(O Level Chem) Macromolecules

1. We have seen in the chapter on Alkenes that ethene gas molecules CH₂=CH₂ can be made to undergo addition polymerization to form the addition polymer, poly(ethene) or polythene, [-CH₂-CH₂]_n where n = 2000 to 20000. There is a sharp increase in the relative molecular mass from that of ethene to that of poly(ethene). There is also a sharp increase in the melting and boiling points because the van der Waals forces between the long-chained poly(ethene) molecules require much more energy to break than the weaker van der Waals forces between the small ethene molecules.

2. Depending on the conditions used, we can produce poly(ethene) of different density. For example, to produce low density (0.91-0.94 g/cm³) poly(ethene) which is used in plastic carrier bags and cling films, a temperature of 2000 ºC and a pressure 2000 atm with a small amount of oxygen impurity are used. To produce high density (0.95-0.97 g/cm³) poly(ethene) which is used to make milk bottles, a temperature of 60 ºC, a pressure of a few atm and catalysts containing titanium compounds are used.

3. A polymer is a macromolecule or large molecule that is made up of many monomers chained up together. A monomer is a molecule that is able to bond in long chains. The monomer of poly(ethene), for example, is ethene. The linking up of monomers to form polymers is called polymerization. There are two ways of linking monomers, namely, addition polymerization and condensation polymerization.

4. Addition polymerization occurs due to the presence of C=C functional group in monomers. One example of addition polymers, other than poly(ethene), is polystyrene which is used to make food containers. The monomer of polystyrene is phenylethene or styrene which contains the C=C bond. Other examples include PVC (polyvinyl chloride or poly(chloroethene)) which is used to make water pipes, PTFE (poly(tetrafluoroethene)) which is used as the non-stick coating on cooking utensils, and PP (poly(propene)). Draw the structures of the monomers of PVC, PTFE and PP. All these monomers contain the C=C bond.

5. Addition polymers contain saturated C-C bonds. The strong C-C covalent bonds require a lot of energy to break making addition polymers generally unreactive. The advantage of unreactivity is that addition polymers are safe and durable. Unfortunately, addition polymers are non-biodegradable which means that they cannot be decomposed naturally by bacteria. If addition polymers are irresponsibly disposed of, they would stay for many years causing land and water pollution, and harming wildlife. Addition polymers are usually disposed of by burying in landfills, burning in incinerators and recycling. All these ways of disposing addition polymers have their disadvantages. Landfills take up precious land, incineration releases carbon dioxide and toxic gases and recycling is difficult and expensive.

6. In condensation polymerization, the monomers have two functional groups at both of its ends. The condensation polymers that we are learning are nylon and Terylene which are man-made fibres used in clothing, curtain materials, fishing line, parachutes and sleeping bags. Carbohydrates, proteins and fats are natural condensation macromolecules which are not in the syllabus.
   

7. There are two monomers of nylon, namely a dicarboxylic acid and a diamine. The dicarboxylic acid has one -COOH functional group on each of its two ends. The diamine has one -NH₂ on each of its two ends. The carboxylic acid and amine functional groups react to form an amide linkage -CONH₂- and a water molecule. Nylon is said to be an example of polyamides because of the amide linkages. Because a small molecule is formed as a by-product, the reaction is described as a condensation reaction. For your information, proteins also have amide linkages but their monomers, called amino acids, have one -COOH group on one end and one -NH₂ group on the other end.

8. Nylon is tough, lightweight and waterproof. Because nylon does not let water vapor through, nylon waterproof clothing traps sweat, making it unpleasant to wear. Is nylon biodegradable? Nylon takes 30 to 40 years to biodegrade but this is still less than 500 years for poly(ethene).

9. There are two monomers of Terylene. One of the monomers is also a dicarboxylic acid, as in nylon. The other monomer is a diol which contains two alcohol -OH groups on both of its ends. We have seen esterification in the chapter on Carboxylic Acids and Alcohols in which an alcohol and a carboxylic acid react to form an ester and a water molecule. Similarly, in the formation of Terylene, ester linkages -COO- and water molecules are formed. Terylene is said to be an example of polyesters because of the ester linkages.
   

