Doc Brown's Chemistry Clinic

Industrial Chemistry Processes

Revision Notes for KS4 Science GCSE/IGCSE Chemistry

Page sub-index: 1. Limestone, lime - uses, thermal decomposition of carbonates, hydroxides and nitrates  *  2. Enzymes and Biotechnology  *  3. Contact Process, the importance of sulphuric acid  *  4. How can metals be made more useful? (alloys of Al,Fe,steel etc.) * 5. The importance of titanium  *  6. Instrumental Methods of Chemical Analysis * 7. Chemical economics of processes and sociological and environmental issues etc.

and other web pages of industrial chemistry: Ammonia synthesis/uses/fertilisers * Oil Products * Extra Organic Chemistry * Extraction of Metals * Halogens - sodium chloride Electrolysis * Transition Metals * Extra Electrochemistry * EMAIL comment?query *

1. Limestone - a very useful material

• GCSE Multiple choice QUIZ on Limestone and its uses:

  • easier Foundation or harder higher  (limestone or marble!)

  • and a GCSE multi-word fill exercise

• Limestone, is a sedimentary rock formed by the mineral and 'shelly' remains of marine organisms, including coral, in warm shallow fertile seas. It is chemically mainly calcium carbonate and is a useful material that is quarried and used directly as a building material. It reacts with acids - 'fizzing' due to carbon dioxide formation - test with 'limewater' - milky white precipitate.

  • Marble is also made of calcium carbonate and is a metamorphic rock formed by the action of heat and pressure on limestone in the Earth's crust. It is a much harder rock than limestone and is used to make highly polished and finely carved stone sculptures, statues etc.

• Chemically, limestone mainly consists of calcium carbonate, CaCO₃, and is a valuable natural mineral resource, quarried in large quantities in many countries (see environmental impact at the end of the metal extraction web page).

• Other uses of limestone rock are outlined below and it is an important raw material in the manufacture of cement and glass and iron.

• Powdered limestone can be used to neutralise acidity in lakes and soils. (neutralisation chemistry). Like lime, it is a safe agri-chemical to use on the land and does produce the controversial side effects of artificial fertilisers, herbicides and pesticides etc.

• When limestone is heated in a kiln at over 900^oC, it breaks down into quicklime (calcium oxide) and carbon dioxide. Both are useful products. This type of reaction is endothermic (heat absorbing) and an example of thermal decomposition (and other carbonates behave in a similar way). 

  • calcium carbonate (limestone) ==> calcium oxide (quicklime) + carbon dioxide

  • CaCO_3(s) CaO_(s) + CO_2(g)

  • This is a reversible endothermic reaction. To ensure the change is to favour the right hand side, a high temperature of over 900^oC is needed as well as the continual removal of the carbon dioxide.

  • The high temperature needed is produced by mixing the limestone with coal/coke (a fuel of mainly carbon) and blowing hot air into the ignited mixture in a rotating kiln for a continuous production line (raw materials in at one end, lime out the other!)  ....

    • C_(s) + O_2(g) ==> CO_2(g) is very exothermic - heat releasing!

  • Note on heating other carbonates - more thermal decomposition.

    • These also show a similar thermal decomposition to calcium e.g.

    • copper(II) carbonate_{(s, green)}  ==> copper(II) oxide_{(s, black)}  + carbon dioxide

    • CuCO_3(s) ==> CuO_(s) + CO_2(g)

    • zinc carbonate_{(s, white)}  ==> zinc oxide_{(s, yellow hot, white cold)}  + carbon dioxide

    • ZnCO_3(s) ==> ZnO_(s) + CO_2(g)

    • Zinc carbonate occurs as the mineral ores calamine/Smithsonite and the resulting zinc oxide can be used to extract zinc metal and zinc oxide itself is used as a whitening agent' in cosmetics and in 'calamine lotion' a mild antiseptic and antipruritic (anti-itching agent) for treating skin irritations.

  • FeCO₃ and MnCO₃ behave in a similar way (so just swap Zn/Ca/Cu with Fe or Mn)

  • Sodium hydrogen carbonate is used in baking powder because on heating it thermally decomposes releasing carbon dioxide gas that gives the 'rising' action in baking.

    • sodium hydrogencarbonate ==> sodium carbonate + water + carbon dioxide

    • 2NaHCO_3(s) ==> Na₂CO_3(s) + H₂O_(l/g) + CO_2(g)

    • This is just one of many chemical process that occur when food is cooked.

• Quicklime reacts very exothermically with water to produce slaked lime (solid calcium hydroxide). 

  • calcium oxide (quicklime) + water ==> calcium hydroxide (slaked lime)

    • this is a very exothermic reaction, the quicklime 'puffs' up and steam is produced!

    • CaO_(s) + H₂O_(l) ==> Ca(OH)_2(s) 

    • with excess water followed by filtration you get calcium hydroxide solution or limewater.

