Complete HSC Chemistry Notes

Transcription

1 Complete HSC Chemistry Notes Module 1 The Production of Materials The Big Picture: The current main way to get polymers is from Crude Oil. The most important part of crude oil is Ethylene as this can make polymers, or make other monomers such as vinyl chloride and styrene which then go on to make polymers. Crude oil Fractional distillation Cracking Ethylene Polymers However Crude Oil is non renewable so we are looking for other sources of the raw materials (ethylene). Cellulose (a condensation biopolymer), is a possible alternate source of ethylene. Ideally, we would go cellulose glucose ethanol ethylene, however it is very difficult to extract the glucose monomer from the cellulose polymers. So instead, we go starch/sucrose glucose ethanol ethylene Ethanol is useful because it can be dehydrated to form ethylene. Ethanol is also useful because it can be used as an alternate fuel. PLA (a biopolymer) doesn t get you ethylene, however it itself is an alternate plastic.

2 Ethylene and Fossil Fuels: Cracking: Cracking is the chemical process of breaking large hydrocarbon molecules into smaller ones. There are two types of cracking. Catalytic cracking and thermal cracking. Catalytic Cracking: Catalytic cracking is the process in which high molecular weight (high boiling point) fractions from crude oil are broken into lower molecular weight (lower boiling point) substances in order to increase the output of high demand products, Ethylene can be produced on an industrial level by the use of catalytic cracking of some of the fractions obtained by the refining of petroleum. Alkanes break down into an alkane and an alkene Catalysts used for cracking alkanes are inorganic compounds called zeolites crystalline aluminosilicates. These are porous, cage-like molecular structures, with cavities that help the reaction occur. 500 C Anaerobic conditions Just above atmospheric pressure. Thermal Cracking: Thermal Cracking is a non-catalytic process in which a mixture of alkanes are decomposed into alkenes and hydrogen gas.

3 Passed through very hot metal tubes ( C) Steam present Just above atmospheric pressure Steam is present as an inert diluent, which keeps the concentration of the reacting gases low enough to ensure that the desired reaction occurs and allows the process to operate at just above atmospheric pressure. Reactivity of Ethylene: Ethylene, like all alkenes, is an unsaturated hydrocarbon and is reactive because of its double bond. Electrophiles attack double bonds because they are electron rich areas, therefore ethylene is highly susceptible to reaction with electrophiles. The double bond means ethylene can easily have further atoms or molecules join the existing chain. Ethylene can be transformed into many useful products because of its high reactivity. While alkenes have similar properties to their corresponding alkanes (boiling points, density, solubility), alkenes are far more chemically reactive than their corresponding alkanes. Some useful products made from ethylene are: polyethylene, vinyl chloride, ethanol. Reactivity of Alkanes vs Alkenes: The reactivity of Alkanes and Alkenes can be compared by using bromine water. For the lab experiment, cyclohexane and cyclohexene were used (rather than say, hexane and hexene) because they are less of an irritant and can be used without a fume cupboard. Initially the bromine water is the bottom layer and is orange, because bromine water is orange and it has a higher density than cyclohexane or cyclohexene. In the Light: In the cyclohexane, the colour shifts to the top layer. This is because Br2 is non polar and so is cyclohexane, meaning they dissolve well, while water is polar. After a long time in the presence of UV light, the solution would go colourless. In cyclohexene, the Br2 reacts with the cyclohexene in an addition reaction. There are no orange bromine molecules anymore, but rather 1,2, dibromocyclohexene and water. This makes the solution colourless.

4 In the Dark: The cyclohexane does not ever react, staying in the split layers with colour on top. Cyclohexane still immediately has the colourless reaction occur, as it does not require the presence of UV light. To test for HBr, which is formed when cyclohexane reacts in the light, you place the mouth of the test tube next to the mouth of a bottle of ammonia. HBr(g) + NH3 (g) à NH4Br (s). This is shown through white smokey wisps produced. Alkanes have substitution reactions. These happen only in the presence of UV light. One hydrogen from the alkane is substituted with one of the atoms of the reactant (eg bromine or chlorine).

5 Alkenes undergo addition reactions. This is where the double bond is removed, allowing for more covalent bonds to occur. The reactant s atoms join to the carbons in these newly created spots. Safety: Bromine water is toxic, an irritant to the eyes and skin, and corrosive. It must therefore be handled carefully with proper safety equipment (goggles, gloves, apron) Cyclohexene and cyclohexane are toxic and flammable. They must be used in small amounts and kept clear of naked flames. They are volatile and produce respiratory irritating vapour, so experiment should be conducted in fume cupboard. Polymerisation: Polymerisation is a chemical reaction in which many identical small molecules combine to form one large molecule. The small molecules are called monomers and the large product molecule is a polymer. Addition Polymerisation: Addition polymers form by monomers adding together without the loss of any atoms. Only one type of monomer is used. This occurs when unsaturated monomers react. Polyethylene is an addition polymer. The monomer ethylene polymerises to form the polymer polyethylene. There are two main types of polyethylene High Density Polyethylene (HDPE) and Low Density Polyethylene (LDPE).

6 Making Polyethylene: Chemical Process: 1. A peroxide initiator (general formula R-O-O-R) is activated. Activation is when the O-O bond is broken, creating reactive radicals due to the electron that is unpaired. R-O-O-R à R-O + O-R 2. Initiation occurs when the double bond is broken, and an electron freed by this break moves over to join the free radical. This creates an ethylene radical, which can react with a nearby ethene molecule. 3. Propagation is the process where the molecule grows on one side, as more ethylene molecules react and join the chain. 4. Termination occurs when either another radical or another propagating chain joins from the other side, pairing up the two unpaired electrons. Propagation ceases. Physical Process: LDPE: GAS PHASE PROCESS: High pressure ( x atmospheric) High temperature (300 degrees) using an initiator. The initiator is an organic peroxide. HDPE: ZIEGLER-NATTA PROCESS: Lower pressure a few times atmospheric Lower temperature: 60 degrees, Metallocene catalyst - titanium and aluminium compopunds