Production of Materials- 1
Production of Materials
1. Fossil fuel products
2. Fossil fuels provide both energy and raw materials such as ethylene, for the production of other substances
Background: Fossil fuels are formed from the remains of organisms that lived on Earth millions of years ago. Fossil fuels are rich in hydrocarbons that can be burnt to release energy or used to make raw materials such as ethylene.
Ethylene is the same substance as ethene. Ethene is the IUPAC name for C2H4 while ethylene is the name that is more commonly used in industry.
Ethylene (C2H4) can be used to produce useful substances such as polyethylene and ethanol.
Polyethylene is the cheapest plastic. The weight of polyethylene produced each year is greater than the total weight of all other plastics. Most plastic food bags, juice and milk containers are made of, or lined with, polyethylene.
· Construct word and balanced formulae equations of chemical reactions as they are encountered.
· An important part of the Preliminary course, that you must continue with throughout the HSC course, is learning how to:
- construct word equations, e.g: methane + oxygen -----> carbon dioxide + water
and
- balance formulae equations, e.g: CH4 + 2O2 -----> CO2 + 2H2O
You must be able to do this for the reactions you encounter in
every module that you study, core and option.
There are three important steps involved:
1. Show all reactants and all products in the word equation.
2. Write the correct formula for each reactant and each product.
3. Balance the formula equation by placing coefficients (numbers) in front of formulas so that you have the same total number of each kind of atom on both the reactant side and the product side. Remember that in chemical reactions atoms are just rearranged, not created or destroyed.
· Identify the industrial source of ethylene from the cracking of some of the fractions from the refining of petroleum.
· Revise fractiontal distilation from Preliminary course.
· Petroleum is a mixture of hydrocarbons. When petroleum undergoes fractional distillation, some fractions, particularly petrol, are in demand and of high economic value. Other fractions, called the ‘feed stock’, consisting of larger molecules than in petrol and of low value, can be passed over a heated catalyst that cracks the larger molecules into smaller molecules. A major by-product of this catalytic cracking is ethylene, also known as ethene.
· The products of cracking include short chain alkanes that can be used as petrol, branched chain alkanes that improve the perofrmance of petrol, alkenes (ethylene and propylene) and hydrogen.
· For example decane is cracked to octane and ethene:
C10H22(g) ® C8H18(g) + C2H4(g)
Steam Thermal Cracking
A process called steam thermal cracking is the main source of ethylene throughout the world. In this process ethane (C2H6) gas from natural gas, or larger hydrocarbons in low value petroleum fractions, are mixed with steam and passed through hot metal coils. The steam removes carbon deposits from the metal coils. The heat from the coils breaks bonds to change the ethane, or the larger hydrocarbons, to ethylene.
· Initial cracking required high temperatures.
· A process called steam thermal cracking is the main source of ethylene throughout the world.
· Initial cracking required high temperatures.
· Temperatures from 450°C to 700°C
Catalytic Cracking
· Initial cracking required high temperatures. The use of catalysts in ‘catalytic cracking’ allows for much lower temperatures.
· Many gas reactions are catalysed using solid inorganic catalysts onto which the gaseaous reactants are adsorbed. This weakens their bonds and reduces the activation energy for the reaction.
· The main catalysts for catalytic cracking are a group of silicate minerals called ‘zeolites’. Zeolites are crystalline substances composed of aluminium, silicon and oxygen. Zeolite crystals have a three-dimensional network structure containing tiny pores. The reactant molecules are adsorbed in these pores where the reactions are catalysed.
· Catalysts are added to the feed stock as a fine powder that is circulated in the catalytic cracker.
· Identify that ethylene, because of the high reactivity of its double bond, is readily transformed into many useful products.
· Ethylene can be transformed into many useful products because of the high reactivity of its double bond.
An explanation
Alkenes are more chemically reactive than their corresponding alkanes. The yellow colour of bromine water, which is due to the presence of bromine, is lost when the bromine water comes in contact with an alkene, but not when in contact with an alkane. This demonstrates the high reactivity of a C=C in an alkene compared with the C-C in an alkane.
· The chemistry of ethene is determined by its reactive double bond.
