much faster rate.
5. The iron catalyst also works well only at a high temperature, which is another reason the temperature cannot be low.

High pressure raises Haber yield, rate and costs

1. There are four moles of gas on the left of the equation and two on the right, so a higher pressure shifts the position of equilibrium towards ammonia.
   1. At $400{}^{\circ }C$, the equilibrium yield rises from about 25% at 100 atmospheres to about 47% at 300 atmospheres.
2. A higher pressure also increases the rate, because the gas particles are closer together and collide more often.
3. Very high pressures need thick-walled equipment and a lot of energy to compress the gases, which is expensive and increases the risk of leaks.
4. About 200 atmospheres is used as a compromise between a good yield and the cost of equipment and energy.

Graphs show how conditions affect the rate

1. On a graph of rate against temperature, the rate rises more and more steeply as the temperature increases.
2. On a graph of rate against pressure for a reaction of gases, the rate rises as the pressure increases, because the gases become more concentrated.
3. Read values from the graph at the conditions being compared, and describe the change using numbers from the axes.
4. A rate graph shows only how fast the reaction goes, so it must be read alongside yield data before conditions are chosen.
   1. The best temperature is where the rate is high enough while the yield has not fallen too far.

Raw material and energy costs affect commercial conditions

1. Nitrogen comes cheaply from the air, but hydrogen is made from natural gas, so the price of gas has a large effect on the cost of ammonia.
2. Only about 15% of the gases react on each pass through the reactor, so the unreacted nitrogen and hydrogen are recycled and no raw material is wasted.
   1. Ammonia is removed by cooling the mixture, because ammonia condenses at a much higher temperature than nitrogen and hydrogen.
3. Heating and compressing the gases uses a lot of energy, so a lower temperature and pressure save fuel and electricity.
4. The heat released by the exothermic reaction is used to preheat the incoming gases, which cuts energy costs.
5. The catalyst gives an acceptable rate at a lower temperature than would otherwise be needed, which saves energy.