(O Level Phy) Practical Electricity Teaching & Learning Notes

1. Power stations supply electricity for households, factories and office buildings. Fuel power stations burn fossil fuels (gas, oil, coal) while nuclear power stations make use of nuclear fission of uranium-235 to produce electrical energy. Fossil fuels and uranium-235 are non-renewable because they cannot be replaced. On the other hand, the hydroelectric, solar and wind power stations make use of renewable sources of energy. Hydroelectric power stations, for example, change kinetic energy of running water into electrical energy. Different solar power stations use different ways of changing solar energy to electrical energy. Wind power stations change kinetic energy of wind to electrical energy. One disadvantage of fuel power stations is that they produce air pollutants. Power stations that use renewable sources of energy do not produce air pollutants. 

2. Different power stations have different energy conversion efficiencies. Power plant efficiency is the ratio between the rate of useful electricity output and the rate of energy supplied by the source. For example, a hydroelectric power station may have an efficiency of 95% while an oil power station may have an efficiency of 45%. Why are their efficiencies so different? The hydroelectric power station uses running water to spin its turbine. The oil power station, on the other hand, uses steam to spin its turbine. Much energy has been lost during energy conversions from the oil to the steam and to the turbine. This makes the oil power station less efficient than the hydroelectric power station. Which power station, the hydroelectric station or the oil power station, do you think, would charge more per unit kWh of energy?

3. Electricity can be converted to other forms of energy by using different electrical components. These electrical components make use of different effects of electricity. A heating element uses the heating effect of electricity to convert electrical energy to heat. Heating elements can be found in electric kettles, electric irons and water heaters. A filament lamp uses the heating and lighting effects of electricity to convert electrical energy to light and heat. An electric motor uses the magnetic effect of electricity to convert electrical energy to mechanical energy. What electrical appliances make use of motors? 

4. The power of an electrical component is the rate of conversion of electrical energy into another form of energy. Its SI unit is the Watt which is equivalent to J/s. For example, an electric lamp that is rated as “60 W” converts 60 J of electrical energy to heat and light in 1 s. 

5. Using the definition of potential difference (V = W/Q), we can show that power (P = E/t) is the product of current and voltage i.e. P = IV. We can further derive P = I²R and P = V²/R by inserting the Ohm's Law relationship. Let's apply these formulas in this question. If a small heater operates at 12 V, 2 A, how much energy will the heater use if it is run for 5 minutes?

6. The SI unit of energy is the Joule. The commercial unit of energy, however, is the kilowatt hour (kWh). The kilowatt is the unit of power while the hour is the unit of time. Since energy = power x time, the kWh is a unit of energy. Knowing that 1 kW = 1000 J/s and 1 h = 3600 s, we can easily convert between J and kWh. Now, let's suppose you have an electric cooker that has a hotplate rated at 1500 W and an electric oven rated at 2000 W. During a day, the hotplate is switched on for a total of 1.0 h and the oven is switched on for 3.0 h hours. What is the cost of using the electric cooker during one day if the cost of electricity is 24 cents per kWh?

7. Electrical hazards include electric shocks and electric fires. Electric shocks are caused by damaged insulation and damp or wet conditions. Short circuits between the live and neutral wire occurs when there is damaged insulation. If you happen to touch the metal casing of an appliance that is “live” due to damaged insulation, you would get an electric shock. Damp conditions make it worst because water lowers the electrical resistance of our body from 500 000 Ω to 1000 Ω, making our body a better conductor of electricity. Damaged insulation can also cause electric fires. The large current that results from short circuits produce enough heat to start a fire. Another cause of electric fires is overloading. A power outlet that is overloaded tends to overheat because it draws in current greater than what the electrical wires can safely carry. Note that thinner wires produce heat faster (P = I²R) than thicker wires because thinner wires have greater resistances (R  = ρl/A).

8. The mains three-pin plug contains the brown Live wire, the yellow-green Earth wire and the blue Neutral wire. It also has a fuse that is connected to the Live wire which is at a high voltage, typically at 240 V. The neutral wire which is at 0 V completes the circuit and enables current to flow through the appliance. The Earth wire is the safety wire that conducts current to the earth in case the metal casing becomes “live”. While the Live and Neutral wire should never be in contact with the metal casing of appliances, the Earth wire is always connected to the metal casing. If the appliance has a double insulation, however, there would not be a need for earthing. That is why you see electric irons have a three-pin plug while the hair dryer which has a plastic casing has a two-pin plug.

9. Fuses and circuit breakers are safety devices used to protect appliances from damage caused by excessively large currents. Inside the fuse, there is a short wire that heats up and melts if current exceeds its fuse rating. Once the fuse melts, it has to be replaced. We should choose a fuse rating which is slightly larger than the current that normally flows through the appliance. Common fuse ratings include 3 A, 5 A and 13 A. Let's suppose you have an appliance that uses 3 A. What is the most suitable fuse rating and in which wire should the fuse be connected?

