CHEMISTRY NOTES TAKE 2 Topic 1

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1 CHEMISTRY NOTES TAKE 2 Topic Fossil fuels provide both energy and raw materials such as ethylene for the production of other substances identify the industrial source of ethylene from the cracking of some of the fractions from the refining of petroleum Ethylene is the main substance used in producing polymers (plastics), obtained as a by-product, or through deliberate means (e.g. refining petrol or deliberate decomposition) Source of ethylene: crude oil (liquid petroleum) / natural gas. There are 2 types of deliberate decomposition: Catalytic Cracking 500 o C in absence of air > atmospheric pressures (prevents explosive and flammable mixtures with air) Uses zeolite catalysts- inorganic crystalline aluminosilicate compounds H-BPT/MWT fractions are broken into lower mwt/bpt fractions using a catalyst Thermal Cracking Higher temp. of C, just above atmospheric pressure (no cat.) Mixture of alkenes and steam Sometimes, feedstock of ethane and propane is used to guarantee production of ethylene identify that ethylene, because of the high reactivity of its double bond, is readily transformed into many useful products Alkenes are much more reactive than alkanes due to unsaturated electron dense double bonds, allowing them to react easily with electronegative species. (Slightly lower MPT/BPT due to dispersion forces i.e. lower mwt) Alkene: Addition reaction: no atoms lost from starter molecule and alkene double bond opens out to allow addition of new atoms to molecule. Alkane: Substitution reaction: reaction in which atom in starter molecule is replaced by another atom/group of atoms. Ethylene s reactivity means it can be used to produce many useful products through reactions with other substances, including- Reaction of ethylene with water: Ethanol - (Alkanol - OH functional group): Ethylene is heated at 300 o C with steam at H-pressure Hydration reaction- addition of water molecule across double bond Ethanol is widely used as a reactant and solvent in synthesis of commercial products 1

2 Reaction of Ethylene with Oxygen- Ethylene Glycol: Ethylene is 1 st reacted with O 2 with Ag cat. to form ethylene oxide This is then converted to ethylene glycol using dilute acid sol n cat when reacted with water. Ethylene glycol is used for manufacturing polymers and automotive antifreeze identify data, plan and perform a first-hand investigation to compare the reactivities of appropriate alkenes with the corresponding alkanes in bromine water Procedure: 1. Pour 5ml of cyclohexane (C6H12) into 2 separately labelled test tubes and add a few drops of bromine water to each test tube 2. Repeat for cyclohexene (C6H10) 3. Stopper tubes and shake so solution is mixed well 4. Place one of alkene and alkane near a lighted window and one in the dark. 5. Leave for 10 min and observe changes. Results: The alkane in the dark did not react with bromine water, the soln staying brown. However, UV light will decolourise the solution from reddish-brown to colourless (must state this change); hence care must be taken transporting the test tubes avoiding excess light. An alkene will always decolourise bromine water from brown to colourless in an addition reaction due to its reactive double bond. A test to determine whether a substitution or addition reaction has occurred is exposing the mouth of each test tube to some conc. NH 3. If HBr is present, a white vapour precipitate will form, and hence a substitution reaction with an alkane has occurred. Why cyclohex(a/e)ne is used: liquid at room temp, easily available at reasonable cost, relatively safe is suitable precautions are taken. Safety Precautions: fume cupboard, keep away from naked flames, standard lab protection identify that ethylene serves as a monomer from which polymers are made Ethylene may undergo addition reactions with other monomer units to form polymers through polymerisation: a chemical reaction in which many identical small molecules (monomers) combine to form one large molecule (polymer) 2

3 term identify polyethylene as an addition polymer and explain the meaning of this Polyethylene is an addition polymer, formed by adding monomers together without the loss of any atoms, where monomer double bonds break/open out, allowing polymer chains to form. This is commonly written like this: outline the steps in the production of polyethylene as an example of a commercially and industrially important polymer Two main processes used to produce different types of polyethylene: Older Gas Phase Process: (LDPE) High pressure ( x atmospheric) High temp. 300 o C Uses organic peroxide/o 2 as initiator molecule Product has significant chain branching, where at some carbon atoms the H-atom is replaced by an alkyl group through back biting - where activated end latches onto an H-atom from a previous carbon, stabilising the end carbon and leaving the newly activated carbon atom reactive to other molecules Consequently, the alkane chains cannot pack together in an orderly fashion, creating Lowdensity polyethylene (LDPE) L-dispersion forces & soft, flexible polymer Uses: disposable plastic bags, clingwrap, milk cartons Ziegler-Natta process (HDPE) Low atmos. pressure Low temp. 60 o C Trialkylaluminum & TiCl 3 compound cat. allows lower temp. & pressure for process. Also affects arrangement of units attached to the main chain and polymer s physical properties e.g. density & heat stability. Forms unbranched polyethylene molecules which can pack closely together in an orderly fashion, dispersion forces & producing a hard, inflexible polymer: High-Density polyethylene (HDPE) Uses: Kitchen utensils, containers, rubbish bins 3

