Ch_14_summary - CHAPTER 14 CHEMISTRY IN INDUSTRY AND...

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Unformatted text preview: CHAPTER 14 CHEMISTRY IN INDUSTRY AND TECHNOLOGY (IB OPTION C) SUMMARY Iron and steel The principal ores of iron are Haematite (Fe2O3) and Magnetite (Fe3O4) In the blast furnace the solid ore, coke (C) and limestone (CaCO3) are fed in at the top and air blown in at the bottom. Starting at the bottom the reactions producing the iron are: C + O2 ⇒ CO2 C + CO2 ⇒ 2 CO FexOy + y CO ⇒ x Fe + y CO2 The limestone is added is to react with sand (SiO2), inevitably a major impurity: CaCO3 ⇒ CaO + CO2 CaO + SiO2 ⇒ CaSiO3 The iron produced in this way contains a great deal of dissolved carbon (~4%), as well as other impurities, which make it very brittle. To remove these pure oxygen is blown through the molten liquid to convert these to the oxides. The volatile ones (such as CO2) escape as gases, the non-volatile ones (such as SiO2) react with the calcium oxide which is also added. The properties of the steel may be modified either by adding other metals or non-metals to form alloys with the iron, or by heat treating it, which affects the crystal structure of the iron. Three common heat treatment processes are: • Annealing, in which the metal is allowed to cool slowly to produce a soft malleable steel • Quenching, in which very hot metal is rapidly cooled so that the high-temperature crystal structure is retained, giving a hard, brittle steel • Tempering, in which the quenched steel is reheated to achieve a hardness intermediate between that achieved by annealing and quenching. Some common steels, along with their composition and uses are: Aluminium Aluminium is obtained from Bauxite – impure, hydrated aluminium oxide. The aluminium oxide is separated from Fe2O3, the main impurity, by dissolving the amphoteric Al2O3 in concentrated sodium hydroxide, filtering off the Fe2O3, then acidifying the mixture to precipitate out pure alumina, Al2O3. Because it is too high in the reactivity series for chemical reduction, aluminium is obtained from alumina by electrolysis: • The Al2O3 is dissolved in molten cryolite (Na3AlF6) which reduces the melting point and improves the conductivity, at about 900°C • Aluminium is formed at the steel lining cathode and the molten metal sinks to the bottom of the cell. • Al3+(l) + 3 e– → Al(l) • At the carbon anode, oxygen, from the oxidation of the oxide ions, reacts with the carbon: o 2 O2–(l) → O2(g) + 4 e– o C(s) + O2 (g) → CO2 (g) • The carbon anodes therefore burn away and require periodic replacement. © IBID Press 2007 1 CHAPTER 14 CHEMISTRY IN INDUSTRY AND TECHNOLOGY (IB OPTION C) SUMMARY Aluminium has a very low density and, owing to a thin impervious layer of its oxide, it is corrosion resistant. Though quite soft it can be made harder by alloying with other metals such as magnesium. The production of both steel and aluminium consume large amounts of energy and use great volumes of water. They both generate CO2, a greenhouse gas, and produce solid waste (slag and Fe2O3 respectively) which requires disposal. In both cases recycling waste metal can greatly reduce the environmental impact. Petrochemicals Currently about 90% of oil is used as a fuel and about 10% for petrochemicals. It is probably easier and, because of the production of greenhouse gases, such as CO2, better for the environment to search for new energy sources to conserve more oil for petrochemical production. Owing to the low reactivity of alkanes, cracking to form alkenes is an important first step in petrochemical production. They all involve heating fractions from the fractional distillation of oil, in the absence of air, for a brief period then rapidly cooling. There are three important types of cracking: • Thermal cracking; in which the gaseous alkane is heated to a very high temperature giving ethene as the major product. • Catalytic cracking; in which the alkane vapour is passed over a zeolite catalyst at a lower temperature to give less ethene and more branch-chained hydrocarbons, which are excellent fuels. • Steam cracking; in which the alkane vapour is mixed with steam before cracking which produces more aromatic hydrocarbons. There are two distinct processes by which ethene polymerises to form polyethene, giving two distinctly different products: LDP (Low Density Polyethene) - carried out at high temperature and very high pressure in the presence of a free-radical initiator (trace of O2 or peroxides). Produces branch chain polymers which cannot form a regular lattice - hence lower density, weaker intermolecular forces and lower melting point. HDP (High Density Polyethene) - carried