10. One common question is “How do the fuse and Earth wire work together as a safety precaution to prevent electric shocks when using electrical appliances?” If the metal casing becomes “live” due to damaged insulation, the low resistance Earth wire draws a very large current from the Live wire causing the fuse to melt and the circuit to be switched off.

11. Unlike the fuse, the circuit breaker, on the other hand, can just be reset and so does not need to be replaced. Inside the circuit breaker, there is an iron lever next to an electromagnet. The electromagnet is a coil of insulated wire that is wound around a soft iron core. The core magnetizes when a current flows through the coil of wire. If the current is large enough, the electromagnet will be strong enough to attract the iron lever. As the iron lever is pulled away from a contact, the circuit is broken. Note that fuses and circuit breakers are wired into the Live conductor that is at a high voltage. Now, design your own circuit breaker. You should include an  electromagnet, an iron lever that can rotate about a pivot and that makes or breaks a contact, and a spring that is attached to the lever.

12. The household electric circuit consists of the lighting circuit and a ring socket circuit that are both parallel circuits and are both connected to the electricity meter and consumer unit. The electricity meter records the electrical energy used in kWh. The consumer unit contains the main switch and some fuses and circuit breakers. The current in the lighting circuit is much lower than the current in the ring circuit. That is why the lighting circuit uses a lower fuse rating (5 A fuse) than the ring circuit (30 A fuse). That is also why thicker wires (2.5 mm²) are used in ring circuits than in lighting circuits (1.5 mm²). In the ring circuit, the Live wire, Neutral wire and Earth wire goes round the household in the loop without connection to one another. The Live and Neutral wires are connected only when an appliance is plugged to a socket and switched on. Now, draw a table to compare between the lighting circuit and the ring socket circuit. 

(O Level Chem) Alcohols and Carboxylic Acids Teaching & Learning Notes

1. We have seen in the chapter on alkenes that ethene C₂H₂ can be changed to ethanol C₂H₅OH by passing a mixture of ethene and steam over H₃PO₄ catalyst at 300 ºC and 70 atm. Ethanol is used as a solvent in perfumes and varnishes, and a useful fuel in many cars manufactured in the US. The ethanol that is present in alcoholic drinks such as wine and beer is made by fermentation of glucose.

2. Ethanol CH₃CH₂OH is the second member of a homologous series of alcohols which has a general formula of C_nH_2n+1OH. All alcohols contain the functional group -OH which is responsible for the chemical properties of alcohols. The first member of alcohols is methanol CH₃OH which contains only one carbon atom (n = 1). The third (n = 3) and fourth (n = 4) members of the alcohols are propanol and butanol respectively. When n is greater than 2, isomerism exists. Complete the following table and try to draw out all the full structural formulas of these alcohols including the isomers of propanol and butanol. 

Name	Number of carbon atoms, n	Molecular formula	Relative molecular mass	Structural formula
Methanol			32
Ethanol	2			CH3CH2OH
Propanol		C3H8O
Butanol				CH3CH2CH2CH2OH

3. Generally, down the series when the molecular mass of the alcohol increases, the boiling point increases. For example, methanol (n = 1) boils at 68 ºC, ethanol (n = 2) boils at 78 ºC and butanol (n = 4) boils at 118 ºC. Isomers however have different boiling points even though they have the same molecular formula. For example, propan-1-ol boils at 97.2 ºC but propan-2-ol boils at 82.3 ºC.

4. Alcohols, like alkanes and alkenes, are flammable. Ethanol, for example, undergoes complete combustion in sufficient supply of oxygen to produce carbon dioxide and water. Write down the balanced equation for the combustion of 1 mole of ethanol. Use the following information to calculate the enthalpy of combustion of 1 mole of ethanol. You should get -1020 kJ/mol. Because ethanol is runny and volatile and its combustion is so exothermic, ethanol can be used as  a fuel for motor vehicles. What is one advantage of using ethanol over fossil fuels to run motor vehicles?

Bond	Bond energy (kJ/mol)
H-C	413
C-C	348
C-O	743
O-H	463
O=O	498
C=O	743

5. Laboratory experiments sometimes use ethanol. For example, in Biology experiments, ethanol is used as a solvent to dissolve the green pigment, chlorophyll, in leaves. Care must be taken to keep ethanol from open Bunsen flames because ethanol is flammable. If you happen to spill ethanol on the laboratory bench and it catches fire, do not pour water because, water, being denser than ethanol would spread the ethanol over a larger area. Instead, you should use the fire blanket to put out the fire.