• Lime (calcium oxide) and slaked lime (calcium hydroxide) are both used to reduce the acidity of soil on land, they are both faster and stronger acting than limestone powder. They are also used to reduce acidity in lakes and rivers due to acid rain. They are also used to neutralise potentially harmful industrial acid waste including sulphur dioxide in the flue gases of power stations.

• In the test for carbon dioxide, calcium hydroxide solution (limewater) forms a white milky precipitate of calcium carbonate (back to where you started!). 

  • calcium hydroxide  + carbon dioxide ==> calcium carbonate + water

  •  Ca(OH)_2(aq) + CO_2(g) ==> CaCO_3(s) + H₂O_(l)

• Formulae of magnesium and calcium compounds (M = metal = Mg or Ca, same group 2, same formula!)

  • IONS: The metal ion in aqueous solution or solid compounds is M²⁺, which combines with other ions such as: oxide O^2-, hydroxide OH^-, carbonate CO₃^2-, hydrogencarbonate HCO₃^-, chloride Cl^-, sulphate SO₄^2-, nitrate NO₃^- to form the calcium or magnesium compounds.

  • COMPOUND FORMULAE: oxide MO, hydroxide M(OH)₂, carbonate MCO₃, hydrogencarbonate M(HCO₃)₂, chloride MCl₂, sulphate MSO₄, nitrate M(NO₃)₂

• The oxides and hydroxides readily react with acids.

  • general word equation: oxide or hydroxide  +  acid ==>  salt  +  water

    • examples ...

    • calcium oxide + hydrochloric acid ==> calcium chloride + water

    • magnesium hydroxide + nitric acid ==> magnesium nitrate + water

    • calcium hydroxide + sulphuric acid ==> calcium sulphate + water

  • since hydrochloric acid gives a chloride salt, nitric acid  gives a nitrate salt, sulphuric acid a sulphate salt ... the symbol equations are ... where M = Mg or Ca (or any other Group 2 metal)

    • MO_(s) + 2HCl_(aq) ==> MCl_2(aq) + H₂O_(l)

    • MO_(s) + 2HNO_3(aq) ==> M(NO₃)_2(aq) + H₂O_(l)

    • MO_(s) + H₂SO_4(aq) ==> MSO_4(aq/s*)  + H₂O_(l)

    • if M(OH)₂ involved, there is a 2H₂O at the end NOT a single H₂O to balance the symbol equation

    • M(OH)_2(s) + 2HCl_(aq) ==> MCl_2(aq) + 2H₂O_(l)

    • M(OH)_2(s) + 2HNO_3(aq) ==> M(NO₃)_2(aq) + 2H₂O_(l)

    • M(OH)_2(s) + H₂SO_4(aq) ==> MSO_4(aq/s*)  + 2H₂O_(l)

    • * the sulphates of e.g. calcium and barium are not very soluble and this slows the reaction down!

• Solubility of calcium compounds (and the chemically similar magnesium):

  • Magnesium and calcium oxides or hydroxides are slightly soluble in water forming alkaline solutions. They readily react and dissolve in most acids (see above).

  • Magnesium and calcium carbonate are insoluble in water but readily dissolve in most dilute acids like hydrochloric, nitric and sulphuric.

  • Equation examples for calcium carbonate (similar for magnesium carbonate) ...

  • calcium carbonate + hydrochloric acid ==> calcium chloride + water + carbon dioxide

    • CaCO_3(s) + 2HCl_(aq) ==> CaCl_2(aq) + H₂O_(l) + CO_2(g)

  • calcium carbonate + nitric acid ==> calcium nitrate + water + carbon dioxide

    • CaCO_3(s) + 2HNO_3(aq) ==> Ca(NO₃)_2(aq) + H₂O_(l) + CO_2(g)

  • calcium carbonate + sulphuric acid ==> calcium sulphate + water + carbon dioxide

    • CaCO_3(s) + H₂SO_4(aq) ==> CaSO_4(aq,s)  + H₂O_(l) + CO_2(g)

    • Calcium carbonate reacts slowly in dilute sulphuric acid because calcium sulphate is not very soluble and coats the limestone.

• Magnesium and calcium hydrogencarbonate are soluble in water and cause 'hardness' i.e. scum with 'traditional' non-detergent soaps. Formulae are Mg(HCO₃)₂ and Ca(HCO₃)₂

• Cement is produced by roasting a mixture of powdered limestone with powdered clay* in a rotary kiln. When cement is mixed with water, sand and crushed rock, a slow chemical reaction produces a hard, stone-like building material called concrete.

  • * Clay is also used directly to make pottery and other ceramics

• Glass is made by heating together a mixture of limestone (CaCO₃), sand (mainly silica = silicon dioxide = SiO₂) and 'soda' (sodium carbonate, Na₂CO₃).