4 1. R-O * + CH 2=CH 2 R-O-CH 2-CH 2 * Steps- Formation of Polymers- LDPE 1. Initiation: 2. R-O-CH 2-CH 2 * + CH 2=CH 2 R-O- CH 2-CH 2-CH 2-CH 2 * 3. R-O-CH 2-CH 2-CH 2-CH 2-O-R - Initiator/catalyst is 1 st split in 2 using UV, creating 2 activated electronegative species, which activates an ethylene monomer by opening up its double bond to form a free radical 2. Propagation: - Ordinary ethylene molecules react with the activated species, adding to the growing polymer chain 3. Termination: 2 activated species collide with each other and join together to form a stable polymer molecule Exchange an H atom to form 2 stable molecules Terminator molecules are added Reaction runs out of monomer units The exothermic polymerisation constantly produces heat, which may decompose the polymer as it forms, yield. Reaction & collision conditions can be controlled so as to achieve a higher yield. Important factors in the production of polyethylene in terms of quality control include: - mwt, density, product purity, additives As polymer chains are not all the same length as different no. of monomers can combine to form diff. length chains quality control is necessary to ensure the product is of uniform high quality with the same composition and purity so customers are confident HDPE/LDPE polyethylene will always have similar properties and continue consumption analyse information from secondary sources such as computer simulations, molecular model kits or multimedia resources to model the polymerisation process Benefits Tactile learning through concrete objects Demonstrates the difference between different molecular bonds Simple representation of 3-D spatial arrangements to aid understanding Limitations Relative sizes and distances between atoms are unrealistic Simplifies the model Dynamic nature of various molecules and their bonds is not shown 4

1.1.8 identify the following as commercially significant monomers: - vinyl chloride (preferred)/ chloroethene (systematic) Cheapest and most widely used polymer (PVC) after polyethylene - Cl side

relatively durable used in raincoats. - Cl bonds make it vulnerable to UV light, & thus extra substances are added to give it beneficial properties, e.g. plasticisers to increase softness and inhibitors to absorb UV light.

5 1.1.8 identify the following as commercially significant monomers: - vinyl chloride (preferred)/ chloroethene (systematic) Cheapest and most widely used polymer (PVC) after polyethylene - Cl side groups provide considerable chain stiffening and polar C-Cl bonds produce relatively strong dipole-dipole intermolecular forces, creating a hard, inflexible polymer - It is insoluble in water and relatively durable used in raincoats. - Cl bonds make it vulnerable to UV light, & thus extra substances are added to give it beneficial properties, e.g. plasticisers to increase softness and inhibitors to absorb UV light. Uses: (soft variant) Electrical insulation, garden hose, (soft and pliable) (hard variant) drainage & sewerage pipes, household guttering (rigid and strong, insoluble in water) - styrene (preferred)/ phenylethene/ ethenylbenzene (systematic) - Large benzene side groups create strong chain stiffening and with only C-C & C-H bonds, is very hard and rigid, stable to heat & UV light and crystalline due to minimal chain branching allowing it to be transformed into transparent products such as CD cases and plastic containers. - Despite its inherent hardness, polystyrene is used to make foam cups, where air is bubbled, trapped and compressed within styrene mixture to form a spongy material Uses: (hard variant): car battery cases, tool handles, modern furniture, CD cases (soft variant): foam packing material, Styrofoam cups, disposable food packages (not chemically reactive and thus will not degrade in UV light) 5

6 1.1.9 describe the uses of the polymers made from the above monomers in terms of their properties In selecting a polymer for a particular use, impt. properties to consider include: Melting/softening point Heat/UV stability Chemical stability Mechanical strength (hardness) Flexibility/ rigidity Cost- cheapest polymer that will fulfil need These properties are determined by: Average MWT: - Reflects no. of monomer units in 1 polymer molecule - Generally, the longer the polymer (Higher mwt), the higher the MPT and hardness Chain Branching: - If polymer forms long unbranched chains, they are more compact and can intertwine, leading to an orderly arrangement with high density & crystalline substance, and thus higher MPT and hardness. - The opp. situation occurs with polymers with lots of chain branching, creating low density, low MPT, greater flexibility and a softer polymer. Chain Stiffening: - This occurs in addition polymers where a large side-group is placed onto a linear chain to reduce flexibility- restricting the ability of the chain to flop around, becoming stiffer and more rigid. Cross Linking: - The rigidity/hardness of a polymer can be increased through crosslinking, a process which >2 linear chains are joined together to form a 2-D network, increasing its hardness due to the additional bonds between polymer chains Stability: - Most polymers are quite stable due to the strong C-C and C-H bonds. - The exception is PVC, which decomposes in UV light or H-temp. creating corrosive HCl 6

