IGCSE Chemistry 0620 Paper 4: Organic Chemistry, Fuels and Polymers

Level: Year 10-11 (IGCSE) | Subject: Chemistry 0620 | Time: 60 minutes | Total: 100 marks | Parts: 29

Paper at a glance

Paper	Time	Marks	Calculator
Paper 4 (Extended, structured)	1 h 15 min	75	Not allowed

Questions

Question 1 (16 marks) – Fractional distillation of crude oil and the properties of alkanes

(a) Describe the principle of the fractional distillation of crude oil and state how the temperature varies inside the fractionating column. (2 marks)

(b) Explain why a fraction with longer carbon chains has a higher boiling point, a higher viscosity and ignites less easily. (3 marks)

(c) 5.80 g of butane (C4H10) burns completely in excess oxygen. Write the balanced equation and calculate the mass of oxygen needed and the mass of carbon dioxide formed (Mr: C4H10 = 58.0, O2 = 32.0, CO2 = 44.0). (4 marks)

(d) Explain the danger of incomplete combustion, name one product of incomplete combustion and state one way of reducing the risk. (4 marks)

(e) 0.0200 mol of methane reacts with excess chlorine in ultraviolet light to give chloromethane as the main product. Write the equation and calculate the mass of chloromethane formed (Mr: CH3Cl = 50.5). (3 marks)

Question 2 (16 marks) – Alkenes and addition reactions

(a) State the general formula of alkenes and explain why they are described as unsaturated hydrocarbons. (2 marks)

(b) Describe how bromine water is used to distinguish ethene from ethane and state the observations. (3 marks)

(c) 5.60 g of ethene reacts completely with hydrogen over a nickel catalyst to form ethane. Calculate the mass of hydrogen needed and the mass of ethane formed (Mr: C2H4 = 28.0, H2 = 2.0, C2H6 = 30.0). (4 marks)

(d) 200 cm3 of ethene is mixed with 300 cm3 of hydrogen and reacts completely at constant temperature and pressure. Calculate the total volume of the mixture after the reaction and the volume of each component. (4 marks)

(e) Explain why ethene decolourises bromine water but ethane does not. (3 marks)

Question 3 (18 marks) – Addition and condensation polymerisation

(a) Compare addition polymerisation with condensation polymerisation. (2 marks)

(b) A sample of poly(ethene) has an average relative molecular mass of 28000. Calculate the average degree of polymerisation (Mr: C2H4 = 28.0) and state the repeat unit of poly(ethene). (4 marks)

(c) A sample of poly(propene) has an average relative molecular mass of 126000. Calculate the average degree of polymerisation (Mr: C3H6 = 42.0) and state how the molecular formula of propene differs from that of propane. (4 marks)

(d) 1.00 mol of hexanedioic acid reacts with 1.00 mol of hexane-1,6-diamine in condensation polymerisation. Explain how the polyamide is formed and calculate the mass of water released (Mr: H2O = 18.0). (4 marks)

(e) Describe two environmental problems caused by waste plastics and suggest one practical solution for each. (4 marks)

Question 4 (17 marks) – Ethanol, fermentation and biofuels

(a) State three conditions needed for the fermentation of glucose to ethanol. (3 marks)

(b) 90.0 g of glucose is fermented completely. Write the equation and calculate the mass of ethanol formed and the volume of carbon dioxide at room temperature and pressure (Mr: C6H12O6 = 180.0, C2H5OH = 46.0; molar volume of a gas at r.t.p. = 24.0 dm3). (4 marks)

(c) 0.500 mol of ethanol burns completely. Write the balanced equation and calculate the mass of oxygen needed and the mass of carbon dioxide formed (Mr: O2 = 32.0, CO2 = 44.0). (4 marks)

(d) Explain why ethanol is often described as a carbon-neutral fuel and state one limitation of this claim. (3 marks)

(e) Give one advantage and one disadvantage of ethanol compared with petrol as a vehicle fuel. (3 marks)

Question 5 (17 marks) – Cracking and the environmental impact of fuels

(a) Define cracking and state two purposes of cracking in the petroleum industry. (3 marks)

(b) One cracking reaction of decane is C10H22 -> C8H18 + C2H4. 1.00 mol of decane cracks completely by this reaction. Calculate the mass of ethene formed and check the result by conservation of mass (Mr: C10H22 = 142.0, C8H18 = 114.0, C2H4 = 28.0). (4 marks)

(c) A batch of crude oil contains 2.0% sulfur by mass. After 1000 kg of the oil is burned, all the sulfur is converted to sulfur dioxide. Calculate the mass of sulfur dioxide formed (Ar: S = 32.0, O = 16.0). (4 marks)

(d) Explain why cracked products are more suitable than the original long-chain fractions as vehicle fuels. (3 marks)

(e) Compare thermal cracking with catalytic cracking. (3 marks)