• Limestone is used to remove acidic oxide impurities in the extraction of iron and in making steel.

• Calcium oxide and calcium hydroxide also react with acids to form salts. You will find details of this kind of reaction on the Acids and Bases page.

• Limestone and hard/soft water are covered on the Extra Aqueous Chemistry page.

• multiple choice test on limestone etc.

• Decomposition of carbonates: see above.

• Decomposition of metal hydroxides:

  • The Group 1 Alkali Metal hydroxides do not readily decompose on heating even 'up to red heat'.

    • Except for lithium hydroxide which forms lithium oxide and water.

      • 2LiOH_(s) ==> Li₂O_(s) + H₂O_(l) 

    • These hydroxides are white solids and soluble in water to give an alkaline solution.

  • On heating the Group II, Lead, Aluminium and Transition Metal hydroxides decompose to form the metal oxide and water vapour.

    • The original hydroxides are usually relatively insoluble solids, white in colour, except copper(II) hydroxide is blue and iron(III) hydroxide is brown.

  • M(OH)_2(s) ==> MO_(s) + H₂O_(l) 

    • M = Mg, Ca, Zn giving white oxide MO (ZnO yellow when hot),

    • M = Cu gives black copper(II) oxide CuO, M = Pb gives yellow lead(II) oxide PbO

    • e.g. if M = Zn: zinc hydroxide ==> zinc oxide + water

  • and 2M(OH)_3(s) ==> M₂O_3(s) + 3H₂O_(l) 

    • where M = Al to give white aluminium oxide, and M = Fe to give reddish-brown iron(III) oxide.

• Decomposition of nitrate salts:

  • The Group 1 Alkali Metal nitrates (NO₃) decompose to form the nitrite (NO₂) salt and oxygen gas.

    • 2MNO_3(s) ==> 2MNO_2(s) + O_2(g) where M = Na or K

    • so when M = Na: sodium nitrate ==> sodium nitrite + oxygen

    • Nitrates are colourless crystals and nitrites are white solids and are all soluble in water giving neutral solutions.

  • The Group II, Lead, Aluminium and Transition Metal nitrates decompose to form the metal oxide, nasty brown nitrogen dioxide gas and oxygen gas when strongly heated.

    • These are all water soluble neutral salts, all colourless crystals except Cu is blue and Fe is pale brown.

    • 2M(NO₃)_2(s) ==> 2MO_(s) + 4NO_2(g) + O_2(g) 

      • where M = Mg, Ca, Zn giving white oxide MO (ZnO yellow when hot),

      • when M = Cu, it gives black copper(II) oxide  CuO, if M = Pb it gives yellow lead(II) PbO.

    • 4M(NO₃)_3(s) ==> 2M₂O_3(s) + 12NO_2(g) + 3O_2(g) 

      • If M = Al, it gives white aluminium oxide, if M = Fe it gives give reddish-brown iron(II) oxide.

  • See gas preparation and collection page for methods.

Aspects of the vitamin, food and drugs GCSE chemistry are on the "Extra Organic Chemistry" page.

Living cells use chemical reactions to produce new materials. Living things produce catalysts called enzymes which allow chemical reactions to occur quite quickly at ordinary temperatures and pressures. Enzymes are powerful 'biochemical catalysts' and are widely used in the food industry and are being used more and more to manufacture many other chemicals. These biological catalysts promote most of the reactions in living tissue. The names of enzymes end in ...ase e.g. amylase, protease, invertase, isomerase etc.

• Cells contain protein molecules that act as biological or biochemical catalysts, they are known as ENZYMES.

• The chemical reactions brought about by living cells are quite fast in conditions that are warm rather than hot.

• This is because the cells use these enzyme catalysts. The 'key and lock' mechanism is explained later on. 

• Enzymes are protein molecules which are usually damaged by temperatures above about 45º C. Although not damaged by lower temperatures, the reactions may be too slow to be of any use. (see rates notes at the end of this section)

• Different enzymes work best at different pH values.

• The enzymes in yeast cells (living organism's) convert sugar into ethanol ('alcohol') and carbon dioxide in the brewing and drinks industry. A similar process is used to convert sugar cane into ethanol and distilled to use as biofuel.

  • e.g. glucose ==> ethanol and carbon dioxide in water and the absence of air.

  • C₆H₁₂O_6(aq) ==> 2C₂H₅OH_(aq) + CO_2(g)

  • This process occurs efficiently between 25 to 55^oC and is called fermentation and is used to produce the alcohol in beer and wine. The carbon dioxide dissolved in the final alcoholic drink produces the fizz!

  • Note on raising agents in cooking: It is this reaction producing bubbles of carbon dioxide which make dough mixtures rise in the kitchen or food industry when yeast is used in baking bread or cake making etc.

