Notes on crude oil and its by-products.

N2(g) + O2(g) → 2NO(g)

Nitrogen oxide is a major pollutant and greenhouse gas

5.12b Recall that sulphur dioxide and nitrogen oxides are pollutant gases which contribute to acid rain, and describe the problems caused by acid rain.

Acid rain is formed when acidic air pollutants such as sulphur dioxide and nitrogen dioxide dissolve in rainwater. The sulphur dioxide and nitrogen oxides mainly come from power stations and factories burning fossil fuels, or from motor vehicles. The acid rain produces many problems. Acid rain can reduce the pH of natural water bodies from between 6.5 and 8.5 to below 4, which will kill fish and other aquatic life. The water is then too acidic to support life. Acid rain also leaches important nutrients from the soil and destroys plants. Without these nutrients, plant growth is stunted. Acid rain reacts with metals and with carbonates in marble and limestone (calcium carbonate). When this happens, metal bridges and stone buildings are damaged, even statues if they're made of limestone. 

5.12 Recall that fractional distillation of crude oil produces more long-chain hydrocarbons than can be used directly and fewer short-chain hydrocarbons than required

With increasing chain length, the hydrocarbons become less flammable, more viscous and therefore less useful. So there isn’t much use for these heavy fractions, but there’s more of it, whilst there’s a higher demand for short-chain hydrocarbons. Smaller hydrocarbon molecules are less viscous and more flammable. Thus they are more useful and in higher demand. (Petrol, a short chain alkane is in higher demand than say, diesel.) Cracking also produces alkenes as a by-product which can be used to make plastics. This brings about a bigger profit.

5.13 Describe how long-chain alkanes are converted to alkenes and shorter-chain alkanes by catalytic cracking, using silica or alumina as the catalyst and a temperature in the range of 600–700°C.

A long chain hydrocarbon is put into a catalytic cracker where there is an absence of air, high temperatures of 600-700°C and a mixed catalyst of aluminium oxide to speed up the process. The result is a mixture of smaller saturated and unsaturated hydrocarbons/alkanes+alkenes. Cracking is an example of thermal decomposition and the break up is random.

Synthetic Polymers

5.14 Recall that an addition polymer is formed by joining up many small molecules called monomers

Monomers are small molecules that can join up to make a very large molecule called a polymer.

e.g. Starch is a polymer made up of small glucose monomers

The monomer in addition polymerisation is unsaturated, i.e. the monomer must have a double bond. This opens up to allow other monomers to join up to form the polymer. 

5.15 Draw the repeat unit of addition polymers, including poly(ethene), poly(propene) and poly(chloroethene)

Polyethene -

Polypropene -

Polychloroethene -

5.16 Deduce the structure of a monomer from the repeat unit of an addition polymer

Put the sticking out bonds back into a double bond. Do not forget the n

5.17 Recall that nylon is a condensation polymer

5.18 Understand that the formation of a condensation polymer is accompanied by the release of a small molecule such as water or hydrogen chloride

Condensation polymers are formed by joining two different monomers so that they react together. When they react, they expel a small molecule, usually water or hydrogen chloride.

5.19 Recall the types of monomers used in the manufacture of nylon  

Dicarboxylic acid and diamine.

5.20 Draw the structure of nylon in block diagram format.

The Industrial Manufacture of Chemicals

5.21 Recall that nitrogen from air, and hydrogen from natural gas or the cracking of hydrocarbons, are used in the manufacture of ammonia

Nitrogen is obtained from the air, hydrogen is obtained from methane or from cracking hydrocarbons

5.22 Describe the manufacture of ammonia by the Haber process, including the essential conditions:

i a temperature of about 450°C

Temperature is a compromise as it cannot be too high because it would favour the backward reaction. It cannot be too low because rate of reaction would be too low to be economically viable.

ii a pressure of about 200 atmospheres

200 atmospheres is a compromise pressure chosen on economic grounds. If the pressure used is too high, the cost of generating it exceeds the price you can get for the extra ammonia produced.

iii an iron catalyst

The yield of ammonia stays the same, you just get it faster because it speeds up the reaction 

The forward reaction is exothermic; the reaction is reversible

5.23 Understand how the cooling of the reaction mixture liquefies the ammonia produced and allows the unused hydrogen and nitrogen to be recirculated

Ammonia is easily liquefied under pressure as long as it isn't too hot, and so the temperature of the mixture is lowered enough for the ammonia to turn to a liquid. The nitrogen and hydrogen remain as gases even under these high pressures, and can be recycled.

5.24 Recall the use of ammonia in the manufacture of nitric acid and fertilisers

5.25 Recall the raw materials used in the manufacture of sulfuric acid

Sulphur, Air, Water (concentrated sulphuric acid also needed)

5.26 Describe the manufacture of sulfuric acid by the contact process, including the essential conditions:

i A temperature of about 450 °C

A compromise as the forward reaction is exothermic so a higher temperature will favour the backward reaction but it cannot be too low because of rate of reaction

ii A pressure of about 2 atmospheres

A compromise as a high pressure will favour the side with fewer moles, the forward reaction. But the conversion is so good at lower pressure anyway, it is not economically viable

iii A vanadium(V) oxide catalyst

Sppeds up the reaction

First, burn sulphur in air:

S(s) + O2(g) → SO2(g)

React the sulphur dioxide with excess air

2SO2(g) + O2(g) ⇌ 2SO3(g)     ∆H= -196 kJ/mol 

React this with concentrated sulphuric acid to produce oleum. Reacting SO3 with water would produce an uncontrollable fog.

H2SO4(l) +SO3(g) → H2S2O7 (l)

This is converted into twice as much concentrated sulfuric acid by careful addition of water. 

H2S2O7(l) + H2O(l) → 2H2SO4(l)

5.27 Recall the use of sulfuric acid in the manufacture of detergents, fertilisers and paints

5.28 Describe the manufacture of sodium hydroxide and chlorine by the electrolysis of concentrated sodium chloride solution (brine) in a diaphragm cell

Sodium hydroxide and chlorine are produced by electrolysis in a diaphragm cell

Chlorine and Hydrogen are discharged at the electrodes leaving a solution with ions Na+ and OH-, which bond to form NaOH(aq).

5.29 Write ionic half-equations for the reactions at the electrodes in the diaphragm cell

Anode: 2CL-(aq) → CL2(g) + 2e-                 Oxidation of Cl-

Cathode: 2H+(aq) + 2e- → H2(g)                Reduction of H+

5.30 Recall important uses of sodium hydroxide, including the manufacture of bleach, paper and soap; and of chlorine, including sterilising water supplies and in the manufacture of bleach and hydrochloric acid.

Sodium Hydroxide → Bleach, Paper, Soap

Chlorine → Sterilising drinking water, Bleach, Hydrochloric Acid