· Reactions of Ethene
Ethene may undergo a large number of addition reactions to produce many useful products:
1. Addition of hydrogen (hydrogenation)
Ethene is converted to ethane by heating it with hydrogen in the presence of a metal catalyst such as nickel, platinum or palladium.
Ni
CH2=CH2(g) + H2(g) ® CH3-CH3(g)
2. Addition of halogens (halogenation)
When halogens are added to ethene the double bond opens out and the addition reaction takes place.
These halogenation reactions are used to distinguish between alkanes and alkenes as alkanes do not readily react with halogens whereas alkenes do.
When a solution of bromine in a non-polar solvent (it has a red-brown colour), the solution discolours as the bromine adds across the double bond.
CH2=CH2 + Br2 ® CH2Br-CH2Br (1,2-dibromoethane)
An aqueous solution of bromine, known as bromine water is also used to distinguish between alkanes and alkenes. Bromine water is a yellow-brown solution which discolours in the presence of alkenes.
CH2=CH2 + Br2(aq) ® CH2OH-CH2Br + HBr(aq)
2-bromoethan-1-ol hydrogen bromide
The addition of halogens to ethene produces some important products such as:
1,2-dichloroethane which is used to manufacture chloroethene which is used to produce the plastic polyvinyl chloride, PVC.
3. Addition of hydrogen halides (hydrohalogenation)
Hydrogen halides such as HCl react with alkanes:
CH2=CH2(g) + HCl(g) ® CH3-CH2Cl(g)
4. Addition of water (hydration)
Ethene is used in the production of ethanol by adding water in the presence of a sulfuric or phosphoric acid catalyst:
H2SO4
CH2=CH2(g) + H2O(l) ® CH3-CH2OH(l)
Ethanol can then be oxidised to form ethanoic acid.
5. Oxidation of ethene
The mild oxidation of ethene produces 1,2-ethanediol, (ethylene glycol) which is used as antifreeze in cooling systems. Ethylene glycol lowers the freezing point and raises the boiling point of water. It is also used in the manufacture of magnetic tapes, photographic film and for making syntheic fibres.
The oxidation of ethene can be achieved by reacting ethene with cold, dilute acidified potassium permanganate (KMnO4) or with oxygen/water:
Cold, dilute
H+/KMnO4
CH2=CH2(g) + ® CH2OH-CH2OH(l)
O2/H2O
CH2=CH2(g) + ® CH2OH-CH2OH(l)
6. Reaction with benzene.
Ethene reacts with benzene to produce styrene which can then be used to make polystyrene.
7. Production of polyethylene
The main use of ethene is to manufacture the polymer, polyethylene.
· Some of the useful products made from ethylene are:
Product / Formula / Usepolyethylene / (CH2)n / plastic
ethylene oxide / (CH2)2O / steriliser
ethanol / C2H5OH / disinfectant
ethanoic acid / CH3COOH / food preservative
Ethylene oxide
· Identify that ethylene serves as a monomer from which polymers are made.
· Identify polyethylene as an addition polymer and explain the meaning of this term.
· A monomer is a repeating unit which reacts to form a long polymer chain.
· The reaction by which monomers become linked to form polymers is known as polymerisation.
· Addition polymerisation:
In addition polymerisation, the monomers simply add to the growing polymer chain in such a way that all the atoms present in the monomer are also present in the polymer.
· Polyethylene is called an addition polymer.
· Ethylene is polymerised to polyethylene
·
· High pressures produce soft, low density polyethylene (LDPE) consisting of tangled chains (with molecular masses < 100 000); used in flexible plastic bags such as those used to store food.
· Low pressures produce harder, high density polyethylene (HDPE) consisting of aligned chains (with molecular masses > 100 000); used in crinkly plastic bags as used for heavy duty garbage bags.
· Outline the steps in the production of polyethylene as an example of a commercially and industrially important polymer.
· Two different forms of polyethylene can be manufactured, depending on the reaction conditions.
· To produce low density polyethylene (LDPE), a peroxide containing an O-O bond that breaks easily forming free radicals is used to initiate the joining of ethylene monomers. The process must occur under high gas pressure to produce LDPE. These production conditions result in molecules with the short branches that characterise LDPE.