    • An alternative to yeast is to use sodium hydrogencarbonate ('sodium bicarbonate' or 'baking soda') in baking. The rising action is also due to carbon dioxide gas formed from its reaction with an acid (e.g. tartaric acid), and nothing to do with enzymes:

      • self-raising baking powder = carbonate base + a solid organic acid, giving

      • sodium hydrogencarbonate + acid ==> sodium salt of acid + water + carbon dioxide

  • A simple laboratory test for carbon dioxide is that it forms a milky precipitate with limewater.

  • However other enzymes in living material can also catalyse oxidation with the oxygen in air. When alcoholic drinks turn sour it is due to the alcohol being oxidised to the weak organic acid ethanoic acid, commonly know as 'vinegar'!

• Enzymes are involved in the following processes in the home

  • bread dough raising (see above)

  • biological detergents may contain protein-digesting protease enzymes and fat-digesting enzymes lipase enzymes.

• In industry, enzymes are used to bring about reactions at normal temperatures and pressures that would otherwise require more expensive and more energy demanding equipment e.g.

  • Proteases break down proteins and are used to 'pre-digest' the protein in some baby foods.

  • Carbohydrases are used to convert starch syrup into sugar syrup.

  • Invertase is used to make the sugar for soft chocolates.

  • Isomerase* is used to convert glucose syrup into fructose syrup, which is much sweeter and therefore can be used in smaller quantities e.g. in slimming foods.

    • (* The name comes from the word 'isomers' which means molecules of the same molecular formula but different structures. Glucose and fructose both have the molecular formula C₆H₁₂O₆).

  • Pectinase breaks down insoluble pectin polysaccharides and so is used in clarify fruit juices.

  • Amylases break down carbohydrates and Lipases break down fats.

  • Enzymes are used in genetic engineering and penicillin production.

  • The dairy industry uses enzymes made by microorganisms (bacteria) to produce yoghurt and cheese from milk.

    • The bacteria enzymes convert the sugar in milk (lactose) into lactic acid.

  • Enzymes in biological detergents help break down staining food materials.

• Successful industrial processes often depend on the immobilization of the enzyme:

  • Methods of immobilisation:

    • It can be trapped in a silica gel lattice or collagen matrix or cellulose fibres.

    • Encapsulating in beads of alginate or polymer microspheres can also be used immobilize enzymes.

  • Advantages of immobilisation:

    • Stabilising the enzyme keeps it functioning for a longer period because it can be easily recovered for further use.

    • To immobilise the enzyme allows a continuous process, this means a continuous input of raw materials and output of product, so can run 24 hours a day for many weeks or months efficiently. A batch process means loading the reactor vessel with reactants, extract the products, clean out the reactor/fermentor, re-load with reactants etc. etc. i.e. less efficient, costs time and so is less economic!

    • There is less contamination of the product with the enzyme because of ease of separation

• KEY in sequence: E = free enzyme, S = free substrate reactant molecule, E-S = enzyme-reactant complex, E-P = enzyme-product complex, E = free enzyme, P = free product

• The enzyme is a complex protein molecule, but there is a particular site where the reactant molecule 'docks in' by random collision. The enzyme is sometimes referred to as the 'lock' and the initial reactant substrate molecule as the 'key', hence this is called the 'key and lock' mechanism. This is also explains why enzymes are very specific - you need the right molecular key for a particular molecular lock.

• Once the 'reactant-enzyme complex' is formed the enzyme function changes the reactant molecule into the new product molecule.

• The 'enzyme-new molecule complex' breaks down to free the new product molecule and the enzyme who's reactive site can now be re-used by another reactant molecule. 

  • Note 1. Compared to the un-catalysed reaction, the enzyme provides a 'chemical change route' with a much lower activation energy, and so this greatly increases the rate of reaction as more molecules have enough kinetic energy to react at the same temperature.

  • Note 2. The products are shown as two molecules, because there are quite often two products for each step of the breakdown of a bigger molecule into smaller molecules e.g. protein to 'smaller protein' + amino acid, or starch to 'smaller starch' plus a glucose molecule etc.  But there can be just one product molecule e.g. when isomerase changes glucose into fructose. There can also be two substrate reactant molecules being combined to form a bigger molecule or a long natural polymer molecule like starch being broken down to small sugar molecules. In other words there are lots of possibilities!

  • Note 3. Many drugs work by blocking the sites normally used by enzymes. The molecular key (the drug) goes onto the reactive enzyme site, but stays there, so inhibiting enzyme activity which promotes harmful chemical-organism effects in the body. The harmful effect might be the production of toxic chemicals from a bacteria or the reproduction of a harmful organism etc.

  • Note 4. "Rates of Reaction Notes" fully explains all the factors affecting the rate of any chemical reaction, including explaining experimental methods and reaction profile diagrams and activation energy.

  • Note 5. Different reactions need different enzymes, and also if enzymes, which bring about the same chemical change, are quite likely to have different optimum rate pH's or temperatures. this phenomena is known as the specificity of enzymes is related to the unique structure of each enzyme and its 'reactivity' limited to interaction with particular substrate molecules.