Question 6 (16 marks) – Energy from combustion and comparing fuels

(a) 0.920 g of ethanol burns completely and raises the temperature of 200 g of water from 20.0 °C to 56.0 °C (specific heat capacity of water = 4.18 J g-1 °C-1). Calculate the heat released and the energy released per mole of ethanol in kJ/mol (Mr: C2H5OH = 46.0). (4 marks)

(b) The enthalpy of combustion of methane is -890 kJ/mol (Mr = 16.0) and that of octane is -5470 kJ/mol (Mr = 114.0). Calculate the energy released per gram of each fuel and state which gives more energy per gram. (4 marks)

(c) State four factors to consider when choosing a vehicle fuel and briefly explain each. (4 marks)

(d) 1000 g of octane burns completely. Calculate the mass of carbon dioxide formed in kg to three significant figures (Mr: C8H18 = 114.0, CO2 = 44.0). (4 marks)

Answers and marking notes

Question 1 – Fractional distillation of crude oil and the properties of alkanes

(a) Crude oil is heated and vaporised before it enters the fractionating column (one mark); the temperature falls from bottom to top, so fractions with different chain lengths condense and are collected at different heights (one mark). [2 marks]

(b) Intermolecular forces become stronger as the carbon chain lengthens, so more energy is needed to separate the molecules and the boiling point is higher (one mark); long chains entangle more easily and flow less freely, giving a higher viscosity (one mark); long chains vaporise less readily and mix more slowly with oxygen, so they ignite less easily (one mark). [3 marks]

(c) 2C4H10 + 13O2 -> 8CO2 + 10H2O (one mark); n(C4H10) = 5.80 / 58.0 = 0.100 mol (one mark); n(O2) = 0.100 x 13 / 2 = 0.650 mol, m(O2) = 0.650 x 32.0 = 20.8 g (one mark); n(CO2) = 0.100 x 4 = 0.400 mol, m(CO2) = 0.400 x 44.0 = 17.6 g (one mark). [4 marks]

(d) When oxygen is insufficient, combustion is incomplete and carbon monoxide is produced (one mark); carbon monoxide is colourless and odourless and binds to haemoglobin, lowering the oxygen-carrying capacity of the blood and can be fatal (one mark); soot particles are also formed and pollute the air (one mark); the risk is reduced by ensuring an adequate air supply, keeping the area ventilated and servicing combustion appliances regularly (one mark). [4 marks]

(e) CH4 + Cl2 -> CH3Cl + HCl (one mark); n(CH3Cl) = 0.0200 mol (one mark); m(CH3Cl) = 0.0200 x 50.5 = 1.01 g (one mark). [3 marks]

Question 2 – Alkenes and addition reactions

(a) The general formula is CnH2n (one mark); the molecules contain a carbon-carbon double bond and hold fewer hydrogen atoms than the maximum, so they can undergo addition and are called unsaturated (one mark). [2 marks]

(b) Bubble each gas through orange bromine water (one mark); ethene decolourises the bromine water (one mark); ethane does not decolourise it (one mark). [3 marks]

(c) n(C2H4) = 5.60 / 28.0 = 0.200 mol (one mark); n(H2) = 0.200 mol, m(H2) = 0.200 x 2.0 = 0.400 g (one mark); n(C2H6) = 0.200 mol (one mark); m(C2H6) = 0.200 x 30.0 = 6.00 g (one mark). [4 marks]

(d) The volume ratio of gases equals the mole ratio, and ethene reacts with hydrogen in a 1 to 1 ratio (one mark); 200 cm3 of ethene needs only 200 cm3 of hydrogen, so 100 cm3 of hydrogen is in excess (one mark); 200 cm3 of ethane is formed (one mark); the total volume afterwards is 200 + 100 = 300 cm3 (one mark). [4 marks]

(e) The double bond of ethene opens and adds a bromine molecule to form colourless dibromoethane (one mark); ethane has only single bonds and cannot undergo addition with bromine at room temperature (one mark); ethane only undergoes slow substitution in ultraviolet light, so the bromine water stays coloured (one mark). [3 marks]

Question 3 – Addition and condensation polymerisation

(a) Monomers in addition polymerisation contain a carbon-carbon double bond and no small molecule is eliminated (one mark); in condensation polymerisation two functional groups react and a small molecule, usually water, is eliminated for every bond formed (one mark). [2 marks]

(b) The repeat unit is -CH2-CH2- (one mark); the degree of polymerisation is 28000 / 28.0 (one mark) = 1000 (one mark), so on average each molecule contains 1000 ethene monomers (one mark). [4 marks]

(c) The degree of polymerisation is 126000 / 42.0 = 3000 (one mark); propene is C3H6 while propane is C3H8 (one mark); they differ by two hydrogen atoms (one mark); the difference arises because propene contains one carbon-carbon double bond (one mark). [4 marks]

(d) The carboxyl group of the acid reacts with the amino group of the diamine to form an amide linkage (one mark); one water molecule is eliminated for every amide linkage formed (one mark); 1.00 mol of the acid and 1.00 mol of the diamine form about 1.00 mol of amide linkages (one mark); about 1.00 mol of water is released, with a mass of 1.00 x 18.0 = 18.0 g (one mark). [4 marks]