· To produce high density polyethylene (HDPE), low gas pressures and a catalysts made of transition metals and organometallic compounds enables more ordered orientation of ethylene to form the long unbranched and aligned molecules in HDPE.
· The difference in properties of the two forms are dependent on the degree of branching of the polymer chains.
· In LDPE the degree of branching is much greater and this reduces the dispersion forces between strands. This results in soft, flexible, low density plastics with relatively low melting points.
This branching means that the polymer chains cannot pack as tightly together. Consequently the density of the product is reduced.
· In HDPE the polymer chains can pack more closely together as a result extensive dispersion forces exist between molecules. This gives HDPE strength, toughness but makes it less flexible than LDPE.
· The degree of branching and hence the density of the polymer is determined by the conditions and catalysts used in the manufacturing process.
Production and Uses of LDPE
· Temperatures range from 100 - 300°C
· Pressures range from 1500 – 3000 atmospheres
· Initiators such as diethyl ether or benzoyl peroxide.
· The polymerisation process consists of three stages: initiation, propagation and termination.
1. Initiation
· The reaction is usually initiated with a catalyst, usually and organic peroxide. These peroxides produce free radicals, a molecule with at least one unpaired electron.
2. Propagation
The free radical is electron deficient and attacks the double bond in the ethene molecule. This then produces an ethyl group with a free radical which can then attack the double bond of another ethene molecule.
R-O· + CH2=CH2 ® R-O-CH2-CH2·
R-O-CH2-CH2· + CH2=CH2 ® R-O-CH2-CH2- CH2-CH2·
A branch occurs when a chain curls back on itself and the free radical removes a hydrogen froming a free radical in the chain.
3. Termination
Termination occurs when two free radical polymers react to form a covalent bond. This is called a chain terminating reaction.
Branching
• A branch occurs when a chain curls back on itself and the free radical removes a hydrogen forming a free radical in the chain.
· LDPE has a variety of uses. One of its main uses is for manufacturing of tough, flexible, transparent film (cling wrap). It can also be moulded into soft, squeezable plastic containers. LDPE is also used for insulation of wires and cables.
Production and Use of HDPE
· The polymerisation of HDPE uses an ionic catalyst called the Ziegler-Natta catalyst. This consists of mixtures of compounds such as TiCl4 and Al(C2H5)3.
In this process ethene molecules are added to the growing polymer molecule on the surface of the catalyst which reduces the amount of branching.
HDPE is used in the manufacture of gas pipes.
Due to its chemical resistance it can be moulded into containers for chemicals, oils, detergents, petrol and solvents.
HDPE is used in toys, plastic buckets and playground equipment. It can also be made into tough films such as freezer bags.
HDPE and LDPE are thermoplastic. They soften on heating.
· Identify the following as commercially significant monomers:
- vinyl chloride
- styrene
by both their systematic and common names
· Vinyl Chloride
· Vinyl chloride is the preferred IUPAC name
· Chloroethene is the systematic name
Styrene
• Styrene is the preferred IUPAC name.
• Ethenylbenzene is the systematic name.
· Describe the uses of the polymers made from the above monomers in terms of their properties.
· The table below provides the systematic and common names for some commercially significant monomers. The table describes the properties that account for the uses of some polymers produced from the selected monomers.
MONOMERS / POLYMERSCommon name / Systematic name / Name / Properties / Used for
ethylene / ethene / LD polyethylene / low density, soft / flexible food bags
HD polyethylene / high density, hard / crinkly garbage bags
vinyl chloride / chloroethene / polyvinylchloride / made rigid and flame resistant with additives, water resistant / rigid pipes and gutters,
flexible raincoats and shower curtains garden hoses, kitchen utensils, furniture covering.
styrene / ethenylbenzene / polystyrene / transparent, due to few crystals,
when gas added forms foam / compact disc cases, heat insulation, floats, Styrofoam cups etc, drinking glasses
· Expanded polystyrene is made by producing gas bubbles inside polystyrene. The low density of expanded polystyrene is used in flotation devices. The trapped gas spaces make it an excellent insulator.
· Gather and present information from first-hand or secondary sources to write equations to represent all chemical reactions encountered in the HSC course.