Extra Advanced chemistry notes on Enzyme structure on the stereochemistry page

Because sulfuric acid has so many uses the industrial development of a country is sometimes measured by the amount of sulphuric acid that is used each year. Sulphuric acid is made starting from the element sulphur which is found in the Earth's crust.

• Sulphuric acid is used as car battery acid and is used to make fertilisers,  dyes and detergents.

  • e.g. ammonia + sulphuric acid ==> ammonium sulphate (a fertiliser salt)

  • 2NH_3(aq) + H₂SO_4(aq) ==> (NH₄)₂SO_4(aq) => evaporation to get crystals

  • Lots more equations on the Acids, Bases, pH and Salts page.

  • Its acid action make it good for cleaning metal surfaces in industry.

• Sulphuric acid is manufactured from the raw materials sulphur, air and water and involves the production of sulphur trioxide in the Contact Process.

• (1) Sulfur is burned in air to form sulphur dioxide (exothermic).

  • In the reaction the sulphur is oxidised (O gain)  (1a) S_(s) + O_2(g) ==> SO_2(g)

  • Sulfur dioxide can also be indirectly obtained from the process of extracting copper from copper sulphide ores e.g. in a copper smelter: (1b) Cu₂S_(s) + O_2(g) ==> 2Cu_(l) + SO_2(g)

• Note: Sulphur dioxide itself is a useful chemical in its own right:

  • It is used as a bleach in the manufacture of wood pulp for paper manufacture

  • and its toxic nature makes it useful as a food preservative by killing bacteria.

• (2) In the reactor, the sulphur dioxide is mixed with air (to give the required SO₂:O₂ 2:1 ratio) and the mixture passed over a catalyst of vanadium(V) oxide V₂0₅ (vanadium pentoxide) at a high temperature (about 450°C) and at a pressure of between one and two atmospheres. It is a 2nd exothermic oxidation and is known as the Contact Process.

• In the reactor the sulphur dioxide is oxidised in the reversible exothermic reaction ...

  •   (2) 2SO_2(g) + O_2(g) 2SO_3(g)

• The reaction forms sulphur trioxide and the equilibrium is very much to the right hand side ...

  • So, despite the reaction being exothermic (95 kJ released per mole of SO₃), a relatively high temperature is used to ensure a reasonable rate of reaction (despite the fact that it favours reverse reaction R to L, from the energy change equilibrium rule, inc. T. favours endothermic direction).

  • The reaction is favoured by high pressure (pressure equilibrium rule, 3 => 2 gas molecules, LHS ==> RHS), but only a small increase in pressure is used to give high yields of sulphur trioxide, because the formation of SO₃ on the right hand side is so energetically favourable (approx. 99% yield, i.e. only about 1% SO₂ unreacted).

  • The use of the V₂O₅ catalyst ensures a fast reaction without having to use too a higher temperature which would favour the left hand side and reduce the yield BUT it does not change the % of sulphur trioxide formed, you simply get there faster.

  • More GCSE notes on reversible reactions and equilibrium rules. Advanced Level notes on ...

• (3) The sulphur trioxide is dissolved in concentrated sulphuric acid to form fuming sulphuric acid (oleum).

  • SO_3(g) + H₂SO_4(l) ==> H₂S₂O_7(l)

• (4) Water is then carefully added to the oleum to produce concentrated sulphuric acid (98%).

  • H₂S₂O_7(l) + H₂O_(l) ==>  2H₂SO_4(l)

  • If the sulphur trioxide is added directly to water an acid mist forms which is difficult to contain because the reaction to form sulphuric acid solution is very exothermic!

  • If you 'add' equations (3) + (4) you get

    • (5) SO_3(g) + H₂O_(l) ==>  H₂SO_4(l)

    • which is how it is usually written in GCSE textbooks, so learn equations (1a), (2) and (5) for the manufacture of sulfuric acid from sulfur.

• Good anti-pollution measures need to be in place since the sulphur oxides are harmful and would cause local acid rain! To help this situation AND help the economics of the process, any unreacted sulphur dioxide is recycled through the reactor.

• Concentrated sulphuric acid can be used in the laboratory as a dehydrating agent.

  • Dehydration is the removal of water or the elements of water from a compound and can be described as an elimination reaction. Usually and adjacent H and OH in a molecule are removed to form the water.

  • When added to some organic compounds containing hydrogen and oxygen, e.g. sugar, concentrated sulphuric acid removes the elements of water from the compound leaving a 'spongy' black carbon residue.

  • If alcohols are heated with conc. sulphuric acid, they are dehydrated to alkenes.

