C.2.1 Compare the use of oil as an energy source and as a chemical feedstock • C.2.2 Compare catalytic cracking, thermal cracking and steam cracking. (Students should include the environmental impact of the processes and their products.) • These are both “woffly”. Its hard to know exactly what information is needed. I will give you some information, but it would be useful to do some background reading as well!
Crude oil is a mixture of compounds. • In order to be useful the crude oil must be refined. • This is done through fractional distillation.
Make sure you can explain how fractional distillation works. • You may want to add the number of carbon atoms present in each fraction.
Lets have a debate. • Motion: Dmitri Mendeleev was correct when he said that burning oil as a fuel was like “firing up a kitchen stove with banknotes”
Some points: • 90% of all oil products are used as fuel. • 10% of oil products are used to produce other products such as polymers, drugs, cosmetics, paints, fertilisers, pesticides, detergents and dyes. • Petrol is a very concentrated and convenient energy source. ( a petrol pump supplies energy at about 34MW ; a medium sized power station produces about 700MW)
Burning hydrocarbons produces smog and global warming (maybe!) • Plastics are non-biodegradable and it can be problematic to dispose of them. • Crude oil is a non-renewable resource • It will last longer if we conserve energy and recycle materials. • Alternative energy sources are becoming available. • Polymers and other organic chemicals can be made from coal or biological materials such as wood, starch or cotton.
There is no right or wrong answer. • Just be prepared to discuss the different considerations.
Cracking • Breaking larger molecules into shorter ones. • This is done because shorter molecules (with 1 to 12 carbon atoms) are in greater demand. • Cracking produces a mixture of alkanes and alkenes.
The total number of carbons is unchanged. • We get a mixture of saturated and unsaturated products.
The short chain alkanes are useful as fuels. • This is because they are easier to burn than longer alkanes. • Why? • Despite what you might think from all the equations (!), cracking often produces branched alkanes. • These are better as car fuel because they prevent “knocking” • They have a higher “octane” number.
The alkenes produced are useful as chemical feedstock. • They are mainly used to produce polymers. • Notice also that polymers don’t form straight lines. • They zigzag. Why? • The carbon atoms each have a tetrahedral structure.
Different ways of cracking • There are (at least) 4 different ways of cracking long alkane molecules. • You need to know the conditions for each one.
Thermal cracking (pyrolysis) • Feedstock: long chain alkanes • Reaction conditions: Temp: 800 – 850 °C Pressure: 70 atm No catalyst Long contact time • Mechanism: Free radical (heterolytic fission) • Products: short chain alkanes (for fuel) and short chain alkenes (esp. ethene for polymerisation)
Catalytic cracking • Feedstock: long chain alkanes • Reaction conditions: Temp: 500°C Pressure: low or moderate Powdered zeolite catalyst (usually a mix of silica and alumina) Short contact time • Mechanism: Complicated – forms an ionic intermediate. • Products: Branched alkanes and alkenes with high octane number (suitable for fuels). Some cyclic compounds are also produced.
Steam cracking • Feedstock: Shorter alkanes – up to 10 carbons • Reaction conditions: Temp: 1250 - 1400°C Pressure: low or moderate No catalyst Very short contact time Feedstock diluted with steam • Mechanism: Free radical • Products: Low molecular mass alkanes and alkenes (esp. ethene)
Hydrocracking • Feedstock: Heavy hydrocarbon fractions (i.e. high Mr) • Reaction conditions: Temp: °C Pressure: 80 atm Zeolite or platinum catalyst Feedstock mixed with hydrogen • Mechanism: • Products: High yield of branched alkanes and cycloalkanes. Some aromatic compounds. No alkenes!
Assessment statements - C3 Addition polymers • C.3.1Describe and explain how the properties of polymers depend on their structural features. ( Students should consider: different amounts of branching in low- and high-density polyethene; different positions of the methyl groups in isotactic and atacticpolypropene. • C.3.2 Describe the ways of modifying the properties of addition polymers. Examples include plasticizers in polyvinyl chloride and volatile hydrocarbons in the formation of expanded polystyrene. • C.3.3 Discuss the advantages and disadvantages of polymer use. (Include strength, density, insulation, lack of reactivity, use of natural resources, disposal and biodegradability. Use polyethene (both LDPE and HDPE), polystyrene and polyvinyl chloride plastics as examples.
We’ve already studied this!! • Prove to me that you can remember it by drawing the repeat unit of polypropene (aka polypropylene)
A few other common polymers: • Polystyrene
Obviously the identity of the monomer changes the properties of the polymer formed. • But there are also a number of other factors that determine the properties of a polymer.
Polyethene can have different properties depending on how it is produced. • One of the factors that makes a difference is the presence of side chains. • This is known as branching. • HDPE has less branching • LDPE has more branching
High density polyethene • HDPEis defined by a density of greater or equal to 0.941 g/cm3. HDPE has a low degree of branching and thus stronger intermolecular forces. • Strong, rigid, high MP (135°C) • HDPE is used in products and packaging such as milk jugs, detergent bottles, margarine tubs, garbage containers, buckets and water pipes.
Low density polyethene • LDPE is defined by a density range of 0.910–0.940 g/cm3. LDPE has a high degree of short and long chain branching, which means that the chains do not pack as well. It has weaker intermolecular forces. • More flexible, lower MP (100°C) • LDPE is used for both rigid containers and plastic film applications such as plastic bags and film wrap
LDPEBranched chainsLimits how close molecules can getWeak intermolecular forces
LDPE is produced at high pressure • The mechanism is free radical. • HDPE is produced at a higher temperature • A catalyst is used.
Extra credit question: • Who is Kilgore Trout?
In any polymer with a “dangling” side group (such as the methyl group in polypropene) The orientation of the side group can affect the polymer properties. • The side groups may be all on the same side • Isotactic • They may alternate sides • Syndiotactic • Or they may have a random arrangement • Atactic
In isotactic PP the polymer chains can pack more closely • So intermolecular forces are higher • So the polymer is tougher • It is used for car bumpers, or is drawn into strong fibres for carpets etc. • Atactic PP is more flexible • It is used in sealants.
The properties of polymers can also be modified by adding other substances. • For example expanded polystyrene (used for packing material and coffee cups – or to make houses!!) • Pentane is added to the mixture during the processing stage. • This doesn’t form a polymer, but as it is volatile it vaporises and expands the polystyrene. • Hence the resulting polymer has low density and is a good thermal insulator
Plasticisers • PVC contains a polar C-Cl bond and hence has high intermolecular forces. • This leads to a very rigid plastic (UPVC or PVCU) which is used in windows, doors, drainpipes etc.
To make the plastic more flexible, we add small molecules known as plasticisers • These reduce intermolecular forces and produce a more flexible polymer • E.g. for use in blood bags or intravenous drips • (not to mention pvc clothes!
A typical plasticiser would be D.O.P. • DiOctylPhthalate – sometimes called DEHP DiEthylHexylPhthalate
This has the disadvantage of mimicking female hormones • It leaches into materials such as oils and fats • The US EPA sets a safe limit of 6ppb in water • It lowers the sperm count in men and carries a high risk for male foetuses and males at or around puberty. • It is also suspected to cause obesity and insulin resistance. • 25% of American women have phthalate levels which are “a concern”
Copolymers • Another common way of modifying polymers is to mix two or more monomers. • The resulting copolymer will have a mix of properties. • By choosing both the identity and relative proportions of the 2 monomers, we can modify the properties of the copolymer to exactly what we require. • E.g. mechanical properties, solubility, crystallinity, glass transition temperature.