out at much lower P and T in presence of complex catalysts (Al(C2H5)3 & TiCl4). Produces a polymer with very little branching that can form a regular lattice - hence higher density, stronger intermolecular forces and higher melting point. Production of LDP involves a free radical mechanism: Initiation R-H + O=O ⇒ R• + HO-O• Propagation R• + H2C=CH2 ⇒ R-CH2-CH2-• Termination 2 R-CH2-CH2-• ⇒ R-CH2-CH2-CH2-CH2-R The radicals (R•) can however remove hydrogen atoms in the middle of a chain, giving rise to branching of the chains: R-CH2-CH2-CH2-CH2-R + R• ⇒ R-CH2-CH•-CH2-CH2-R + R-H Production of HDP involves the formation of a complex with the catalyst. Ethene molecules then insert themselves into this very polar (“ionic”, Cδ--Tiδ+) bond resulting in polymer chains without any branching. The following equation is an oversimplification of this: TiCl4 + Al(C2H5)3 ⇒ C2H5-TiCl3 + Cl-Al(C2H5)2 CH2=CH2 + C2H5-TiCl3 ⇒ C2H5-CH2-CH2-TiCl3 etc. © IBID Press 2007 2 CHAPTER 14 CHEMISTRY IN INDUSTRY AND TECHNOLOGY (IB OPTION C) SUMMARY When propene polymerises to form polypropene, there are again two distinct products: Isotactic - the methyl groups are on alternate carbons and are on the same side of the hydrocarbon chain. This allows a regular lattice structure, so stronger intermolecular forces and a higher melting point. Atactic - there is a random distribution of methyl groups and they are on different sides of the hydrocarbon chain. This interferes with regular lattice structure, so weaker intermolecular forces and a lower melting point. Condensation polymers result from the joining of monomers that have two functional groups on each monomer and a small molecule (often water) is formed for each bond between the monomers. Usually two monomers are involved, each having two identical functional groups. Common examples are give in the text. The properties of polymers are very dependent on their structure and hence may be altered by modifications to the structure. Some examples are: • Because of the polar nature of chlorine, PVC has strong forces between the polymer chains hence it is quite rigid. Adding plasticisers (smaller molecules that come between the polymer chains) reduces the intermolecular forces giving a much more flexible product. • If volatile hydrocarbons are present when styrene (phenylethene) polymerises then they vapourise to leave large gaps between the polymer chains. This produces a very low density polymer (expanded polystyrene) with excellent thermal insulation properties. • If water is added during the polymerisation of polyurethane it reacts with the isocyanate to produce carbon dioxide gas (-NCO + H2O ⇒ -NH2 + CO2) which results in a foam suitable for soft furnishings. • Polyacetylene contains conjugated double bonds (that is alternate single and double bonds), but in spite of the possible extensive delocalisation it is only a poor conductor unless doped with iodine. The structure of the polymer determines its properties, hence phenol-methanal polymers are very rigid owing to the extensive cross-linking between the chains and in Kevlar, used for bullet-proof vests, there is strong hydrogen bonding between the polymer chains. Kevlar molecules (see structure above) has rigid rod-shaped molecules with strong intermolecular hydrogen bonds between the chains. As a result it can give rise to a lyotropic liquid crystal with a very strong, ordered structure. If concentrated sulfuric acid is added to Kevlar, then the O and N atoms are protonated, destroying the hydrogen bonded structure. The feel of fibres made from some polyesters can be rather harsh and they are not easy to dye. Blending with other fibres, either natural (like cotton) or synthetic (like polyamides) can improve these properties. Some specific examples of desirable properties are phenol-methanal polymers which are rigid and are excellent electrical insulators making them ideal for power sockets etc. Polyurethane foams have a low density and high elasticity making them excellent for use as padding in mattresses and soft furnishings. PET has a high tensile strength and low gas permeability making it ideal for “fizzy” drink bottles, plus it is easily recycled. Homogeneous catalysts (such as an acid in esterification, or Fe2+ in the reaction of H2O2 & I-) are in the same phase as the reactants. They react with one of the reactants to produce an intermediate, which is consumed at a later stage to regenerate the catalyst. These stages have lower activation energies than the uncatalysed process. Homogeneous catalysts can however be difficult to separate from the final product. © IBID Press 2007 3 CHAPTER 14 CHEMISTRY IN INDUSTRY AND TECHNOLOGY (IB OPTION C) SUMMARY Heterogeneous catalysts are in a different phase to the reactants and hence are easy to separate. They provide an active surface on which the reaction can take place with a reduced activation energy. It is important that the product does not bond to the surface so the surface is freed for other reactants. As it is a surface phenomenon heterogeneous catalysts must have high surface areas. Most industrial processes involve catalysts because they speed up the reaction (greater efficiency) and can also increase the yield of the desired product rather than by-products (greater selectivity). Some disadvantages are the way conditions can reduce the activity of catalysts (denaturing of enzymes, surfaces deactivated by “poisons”) and the environmental impact of the escape of toxic catalysts. The highly exothermic reaction between hydrogen and oxygen can be harnessed to produce electricity, and hence the production of hydrogen used to store energy, in a fuel cell: Electrode Negative Positive Alkaline conditions 2 H2(g) + 4 OH–(aq) → 4 H2O(l) + 4 e– O2(g) + 2 H2O(l) + 4e– → 4 OH–(aq) Acidic conditions 2 H2(g) → 4 H+(aq) + 4 e– O2(g) + 4H+(aq) + 4e– → 2H2O(l) Energy can also be stored through rechargeable batteries the reactions below are those that take place when they are delivering a current: Lead-acid cell (2.0 V) NiCad cell (1.4 V) Lithium ion (3.0 V) Negative electrode Positive Pb(s) + SO42-(aq) → PbSO4 (s) + 2 e– PbO2(s) + 4H+ (aq) + SO42–(aq) + 2e– → PbSO4(s) + 2H2O(l) Negative electrode Cd(s) + 2OH–(aq) → Cd(OH)2(s) + 2e– Positive electrode NiO(OH)(s) + H2O(l) + e–→ Ni(OH)2(s) + OH–(aq) Negative electrode LiC6 → Li+ + 6C + e– Positive electrode Li+ + e– + MnO2 → LiMnO2 Fuel cells and rechargeable batteries are both ways of converting the chemical energy of an exothermic reaction into electrical energy. The major difference is that the reactions in rechargeable batteries have to be reversible. Liquid crystals are fluids that have anisotropic physical properties (electrical, optical and elasticity), if the molecules of the fluid all have the same orientation. Many natural materials, such as cellulose, DNA and spider silk display this. There are two types of liquid crystal: Thermotropic liquid-crystals - are pure substances, such as biphenyl nitriles, that show liquid-crystal behaviour over a temperature range between the solid and liquid states. Lyotropic liquid crystals - are solutions, such as soap and water, that show the liquid crystal state at certain concentrations. In the nematic phase rod-shaped molecules are distributed randomly but tend to align themselves giving rise to anisotropic properties. If the temperature is raised, the increased thermal agitation disrupts this directional order until it is lost at the temperature at which the normal liquid phase is formed. © IBID Press 2007 4 CHAPTER 14 CHEMISTRY IN INDUSTRY AND TECHNOLOGY (IB OPTION C) SUMMARY Liquid-crystal displays (LCDs) are used in digital watches, calculators and laptops In these the orientation of the polar molecules can be controlled by the application of a small voltage across a thin film of the material. The ordered areas of the display have anisotropic optical properties, hence the light and dark areas can be controlled by the applied voltage to a grid of electrodes. In order to be useful, a liquid crystal must be stable, maintain the liquid crystal state over a large temperature range, be polar so the orientation can be controlled by an electrical field and be capable of rapidly changing orientation. Biphenyl nitriles (R- - -CN)produce an effective liquid crystal state because the biphenyl group makes the molecule more rigid and rod-shaped, whilst the nitrile group makes the molecule polar, so that the intermolecular forces are strong enough to make the molecules align. Nanotechnology involves research and technology development at the 1 nm to 100 nm range. It creates and uses structures that have novel properties because of their small size and builds on the ability to control or manipulate at atomic scale. Scanning probe microscopes are able to move individual atoms on a surface one atom at a time, whereas chemical reactions allow atoms to be positioned at a particular site in a molecule. Nanotubes are cylinders made only from carbon hexagons (like a looped round graphite sheet). The closed ends of the tube also involve pentagons to produce the curvature (as in fullerenes). Nanotubes can be either single or multiple, comprising a series of concentric single nanotubes. Nanotubes have anisotropic properties with high tensile strength along their axes. As the surface of nanotubes allows the flow of electrons (like graphite), the electrical conductivity of nanotubes increases with their length. The threat to health of nanoscale particles is largely unknown, hence there are people who have significant concerns in this regard. It is an issue that requires industrial responsibility and strong political leadership. Silicon, a semiconductor, has four electrons in its valence shell, meaning that the lowest electron band in the lattice structure is totally filled. It therefore cannot conduct except when electrons gain enough energy to jump into the next unfilled band (hence conductivity increases with temperature). If traces of atoms with one more electron (e.g. As, Sb) are added to the structure (doping) then the extra electrons have to go in the unfilled band, increasing conductivity (n-type semiconductors). Similarly if traces of atoms with one less electron (e.g. Ga, In) are added to the structure then this creates a space (“hole”) in the filled band, again increasing conductivity (p-type semiconductors). If p-type and n-type conductors are placed next to each other electrons flow from the surface of the n-type conductor to the p-type conductor, producing an electric field. This allows a p→n flow, but not a n→p flow, because the extra electrons just inside the p-type repel other electrons. If solar energy excites electrons in silicon into the conducting band then they can move p→n, but not the other way, hence the n-type semiconductor becomes the negative terminal of as solar cell. Electrons can flow through the external circuit back to the p-type (which acts as the positive terminal because it has lost electrons). In a LCD display each pixel contains a liquid crystal sandwiched between two scratched glass plates, which have a polarising film aligned with the direction of the scratches. The liquid crystal molecules in contact with the glass line up with the scratches, and because the scratches in the two plates are at 90o to each other, the molecules form a twisted arrangement © IBID Press 2007 5 CHAPTER 14 CHEMISTRY IN INDUSTRY AND TECHNOLOGY (IB OPTION C) SUMMARY between the plates, stabilised by intermolecular forces between the chains. These molecules rotate the plane of polarisation of plane-polarized light so that light will pass through the film and the pixel will appear bright. If a voltage is applied across the film, the polar molecules will align with the field, rather than the scratches, so the twisted structure is lost and the plane of the plane-polarized light is no longer rotated. As a result, the crossed polarising films cause the pixel to appear dark. The electrolysis of brine (aqueous NaCl) produces chlorine, hydrogen and sodium hydroxide: At cathode 2 H2O(l) + 2 e– → 2OH–(aq) + H2(g) At anode 2 Cl– → Cl2(aq) + 2e– (but at low [Cl-]) 4 OH–(aq) → 2H2O(l) + O2(g) + 4 e–) As a result Na+ and OH- remain in the solution, hence sodium hydroxide. It is important to keep the products apart, otherwise they react to form bleach: Cl2(aq) + 2 OH–(aq) → 2 Cl–(aq) + ClO–(aq) + H2O(l) In the membrane and diaphragm cells this is done by physically separating the two electrode compartments with. In the diaphragm cell the flow from anode to cathode is ensured by a pressure differential. In the membrane cell the compartments are separated by a membrane that allows cations (Na+) to pass through, but not anions (Cl- and OH-), hence no OH- can travel back to the anode compartment. An alternative is the mercury-cell in which mercury is used as the cathode, which results in the formation of a sodium amalgam rather than hydrogen. This amalgam is then pumped to a separate vessel where it is allowed to react with water to give the sodium hydroxide: Na+(aq) + e– → Na(Hg) 2 Na(Hg) + 2 H2O(l) → 2 Na+(aq) + 2 OH–(aq) + H2(g) Though it produces purer products than the diaphragm cell, the mercury-cell is being phased out in favour of the membrane cell because of concerns of brain damage that can result from the escape of mercury and its concentration in the food chain. Chlorine is used in the production of plastics (Polychloroethene - PVC), treating drinking water to kill bacteria, bleaching paper, producing chlorinated solvents, antiseptics and insecticides. Sodium hydroxide is used in the production of wood pulp for paper, the purification of bauxite, soap manufacture and as a general alkali in the chemical industry. (Shaded areas indicate AHL material) © IBID Press 2007 6 ...
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This note was uploaded on 09/23/2010 for the course CS 001 taught by Professor Jix during the Spring '10 term at Riverside Community College.

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