    • e.g. ethanol ==> ethene + water

    • CH₃CH₂OH ==> CH₂=CH₂ + H₂O

  • When added to blue copper sulphate crystals concentrated sulphuric acid removes the water of crystallisation leaving white anhydrous copper sulphate. In this case the water already exists BUT not in a mixture and so the following reaction is classified as a chemical change.

    • CuSO₄.5H₂O_(s) CuSO_4(s) + 5H₂O_{(absorbed into the H2SO4 which it reacts with)}

  • Conc. H₂SO₄ catalyses the reaction between an alcohol and carboxylic acid to form an pleasant smelling ester liquid but it isn't considered a dehydration reaction (H comes from one molecule and OH from the other).

    • e.g. the esterification ethanoic acid + ethanol ==> ethyl ethanoate + water

    • CH₃COOH + CH₃CH₂OH ==> CH₃COOCH₂CH₃ + H₂O

  • Concentrated sulphuric acid can be used as a drying agent e.g. in the preparation of gases.

    • The prepared gas is bubbled through a dreschel/dreschler bottle (illustrated on the right), containing the concentrated sulphuric acid. In this case the water vapour is just a component in a gaseous mixture. Most gases can be dried in this way except the alkaline gas ammonia which will exothermically react to form a solid salt. In this case the water vapour is just a component in a gaseous mixture.

• *

 Extraction details page: Aluminium can be made more resistant to corrosion by a process called anodising. Iron can be made more useful by mixing it with other substances to make various types of steel. Many metals can be given a coating of a different metal to protect them or to improve their appearance.

• Aluminium is a reactive metal but it is resistant to corrosion. This is because aluminium reacts in air to form a layer of aluminium oxide which then protects the aluminium from further attack.

  • This is why it appears to be less reactive than its position in the reactivity series of metals would predict.

• For some uses of aluminium it is desirable to increase artificially the thickness of the protective oxide layer in a process is called anodising.

  • This involves removing the oxide layer by treating the aluminium sheet with sodium hydroxide solution.

  • The aluminium is then placed in dilute sulphuric acid and is made the positive electrode (anode) used in the electrolysis of the acid.

  • Oxygen forms on the surface of the aluminium and reacts with the aluminium metal to form a thicker protective oxide layer. 

• Aluminium can be alloyed to make 'Duralumin' by adding copper (and smaller amounts of magnesium, silicon and iron), to make a stronger alloy used in aircraft components (low density = 'lighter'!), greenhouse and window frames (good anti-corrosion properties), overhead power lines (quite a good conductor and 'light'), but steel strands are included to make the 'line' stronger and poorly electrical conducting ceramic materials are used to insulate the wires from the pylons and the ground.

  • There is a note about the bonding and structure of alloys on another page.

• The properties of iron can be altered by adding small quantities of other metals or carbon to make steel. Steels are alloys since they are mixtures of iron with other metals or with non-metals like carbon or silicon.

• Making Steel:

  • (1) Molten iron from the blast furnace is mixed with recycled scrap iron

  • (2) Then pure oxygen is passed into the mixture and the non-metal impurities such as silicon or phosphorus are then converted into acidic oxides (oxidation process) ..

    • e.g. Si + O₂ ==> SiO₂, or 4P + 5O₂ ==> P₄O₁₀

  • (3) Calcium carbonate (a base) is then added to remove the acidic oxide impurities (in an acid-base reaction). The salts produced by this reaction form a slag which can be tapped off separately.

    • e.g. CaCO₃ + SiO₂ ==> CaSiO₃ + CO₂ (calcium silicate slag)

  • Reactions (1)-(3) produce pure iron.

  • Calculated quantities of carbon and/or other metallic elements such as titanium, manganese or chromium are then added to make a wide range of steels with particular properties.

  • Because of the high temperatures the mixture is stirred by bubbling in unreactive argon gas!

  • Economics of recycling scrap steel or ion: Most steel consists of >25% recycled iron/steel and you do have the 'scrap' collection costs and problems with varying steel composition* BUT you save enormously because there is no mining cost or overseas transport costs AND less junk lying around! (NOTE: * some companies send their own scrap to be mixed with the next batch of 'specialised' steel they order, this saves both companies money!)

• Different steels for different uses:

  • High % carbon steel is strong but brittle.

  • Low carbon steel or mild steel is softer and is easily shaped and pressed e.g. into a motor car body.

  • Stainless steel alloys contain chromium and nickel and are tougher and more resistant to corrosion.

  • Very strong steels can be made by alloying the iron with titanium or manganese metal.

• There is a note about the bonding and structure of alloys on another page.

• Steel can be galvanised by coating in zinc, this is physically done by dipping the object into a bath of molten zinc. On removal and cooling a thin layer of zinc is left on. The zinc chemically bonds to the iron via the free electrons of both metals - its all the same atoms to them! It can also be done by electroplating (details below).

• Steel (and most metals) can be electroplated.

  • The steel object to be plated is made the negative electrode (cathode) and placed in a solution containing ions of the plating metal.

