Chemistry Notes Daniel P
Contents 1 Introduction 3 2 Production of Materials 4 2.1 Ethylene and its Uses...................................... 4 1. Chemical Equations................................... 4 2. Industrial Sources of Ethylene.............................. 4 3. Reactivity of Alkanes and Alkenes........................... 5 4. Ethylene as a Monomer for Polymerisation...................... 5 5. Addition Polymers.................................... 6 6. Polyethylene....................................... 6 7. PVC and Polystyrene.................................. 7 8. Properties and Uses of Polymers............................ 8 2.2 Biomass.............................................. 9 1. The Need for Renewable Resources........................... 9 2. Condensation Polymers................................. 10 3. Condensation Polymerisation.............................. 10 4. Cellulose......................................... 10 1
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Chapter 1 Introduction These notes have been prepared for HSC Syllabus in NSW (2001-2018) 3
Chapter 2 Production of Materials 2.1 Ethylene and its Uses 1. Chemical Equations Construct word and balanced equations of chemical reactions as they are encountered Word equations use words to represent the reaction Symbolic equations use chemical formulae and symbols to represent chemical reactions Note: 1. The formula for the reactants and products must be correct, and the net charge of the equation must be preserved 2. The number of atoms for reactants in all chemical equations must equal the number of products by placing whole numbers in front of the formula, to ensure that the Law of Conservation of Matter holds true (except for nuclear reactions) 3. A balanced equation must include state symbols (s) solid, (l) liquid, (aq) aqueous and (g) gas written as subscripts 2. Industrial Sources of Ethylene Identify the industrial source of ethylene from the cracking of some of the fractions from the refining of petroleum Petroleum is composed of crude oil and natural gas, and is separated using fractional distillation which separates the components of crude oil according to their molecular weight i.e. boiling point. Generally fractional distillation produces larger quantities of less useful higher molecular weight alkanes than the lower molecular weight alkanes. Cracking is the process by which these higher molecular weight alkanes are then split into lower molecular weight alkanes, which are more in demand because of their usefulness as an accessible fuel source. Small alkenes such as ethylene are produced as by-products which can be used in the manufacture of plastics. 4
Examples: i. The catalytic cracking of dodecane produces octane and but-1-ene: C 12 H 12 (l) C 8 H 18 (l) + C 4 H 8 (g) Note: This is conducted at 500 C, 140 kpa and using a zeolite catalyst ii. The thermal cracking of natural gas, a mixture mostly of ethane and propane: C 2 H 6 (g) C 2 H 4 (l) + H 2 (g) and C 3 H 8 (g) C 2 H 4 (l) + CH 4 (g) Catalytic Cracking Catalytic cracking is a process to break large alkanes into smaller alkanes producing small alkenes in the process. It is undertaken in a cat cracker column at 500 C and pressures just above atmospheric pressure i.e. 100-140 kpa. The catalyst used is a porous aluminosilicate referred to as a zeolite. The structure of the catalyst means it has a larger surface area allowing the reaction to occur faster. Thermal Cracking Thermal cracking, also called steam cracking, is a process that using high temperatures (700-1000 C) to directly decompose high molecular weight hydrocarbons into useful alkenes, and hence does not require the use of a catalyst. Alkanes are passed with steam through hot metal tubes at just above atmospheric pressure. Thermal cracking produces hydrogen gas and also small alkenes, such as ethylene or propylene. 3. Reactivity of Alkanes and Alkenes Identify that ethylene, because of the high reactivity of its double bond, is readily transformed into many products Ethane and ethylene both are non-polar hydrocarbon molecules with weak dispersion forces resulting in similar physical properties. However, their chemical properties are different. Ethane, as are all alkanes, are saturated compounds that is no additional atoms can be incorporated into its molecular structure and therefore requires the input of energy to react with other substances. Ethylene, as are all alkenes, are unsaturated compounds and so additional atoms can be incorporated into its structure because they have a high reactive double bond, which is able to open up to form two single bonds. This carbon-carbon double bond in alkenes significantly more reactive than alkanes and allows them to be transformed into many products such as polyethylene, ethanol and polyvinyl chloride. 4. Ethylene as a Monomer for Polymerisation Identify that ethylene serves as a monomer from which polymers are made Ethylene serves as a monomer from which polymers are made and is the starting material for polyethylene It is also the starting material for other commercially significant polymers such as those formed from monomers derived from ethylene including chloroethene (vinyl chloride) and ethenylbenzene (styrene). 5