(e) Plastics are not readily broken down by microorganisms and persist in landfill sites, occupying land (one mark); a solution is to sort and recycle them (one mark); burning poly(chloroethene) releases toxic gases such as hydrogen chloride and pollutes the air (one mark); a solution is to sort and recycle it, or to use an energy recovery plant fitted with gas cleaning equipment (one mark). [4 marks]

Question 4 – Ethanol, fermentation and biofuels

(a) Yeast must be present (one mark); the conditions must be anaerobic, that is oxygen-free (one mark); the temperature must be kept at about 25 to 35 °C (one mark). [3 marks]

(b) C6H12O6 -> 2C2H5OH + 2CO2 (one mark); n(glucose) = 90.0 / 180.0 = 0.500 mol (one mark); n(C2H5OH) = 1.00 mol, m = 1.00 x 46.0 = 46.0 g (one mark); n(CO2) = 1.00 mol, V = 1.00 x 24.0 = 24.0 dm3 (one mark). [4 marks]

(c) C2H5OH + 3O2 -> 2CO2 + 3H2O (one mark); n(O2) = 0.500 x 3 = 1.50 mol, m(O2) = 1.50 x 32.0 = 48.0 g (one mark); n(CO2) = 0.500 x 2 = 1.00 mol (one mark); m(CO2) = 1.00 x 44.0 = 44.0 g (one mark). [4 marks]

(d) The plants used to make ethanol absorb carbon dioxide as they grow (one mark); the carbon dioxide released when ethanol burns is roughly equal to the amount absorbed (one mark); the limitation is that growing, harvesting, distilling and transporting the crop uses fossil fuels, so there is still a net release of carbon dioxide (one mark). [3 marks]

(e) An advantage is that ethanol can be made from renewable crops, burns more completely and produces less carbon monoxide (one mark); a disadvantage is that it has a lower energy content per unit volume, so more fuel is needed for the same distance (one mark); it also competes with food production for land (one mark). [3 marks]

Question 5 – Cracking and the environmental impact of fuels

(a) Cracking is the breakdown of long-chain hydrocarbons into shorter chains at high temperature, with or without a catalyst (one mark); one purpose is to increase the supply of fuels such as petrol (one mark); another is to produce feedstock chemicals such as ethene (one mark). [3 marks]

(b) n(C2H4) = 1.00 mol, m = 1.00 x 28.0 = 28.0 g (one mark); n(C8H18) = 1.00 mol, m = 1.00 x 114.0 = 114.0 g (one mark); the total mass of reactant is 1.00 x 142.0 = 142.0 g (one mark); the products total 114.0 + 28.0 = 142.0 g, equal to the reactant, so mass is conserved (one mark). [4 marks]

(c) Mass of sulfur = 1000 kg x 2.0% = 20.0 kg (one mark); n(S) = 20000 g / 32.0 = 625 mol (one mark); S + O2 -> SO2, so n(SO2) = 625 mol (one mark); m(SO2) = 625 x 64.0 = 40000 g = 40.0 kg (one mark). [4 marks]

(d) Short-chain hydrocarbons have weaker intermolecular forces and lower boiling points, so they vaporise more readily (one mark); they mix more evenly with air in the engine and burn more completely (one mark); they have higher octane ratings and burn more smoothly, reducing knocking (one mark). [3 marks]

(e) Thermal cracking uses only high temperature and pressure, around 700 to 900 °C (one mark); catalytic cracking uses a zeolite catalyst at a lower temperature of about 500 °C and is faster (one mark); catalytic cracking can be directed more selectively towards high-octane petrol (one mark). [3 marks]

Question 6 – Energy from combustion and comparing fuels

(a) Q = mcDT = 200 x 4.18 x 36.0 = 30096 J (one mark); n(C2H5OH) = 0.920 / 46.0 = 0.0200 mol (one mark); energy per mole = 30096 / 0.0200 = 1504800 J (one mark); that is about 1505 kJ/mol (one mark). [4 marks]

(b) Methane: 890 / 16.0 = 55.6 kJ/g (one mark); octane: 5470 / 114.0 = 48.0 kJ/g (one mark); methane releases more energy per gram (one mark); methane has a higher hydrogen content, so each gram produces more water (one mark). [4 marks]

(c) Energy per gram or per litre, which affects how far the vehicle can travel (one mark); ignition and storage safety, which affects transport and fuel tank design (one mark); combustion products, which affect air quality and health (one mark); cost and security of supply, which affect everyday feasibility (one mark). [4 marks]

(d) n(C8H18) = 1000 / 114.0 = 8.77 mol (one mark); C8H18 + 12.5O2 -> 8CO2 + 9H2O, so n(CO2) = 8 x 8.77 = 70.2 mol (one mark); m(CO2) = 70.2 x 44.0 = 3088 g (one mark); that is 3.09 kg (one mark). [4 marks]

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