6. Besides undergoing combustion, alcohols can also be oxidized by atmospheric oxygen to form weak acids called carboxylic acids. This oxidation can also be achieved by other oxidizing agents such as acidified potassium dichromate or acidified potassium permanganate. For example, ethanol C₂H₅OH can be oxidized by acidified potassium dichromate to ethanoic acid CH₃COOH. In this redox reaction, the orange dichromate is changed to green Cr³⁺(aq). This reaction is used in breathalyzers to test for alcoholic breathe in drivers. Why is this a redox reaction? If potassium permanganate is used, what do you think would be observed?

7. Ethanoic acid is the second member of another homologous series called carboxylic acids which has the general formula, C_nH_2n+1COOH. The first member, methanoic acid HCOOH (n = 0), commonly called formic acid, boils at 101 ºC. Note that in this case, n for the first member is not 1. Ethanoic acid CH₃COOH (n = 1), commonly called acetic acid, boils at 118 ºC while propanoic acid CH₃CH₂COOH (n = 2) boils at 141 ºC. Therefore, like other homologous series, the boiling point of carboxylic acids increases as relative molecular mass increases. Try to draw out the full structural formulas of the first few members of the carboxylic acids.

8. The first few members of carboxylic acids are soluble in water. Down the series, however, the solubility of carboxylic acids decreases. From n = 9 onwards, the carboxylic acids become insoluble in water. The insoluble carboxylic acids which have long chain of carbon atoms are often referred to as fatty acids.

9. Fatty acids are said to be saturated if they contain only C-C single bonds, and are said to be unsaturated if they contain C=C double bonds. Do you still remember, from the topic on alkenes, how you can change unsaturated fatty acid to saturated fatty acid? What catalyst do you use?

10. The functional group of carboxylic acid is -COOH which is responsible for its acidic properties. Therefore, carboxylic acids can react with alkalis to form a salt and water, with metals to evolve hydrogen gas, and with carbonates to evolve carbon dioxide. Make sure you know how to describe the tests for hydrogen gas and carbon dioxide gas. Try to write out the chemical equations for the reaction between carboxylic acid with sodium hydroxide, with magnesium, and with calcium carbonate. Note that ethanoate CH₃COO^- has a charge of +1.

11. Carboxylic acids are, however, weak acids meaning that they dissociate partially to give H⁺(aq). In an aqueous solution of ethanoic acid, for example, only about 0.4% of the ethanoic acid molecules dissociate to become ethanoate ions CH₃COO^-(aq) and protons H⁺(aq). Vinegar, which contains about 1.0 mol dm^-3 of ethanoic acid, has a pH of 2.4. 

(12 and 13 are only for Pure Chemistry)

12. If we heat ethanoic acid and ethanol in the presence of sulfuric acid catalyst under reflux as shown below, we will get a sweet-smelling compound, called ethyl ethanoate CH₃COOC₂H₅, that floats on top of the reaction mixture. Ethyl ethanoate is a class of compounds called esters which contain the -COO- ester linkage. The reaction that forms esters is called esterification. At the same time, water is formed as a by-product. Since a small molecule is formed as a by-product, esterification is also described as a condensation reaction. Note that the reaction is reversible. To prevent the reaction from going backward reforming the reactants, concentrated sulfuric acid, which is a dehydrating agent, is used to absorb the water.

13. We can produce different esters by heating different alcohols and carboxylic acids using the same conditions. Try writing chemical equations for the formations of different esters. Esters tend to smell very pleasant and so they are used as perfumes and flavorings. They are also used as solvents such as in inks and adhesives as they can dissolve many organic compounds. 

Now, try to draw out the flow chart of reactions involving ethane, ethene, ethanol and ethanoic acid.

(O Level Chem) Alkanes and Alkenes Teaching & Learning Notes

1. Carbon is a Group IV and Period 2 element and so it has four valence electrons and two shells. Using its four valence electrons, a carbon atom is able to form a maximum of four covalent bonds. After forming four bonds, its shell is completely full and so the carbon atom can never form five or more bonds. Carbon is well known for its ability, called catenation, to form covalent bonds with other carbon atoms to form large structures such as diamond and graphite and long chains of organic compounds. Living things are made of carbohydrates, proteins and fats that are long-chain carbon compounds.