5. Addition Polymers Identify polyethylene as an addition polymer and explain the meaning of this term Polyethylene is an example of an addition polymer. Addition polymers are formed when small monomers react in a series of addition reactions as a continuous reaction forming a long chain of molecules to form the polymer. No other products are released in this reaction An addition reaction is when the double bond in a unsaturated compound is broken and additional atoms, typically hydrogen, halogens or alkyl groups, bond resulting in a saturated compound. 6. Polyethylene Outline the steps in the production of polyethylene as an example of a commercial and industrially important polymer Ethylene because of the carbon-carbon double bond present, is able to bond with other ethylene molecules to form a long chain in a chain reaction in a process called addition polymerisation. Addition polymerisation is when monomers are bonded together to form long chains called polymers, through means of an addition reaction. In general, the process is as follows: The catalyst or initiator activates the ethylene molecule by decomposing into free radicals. These free radicals then cause the double-bond in the ethylene molecule to open, resulting in a new activated species, which is able to keep adding one ethylene molecule after each other in a chain reaction. The process stops when two activated chains collide and either join or exchange hydrogen to form two polyethylene molecules. The addition of an inhibitor can stop the formation of radicals and hence stop the process. This allows the size and molecular weight of polymer chains to be regulated. 6
Low Density Polyethylene LDPE (low-density polyethylene) has significant chain branching and relatively low density with weak dispersion forces between the molecules. These factors are responsible for some of its properties including its good flexibility, good and transparency. Some of its uses related to its properties include cling wrap, carry bags and some bottles. LDPE is formed using the gas-phase process, which is conducted at high temperatures (300 C) and very high pressures (100,000 300,000 kpa). Organic peroxide is used as an initiator i.e. it is not a catalyst but is used to start the chain reaction. High Density Polyethylene HDPE (high-density polyethylene) has minimal chain branching and has a higher density than LDPE with stronger dispersion forces that result in stronger, more compact, more rigid form of polyethylene than LDPE. Some of its uses related to its properties include kitchen utensils, some toys and even as a building material. HDPE is formed using a newer process, called the Ziegler-Natta process. This process can be conducted at 60 C and at normal atmospheric pressure (100 kpa) and uses a transition metal catalyst referred to as Ziegler-Natta catalyst (which is made of titanium chloride and a tryalkylaluminium compound). 7. PVC and Polystyrene Identify the following as commercially significant monomers: vinyl chloride styrene by both their systematic and common names Both vinyl chloride (chloroethene) and styrene (ethenylbenzene) are used as monomers in the production of polyvinyl chloride (PVC) and polystyrene respectively, by the process of polymerisation. These polymers have many uses in commercial industry. 7
8. Properties and Uses of Polymers Describe uses of the above properties from the above monomers in terms of their properties Some of the physical and chemical properties thare are considered when selecting a polymer for a suitable application include: melting point mechanical strength, flexibility and rigidity chemical stability and stability when exposed to heat or light solubility These relate to the: Average Molecular Weight: The higher molecular weights (longer chains) have higher melting points. Chain Branching: Polymers with minimal branching are denser because they can intertwine and align closely in an orderly arrangement. As a result, such polymers have a high melting point are relatively hard and crackle when crumpled. Polymers with high branching are more spaced out resulting in a larger volume. As a result, such polymers have a lower density, lower melting point, greater flexibility and softness. Chain Stiffening: The addition of a larger functional-group such as a chloride or benzene results in a less flexible polymer. Cross-linking: Cross-linking increases the hardness or rigidity of the substance. For example the addition of sulfur cross-links to rubber can improve its elasticity, as the cross-links return to original shape, and adding lots of sulfur cross-links