  • The positive electrode (anode) is made of the pure plating metal (which dissolves and forms the fresh deposit on the negative electrode).

  • Nickel, zinc, copper, silver and gold are examples of plating metals.

  • The details of copper purification amount to copper plating, so all you have to do is swap the pure negative copper cathode with the metal you want to coat (e.g. Ni, Ag or Au or any material with a conducting surface). Swap the impure positive copper anode with a pure block of the metal you want to form the coating layer. The electrodes dip into a salt solution of nickel, zinc, copper, silver or gold ions etc. and a low d.c. voltage passed through. If M = Ni, Cu, Zn ....

    • At the positive (+) anode, the process is an oxidation, electron loss, as the metal atoms dissolve to form metal(II) ions.

    • M_(s) ==> M²⁺_(aq) + 2e^-

    • at the negative (-) cathode, the process is a reduction, two electron gain by the attracted metal(II) ions to form neutral metal atoms on the surface of the metal being coated.

    • M²⁺_(aq) + 2e^- ==> M_(s)

    • For silver plating it is Ag⁺, Ag and a single electron change.

    • Any conducting (usually metal) object can be electroplated with copper or silver for aesthetic reasons or steel with zinc or chromium as anti-corrosion protective layer.

• Many other metals have countless uses e.g. zinc

  • Zinc is used to make the outer casing of zinc-carbon-weak acid batteries.

  • Zinc is alloyed with copper to make the useful metal brass (electrical plug pins). Brass alloy is stronger and more hardwearing than copper AND not as brittle as zinc.

• Titanium manufacture and uses are described below in section 5. below

• More GCSE notes on metals:

  • Metal Extraction and Metal Reactivity

  • Transition Metals - their physical and chemical properties and uses and mention of uses of other metals.

  • Uses of sodium are mentioned on the Group 1 Alkali Metals page.

5. What is titanium and how is it produced ?

Titanium is a very important metal for various specialised uses. It is more difficult  to extract from its ore than other, more common metals.

• Titanium is a transition metal and is strong and resistant to corrosion.

  • Titanium alloys are amongst the strongest of metal alloys.

  • There is a note about the bonding and structure of alloys on another page.

  • It is used in aeroplanes, in nuclear reactor alloys and for replacement hip joints.

  • It is one of the main components of Nitinol 'smart' alloys. Nitinol belongs to a group of shape memory alloys (SMA) which can 'remember their original shape'. For example they can regain there original shape on heating (e.g. used in thermostats in cookers , coffer makers etc.) or after release of a physical stress (e.g. used in 'bendable' eyeglass frames, very handy if you tread on them!). The other main metal used in these very useful intermetallic compounds is nickel.

    • Nitinol is an acronym for 'Nickel Titanium Naval Ordinance Laboratory' betraying, like so many technological developments, its military origins, but now acquiring many 'peaceful' uses.

• Titanium is extracted from the raw material is the ore rutile which contains titanium dioxide.

• The rutile titanium oxide ore is heated with carbon and chlorine to make titanium chloride

  • TiO₂ + 2Cl₂ + C ==> TiCl₄ + CO₂

• After the oxide is converted into titanium chloride TiCl₄, it is then reacted with sodium or magnesium to form titanium metal and sodium chloride or magnesium Chloride. This is an expensive process because sodium or magnesium are manufactured by the costly process of  electrolysis (electricity is the most costly form of energy).

  • This reaction is carried out in an atmosphere of inert argon gas so none of the metals involved becomes oxidised by atmospheric oxygen.

  • TiCl₄ + 2Mg ==> Ti + 2MgCl₂ or TiCl₄ + 4Na ==> Ti + 4NaCl

  • These are examples of metal displacement reactions e.g. the less reactive titanium is displaced by the more reactive sodium or magnesium.

  • Overall the titanium oxide ore is reduced to titanium metal (overall O loss, oxide => metal)

6. Why are instrumental methods of detection so useful?

Typical chemical tests are on a separate web page  *  Mass spectrometry

Instead of testing for chemicals using standard laboratory equipment such as test tubes etc. Special instruments have been developed to carry out such testing. These are quick, accurate and can be used on very small samples.

• Elements and compounds can also be detected and identified using a variety of instrumental methods. Some instrumental methods are suited to identify elements while other instrumental methods are suited to the identification of compounds.

• Instrumental methods are accurate, sensitive and rapid and are particularly useful when the amount of a sample is very small.

• Mass spectroscopy can be used to identify elements and their relative ratio of isotopes and for molecules it can help to determine a molecular structure (its expensive, and nmr is much better for molecular structure analysis - especially organic molecules, see below).

  • The atoms or molecules are vapourised and converted to positive ions (based on a single atom or molecular fragment)  by bombardment with high energy electrons. The gaseous ions (e.g. Na⁺ or CH₃⁺ etc.) are analysed according to their mass in a powerful magnetic field. 