2. Fossil fuels include natural gas, crude oil and coal. They are called fossil fuels because they were made naturally from decomposing prehistoric plant and animal remains. Since living things are made of compounds of carbon, fossil fuels are made of compounds of carbon too. Coal is a black colored rock mainly made of carbon. Natural gas is a mixture of hydrocarbons consisting mainly of methane. Crude oil is a thick, often black, liquid consisting of various hydrocarbons of different relative molecular masses.

3. Hydrocarbons are covalent compounds. Between atoms in the hydrocarbon molecules, there are strong covalent bonds. Between molecules, however, there are weak van der Waals forces. The larger the hydrocarbon molecule, the greater the van der Waals forces which require more energy to overcome. That is why hydrocarbons with greater molecular masses have higher melting and boiling points than hydrocarbons with lower molecular masses. Now, arrange the compounds, butane C₄H₁₀, heptane C₇H₁₆, and methane CH₄, in order of increasing melting and boiling points.

4. Crude oil needs to be separated into fractions that are useful to us. Since it is a mixture of hydrocarbons with different boiling points, crude oil can be separated using fractional distillation. At the oil refinery, crude oil is heated to about 350 ºC to evaporate most of its compounds. The gases are pumped into the bottom of a fractionating tower about 10 m high. Hydrocarbons with higher boiling points will condense first and are collected at the lower parts of the tower. Hydrocarbons with lower boiling points will rise to the cooler top of the tower before they condense. The fractions obtained include petrol or gasoline (C₅-C₁₀, bp~120 ºC) used as a fuel in cars, naphtha (C₅-C₉, bp~70 ºC) used as feedstock for the chemical industry, kerosene or paraffin (C₁₀-C₁₆, bp~170ºC) for heating and cooking and for aircraft engines, diesel (C₁₄-C₂₀, bp~270 ºC) for diesel engines, lubricating oil (C₂₀-C₅₀) used as lubricants, polishes and waxes. The residue (>C₇₀), bitumen, would be used for making road surface.

5. A homologous series is a group of compounds with a general formula and similar chemical properties. The compounds in a homologous series show a gradual change in physical properties. For example their melting points and viscosity increase and flammability decreases as their molecular mass and size of molecules increase. The homologous series that we will study in O-level are alkanes, alkenes, alcohols and carboxylic acids. Alkanes and alkenes are hydrocarbons because they contain hydrogen and carbon only. Alkanes are also called saturated hydrocarbons because they only contain C-C single bonds, while alkenes are called unsaturated hydrocarbons because they contain C=C double bonds.

6. Alkanes is a homologous series of saturated hydrocarbons with a general formula of C_nH_2n+2. The first member of alkanes is methane CH₄ which has the lowest melting and boiling points. Methane is easily burned and so we say it is very flammable. The next three members are ethane C₂H₆, propane C₃H₈ and butane C₄H₁₀ which are also flammable gases. Pentane C₅H₁₂ and hexane C₆H₁₄ are flammable liquids. The greater the molecular mass of the liquid alkanes, the more viscous and the less flammable they are. The following diagram shows the full structural formulas of straight-chained alkanes up to butane. In drawing full structural formulas, all the bonds must be shown. Now, it is your turn to draw the straight-chained pentane and hexane. Check that all the structures of alkanes agree with the general general formula C_nH_2n+2.

7. Straight-chained alkanes are unbranched alkanes. From C₄ onwards, branched alkanes exist. This is an example of isomerism. Isomers are molecules with the same molecular formula but different structural formulas. Butane C₄H₁₀, for example, can only form two isomers, n-butane which is the unbranched butane, and isobutane which is the branched butane. Pentane C₅H₁₂can form three isomers, namely n-pentane, iso-pentane and neo-pentane. The more carbon atoms there are in the alkane, the more isomers it can form. Try drawing the isomers of hexane C₆H₁₄. How many isomers can hexane form?

8. Alkanes are generally less reactive than other classes of organic compounds because it requires a lot of energy to break their strong C-H and C-C covalent bonds. Alkanes can undergo two kinds of reactions, namely, combustion and free radical substitution. In plentiful supply of oxygen, methane burns to form carbon dioxide and water. If the supply of oxygen is limited, then incomplete combustion occurs and the poisonous carbon monoxide is produced. Use the following values to calculate the enthalpy of combustion of 1 mole of methane. The incomplete combustion of methane is less exothermic than its complete combustion. 