makes the rubber rigid. Properties and Uses of Polyvinyl Chloride Polyvinyl chloride is a thermoplastic which means that it can be heated, and hence reshaped. PVC is non-corrosive, water-resistant and rigid (as C-Cl bonds have strong dispersion forces) and hence is used on many outdoor uses such as parts to windows, cable insulation and pipes. PVC is therefore combined with additives, called plasticiser to make it more flexible and hence suitable for increased applications. However, since pure PVC is susceptible to UV light, so it is typically used in underground applications, such as underground pipes or cables. PVC is therefore combined with additives to make it more resistant to UV light, and to prolonged periods of heat exposure. PVC has good insulation properties, but in order to be most effective for use in cables, combining PVC with additives is recommended. However PVC has the disadvantage of decomposing to HCl which is corrosive and a potential hazard. 8
Properties and Uses of Polystyrene Crystal Polystyrene: Crystal polystyrene is a very stiff polymer due to the presence of a benzene ring, which makes it suitable for rigid items such as car battery cases and screwdriver handles. Crystal polystyrene has a high refractive index and is fairly transparent polymer, which makes it suitable for drinking glasses and CD cases. Due to its low softening temperature, its uses in medicine are limited to disposable items. Expanded Polystyrene (Styrofoam): Expanded polystyrene is white, and is useful for sound insulation. It also has good thermal insulation properties and hence is used for holding hot food and beverages and also in the chemical and electrical industry as a calorimeter. 2.2 Biomass 1. The Need for Renewable Resources Discuss the need for alternative sources of the compounds presently obtained from the petrochemical industry Fossil fuels are non-renewable resources. This means that these resources will run out. Alternative sources that are renewable and able to produced more cheaply may be required in the future, to meet the needs of the petrochemical industry. Fossil fuels are also non-biodegradable, meaning they contribute to landfill waste, and are an increasing source of atmospheric pollution. Biodegradable materials are sought to replace fossil fuels as they can be broken down into its raw components by bacteria and enzymes, and hence do not contribute to landfill waste Biomass Biomass is organic material, derived from plant and animal sources including animal dung, organic waste from domestic and industrial sources, and plant matter such as crops and trees. One advantage it has over petroleum based sources, because it is a renewable resource and is biodegradable. Natural polymers referred to as biopolymers, are typically present in biomass. Some examples include: polysaccharides, such as cellulose and starch, which are based on saccharides (carbohydrates such as glucose) proteins, such as wool, hair and silk, which are based on amino acids nucleic acids, such as DNA and RNA 9
2. Condensation Polymers Explain what is meant by a condensation polymer Cellulose is an example of a condensation polymer. Condensation polymers refer to the product formed when two distinct molecules bond together resulting in the release of small molecules such as water, for every bond formed. For a condensation polymer to be formed, the two monomers must be symmetrical on both ends, as typically it is the presence of two different functional groups which react to produce the polymer. 3. Condensation Polymerisation Describe the reaction involved when a condensation polymer is formed Condensation polymerisation is a process typically where the functional groups react forming one bond and releasing one small molecule (typically water). In the polymerisation of β-glucose, the individual monomers first link together to form a maltosedimer, releasing water as a by-product. These maltose-dimers then link together to form a longer chain, liberating water. Many glucose units condense in this process, proceeding as a chain reaction to form long, unbranched cellulose, a polysaccharide. 4. Cellulose Describe the structure of cellulose and identify it as an example of a condensation polymer found as a major component of biomass Cellulose is the most abundant polymer and a major constituent of biomass particularly as it is major structural component of plant cell walls and also woody plants and natural fibres such as cotton, flax and hemp It is formed from β-glucose, where the glucose molecules are bonded together Properties of Glucose Cellulose is made of many molecules of β-glucose bonded together by covalent bonds: Hydrogen bonding present in cellulose makes it rigid, strong and resistance to chemicals. Within cellulose are β-linkages between two glucose molecules which result in cellulose being a flat ribbon-like strand, and also add to its rigid structure and strength. 10