7 1. 2. Some scientists research the extraction of materials from biomass to reduce our dependence on fossil fuels discuss the need for alternative sources of the compounds presently obtained from the petrochemical industry The need for alternative sources of petrochemical compounds stems from issues of scarcity and detrimental environmental impact. Scarcity: Petrochemical products are derived from non-renewable sources of crude oil. At the current unsustainable rates of consumption and production, the base materials used in current production of petrochemical products will become unavailable. Environmental Impact: As the majority of crude oil is used as fuel, the consumption of such products has an enormous impact on the environment. Current petrol products burn relatively uncleanly, contributing to the greenhouse effect and acid rain. The production and consumption of non-biodegradable petrochemical products also contributes to landfill and pollution. Thus it is imperative that alternative sources of petrochemical products must be found. Alternative fuels also mean petrochemicals can be reserved for the production of polymers use available evidence to gather and present data from secondary sources and analyse progress in the recent development and use of a named biopolymer. This analysis should name the specific enzyme(s) used or organism used to synthesise the material and an evaluation of the use or potential use of the polymer produced related to its properties One method to alleviate problems of scarcity and environmental degradation is to use biopolymers: polymers made mostly/all by living organisms Biopolymers solve another key issue of petroleum-based polymers in their non-biodegradability i.e. not decomposed by fungi/bacteria- ruining the landscape. Solutions include: Alternating biopolymer monomers with synthetic monomers in polymer molecule (biological decay of bio section disintegrates entire polymer) Developing biopolymers to have synthetic properties One example is PHB or Poly(3-hydroxy)butanoate (commercially sold as Biopol), using Alcaligenes Eutrophus to modify natural polymers using living organisms to obtain similar properties to polypropylene. - 1 st developed in 1925 by Maurice Limoigne Monomer: 3-hydroxybutanoic acid 7

8 Production method: 1. Alcaligenes Eutrophus (bacteria) is fed nutrient-rich diet, multiplying rapidly into large colonies 2. Glucose/nitrogen is withdrawn from diet and bacteria naturally secrete PHB as energy storage (similar to body fat), which is harvested (30-80% of body weight). Properties and Uses: Naturally occurring:- non-toxic & renewable Biodegradable:- decomposes into CO 2 & H 2O RENEWABLE Physically similar to polypropylene: tough, flexible, crystalline, lightweight, H-MPT, insoluble in water, UV & chemically stable Biocompatible: used in medical industry with medical sutures due to PHB s compatibility with biological systems in intimate contact with living tissue does not react with biological systems Other uses: disposable nappies, bandages and sutures, plastic bags & disposable cutlery Development and Impact: Scientists have located the gene resp. for producing PHB & transferred this to E. Coli, speeding production due to genetically-engineered bacteria that has faster growth, better yields and production of less waste biomass. Genetically engineering plants such as cress and potatoes to produce biodegradable plastics rather than starch Whilst PHB is more ecologically friendly, it is not economically viable outside of the medical industry, most evident in the initiation and termination of PHB shampoo bottle production Only when petroleum prices increase will PHB become cost-effective and a viable alternative to synthetic polymers explain what is meant by a condensation polymer Condensation polymers: Polymers that form by the elimination of a small molecule when monomers with >2 functional groups join together e.g. cellulose, nylon, cotton, etc describe the reaction involved when a condensation polymer is formed Cellulose forms when alternating beta-glucose monomers form a chain and condense out a water molecule, unlocking the monomer and enabling polymerisation. For example, when two glucose monomers react through two hydroxyl groups -OH, an H-OH molecule is condensed out, to allow the glucose monomers to bond through the oxygen atom. The first two glucose molecules to join condense out an H-OH, and every glucose molecule added to the growing chain then condenses out another H-OH. 8

9 1.2.5 describe the structure of cellulose and identify it as an example of a condensation polymer found as a major component of biomass Cellulose structure: Monomer glucose units contain 5 carbon atoms and one oxygen atom, forming a puckered ring, with OH groups on 5 of the C atoms, forming linear chains with intermolecular H-bonding Cellulose is the major component of biomass, that is: - material produced by living organisms, mainly plant material, including animal excretions and algae production most abundant carb on Earth Both starch and cellulose can be used to source glucose and thus ethanol. However, plants produce a greater amount of cellulose than starch, making it a more attractive option. However there are 2 processes for converting cellulose to glucose Digestion by cellulose enzymes - Cellulose is finely ground to increase surface area and thus greater penetration - It is then treated with NaOH/hot water to swell/open up the cellulose fibres then digested with cellulase to obtain glucose solution Digestion with strong acid - Suspension of cellulose with mod. conc. H 2SO 4 is heated to break cellulose into glucose - Insoluble matter and impurities are filtered out and acid is neutralised to produce glucose sol n Glucose solution is then fermented with yeast to produce ethanol, which can then be dehydrated to obtain ethylene. The main disadvantage for these processes is that much of the energy required (heat and electricity) comes from fossil fuels, where potentially more oil is used digesting cellulose then would be directly cracking the crude oil to obtain ethylene itself. - It is also significantly more expensive than converting starch into sugar, due to the H-bonds in cellulose requiring many energy-intensive processes, as opposed to directly cracking crude oil. 9

Chemistry Notes. Daniel P

Chemistry Notes. Daniel P Chemistry Notes Daniel P Contents 1 Introduction 3 2 Production of Materials 4 2.1 Ethylene and its Uses...................................... 4 1. Chemical Equations...................................