Bond	Bond energy (kJ/mol)
O=O	498
C-O	358
O-H	463
C=O	732
C-H	413

9. If methane and chlorine gases are mixed together in the dark at room temperature, there is no reaction. In the presence of ultraviolet, however, a chain reaction occurs. The ultraviolet has provided the activation energy for the reaction to occur by breaking the Cl-Cl bond to form the reactive chlorine atoms. This is the initiation stage. The Cl atom is called a free radical because it has an unpaired valence electron. A chlorine radical is written as Cl•. With the Cl• radical, the propagation to change methane CH₄ into methyl chloride CH₃Cl occurs. The Cl• is reformed to attack more CH₄ molecules. In effect, the Cl• radical acts as a catalyst to change CH₄ to CH₃Cl. The reaction stops or terminates when radicals combine together to form molecules. Write down the overall equation for the chain reaction by combining equations 2 and 3 together.

10. Alkenes are unsaturated hydrocarbons with the general formula C_nH_2n. The functional group of alkenes is C=C. Alkenes are called unsaturated because the carbon atoms are not bonded to the maximum number of 4 atoms. The first member of the alkenes has two, not one, carbon atoms and it is called ethene C₂H₂. Note the difference that the names of alkanes end with -ane while the names of alkenes end with -ene. Draw the structural formulas of unbranched but-1-ene, pent-1-ene and hex-1-ene. The number “1” in the names indicate that the C=C is at the first carbon atom. The melting and boiling points and viscosity increase down the series of alkenes, just like the trend in alkanes.

11. Like alkanes, alkenes undergo combustion to form carbon dioxide and water. Practice writing out the chemical equation for the complete combustion of ethene. Alkenes typically undergo a type of reactions called addition since two more atoms can be attached to the carbon atoms of each C=C bond. Although the C=C bond (bond energy = 610 kJ/mol) is stronger than the C-C bond (bond energy = 350 kJ/mol), alkenes are more reactive than alkanes. This is because one of the two bonds in C=C is easily broken.

12. When ethene and hydrogen are passed over nickel catalyst at 150 ºC, the hydrogen is added to the C=C and the saturated ethane is formed. This addition reaction is also called hydrogenation. Solid margarine is manufactured from liquid vegetable oils by hydrogenation too. Vegetable oils are polyunsaturated meaning that the chain of molecules contains several C=C bonds. 

13. When a mixture of gaseous ethene and steam is passed over a catalyst of phosphoric(V) acid at 300 ºC and 60-70 atm, ethanol C₂H₅OH is formed. This is the industrial manufacture of ethanol from ethene. In the chapter on alcohols, you will learn about fermentation which is another method of making ethanol. Note that ethanol can also be changed back to ethene by dehydration using concentrated sulfuric acid at 170 ºC.

14. How do we test for the presence of alkenes? In other words, how do you test for unsaturation in an organic compound? We use bromine. When ethene, for example, is blown into bromine that is  dissolved in an organic solvent such as tetrachloromethane under standard laboratory conditions with no ultraviolet, the reddish-brown liquid is decolorized. 1,2-dibromoethane, which is the addition product, is formed. Note that if aqueous bromine is used instead, the product is 2-bromo-ethanol.

15. How is ethene manufactured? Crude oil, unfortunately, contains too many large hydrocarbons which are useless because they are viscous and are not flammable. There is not enough small hydrocarbons to meet demand. Cracking is needed to break large hydrocarbon molecules to produce small hydrocarbons such as ethene. For example, C₁₅H₃₂ is vaporized and passed over a very fine zeolite catalyst powder at 500 ºC to form ethene, propene and octane C₈H₁₈. Try to write down the equation for this reaction which is an example of thermal decomposition. Cracking can also be done in the laboratory. 

16. The ethene and propene produced from the cracking of C₈H₁₈ are useful for making plastics. At high pressure of 10-80 atm and 70-300 ºC in the presence of an aluminum-based catalyst, the gaseous ethene is converted to white solid pellets of poly(ethene) which is our familiar plastic. The ethene molecules have joined together by addition reaction. Each poly(ethene) molecule can be made by 1000 to 1,000,000 ethene molecules and so there is a sharp increase in molecular mass from reactant to product. Poly(ethene) is an example of polymers which are very long molecules made up of repeating units called monomers. The formation of polymers by the addition of unsaturated monomers is called addition polymerization. Try to show the equation for the addition polymerization of propene. Use “n” to denote a very large number.

Draw a flow chart to show all the reactions of ethane and ethene that you have learned.