• Atomic emission spectroscopy can be used to identify elements and analyse element mixtures. 

  • Basically atomic spectroscopy is about 'exciting atoms' with heat or electrical energy until they emit the absorbed energy as visible light. You see this effect when fireworks go off, most of the colour comes from the 'excited' metal atoms in the salts added to the explosive powder mixture.

  • In a simple way flame colour tests in the school laboratory are used to identify elements e.g. sodium is yellow, barium green etc. BUT these colours are formed from many specific frequencies of visible light added together, so how do you sort out e.g. two shades of greens from copper or barium?

  • The answer is that detailed analysis of the different emitted frequencies of visible light (e.g. using a prism) gives a 'finger print pattern' by which to identify elements.

  • AND the greater the relative intensity of light frequency the more there is of that element.

  • So atomic spectroscopy is used to identify elements and analyse a mixture of elements or detect traces of elements in a solid or solution.

  • This analytical method has many applications e.g.

    • Its used in the steel industry to monitor the composition of steel as the molten mixtures are being made

    • Astrophysicists can identify elements in distant stars from the light emitted.

    • Tiny traces of metal ions can be detected in water e.g. for pollution monitoring.

• Nuclear magnetic resonance spectroscopy (nmr) is one of the most powerful analytical tools for determining the molecular structure of an organic compound.

  • Its very expensive for routine analysis but is invaluable in designing and analysing new molecules or finding the structure of natural molecules that the drug industry might find useful in developing new pharmaceutical products.

• Infra-red spectroscopy can help to determine molecular structure and identify an organic compound.

  • Each molecule has a 'fingerprint' pattern of absorption of different infrared frequencies. Can be used to determine alcohol concentrations in breath!

• Ultra-violet spectroscopy can be used to the determine purity or concentration of solution of a substance that absorbs uv light.

• Gas-liquid chromatography (gc/glc) can be used to analyse liquid mixtures which can be vapourised (e.g. petrol, blood for alcohol content). The instrument is called a gas chromatograph.

  • 

  • A sample of the substance under investigation is injected and vapourised into a tube containing a carrier gas (called the mobile phase, it moves). The gas carries the vaporised substance through a long 'separating' tube or column wound around inside a thermostated oven.

  • The substances in the mixture are partially absorbed by an absorbent material held in the or column (called the immobile phase, doesn't move), but only temporarily. However different substances are held back, or 'retained', for different times so that the mixture separates out in the carrier gas stream.

    • The column is filled with a porous solid so gas can get through but passes over a large surface area OR it is coated in a very high boiling organic liquid which can also provide a large absorbing surface but still allows gas flow.

  • The gases emerge from the oven into a detector system which electronically records the different signal as each substance comes through. A printout or computer display of the results from the gas chromatograph, called the gas chromatogram, shows a series of peaks in the graph line imposed on a steady baseline when only the carrier gas is passing through the detector.

  • The time it takes for a substance to come through is called the retention time and is unique for each substance for a particular set of conditions (flow rate, length of separating column, nature of separating column material, temperature etc.). Generally speaking, the greater the molecular mass of the mixture molecule, the longer the retention time. This is because the component molecule - immobile phase intermolecular force of attraction increases with the size of the component molecule, so it is absorbed/retained temporarily a bit more strongly (see right of diagram).

  • The height of the peak, or more strictly speaking, the area under the peak, is proportional to the amount of that particular substance in the mixture.

  • Therefore it is possible to identify components in a mixture and calculate their relative proportions in the mixture.

  • The chromatogram shown above (right of diagram) illustrates the separation of some alkane hydrocarbons in petrol (in reality it is far more complicated with dozens of hydrocarbon molecule peaks on the chromatogram). The different peak heights give the relative proportions i.e. hexane >pentane > heptane. The retention time order follows the trend of increasing molecular mass gives increasing retention time i.e. in time heptane C₇H₁₆ > C₆H₁₄ > C₅H₁₂

  • The gas chromatographic instrument can be calibrated with known amounts of known substances.

  • Don't confuse with 'non-instrumental' paper/thin layer chromatography.

• Industry requires rapid and accurate methods for the analysis of its products. There have also been increasing demands from society for safe and reliable monitoring of our health and environment. The development of modem instrumental methods has been aided by the rapid progress in technologies such as electronics and computing.

• Various factors have influenced the development of instrumental methods. With modern methods you get ...

  • greater sensitivity i.e. smaller amounts of material can be used OR much smaller amounts of a trace element or compound can be detected in a bulk mixture (drug testing of athletes)

  • more accurate data (perhaps analysed by computer)

  • automation of analysis, multi-samples efficiently analysed

  • a greater range of analytical techniques, today's laboratory is far more versatile these days

  • greater reliability and consistency once the instrument is set up and procedures in place and checked.