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GCSE · Chemistry

C7a Alkanes, Alkenes and Crude Oil

AQA 8464 · Calculator allowed · about 85 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

Crude oil is a mixture of hydrocarbons found trapped in porous rocks. It is a finite resource.

(a)Define the term 'hydrocarbon'.(1)

(b)State what is meant by describing crude oil as a 'finite resource'.(1)

(Total for Question 1 is 2 marks)

Alkanes are a homologous series of saturated hydrocarbons. Complete the table by giving the missing name or molecular formula for each alkane.

Carbon atoms	Name	Molecular formula
1	methane	CH4
2	ethane	(a)
3	(b)	C3H8
4	butane	(c)
5	(d)	C5H12

(a)Give the molecular formula of ethane.(1)

(b)Name the alkane with molecular formula C3H8.(1)

(c)Give the molecular formula of butane.(1)

(d)Name the alkane with molecular formula C5H12.(1)

(Total for Question 2 is 4 marks)

Alkanes belong to a homologous series with the general formula CnH2n+2.

(a)Use the general formula to deduce the molecular formula of heptane, the alkane with 7 carbon atoms.(1)

(b)Draw the displayed structural formula of propane (C3H8), showing all atoms and all covalent bonds.(2)

(Total for Question 3 is 3 marks)

Required practical (technique): crude oil is separated into fractions using fractional distillation, the same distillation technique and apparatus used elsewhere in the course to separate mixtures of liquids.

(a)State the physical property of the hydrocarbons that fractional distillation separates them by.(1)

(b)Explain, in terms of intermolecular forces, why long-chain hydrocarbons condense lower down a fractionating column (nearer the bottom) than short-chain hydrocarbons.(3)

(c)A technician, Priya, is modelling fractional distillation in the laboratory using a mixture of three liquids, X (boiling point approximately 40 degrees C), Y (boiling point approximately 98 degrees C) and Z (boiling point approximately 174 degrees C). Describe how Priya could use simple distillation apparatus to separate this mixture into the three separate liquids.(3)

(Total for Question 4 is 7 marks)

Crude oil is separated into fractions, such as refinery gases, petrol, diesel and bitumen, which contain hydrocarbons of different chain lengths.

(a)Describe the trend in (i) boiling point and (ii) viscosity as the chain length of the hydrocarbons in a fraction increases.(2)

(b)Explain why fractions with shorter carbon chains are generally easier to ignite (more flammable) than fractions with longer carbon chains.(2)

(Total for Question 5 is 4 marks)

Complete combustion of a hydrocarbon fuel occurs when there is a plentiful supply of oxygen.

(a)Complete and balance the equation for the complete combustion of methane, including the missing state symbol: CH4(g) + ___O2(g) -> ___CO2(g) + ___H2O(_)(2)

(b)Complete and balance the equation for the complete combustion of propane: C3H8 + ___O2 -> ___CO2 + ___H2O(2)

(Total for Question 6 is 4 marks)

If there is a limited supply of oxygen, a hydrocarbon fuel can undergo incomplete combustion instead of complete combustion.

(a)State two products, other than water, that can form when a hydrocarbon fuel undergoes incomplete combustion.(2)

(b)Crude oil often contains small amounts of sulfur impurities. Explain how burning fuels derived from crude oil can lead to acid rain.(2)

(c)State one environmental effect of acid rain.(1)

(Total for Question 7 is 5 marks)

Propane (C3H8) undergoes complete combustion according to the equation: C3H8 + 5O2 -> 3CO2 + 4H2O. (Relative atomic masses: Ar(C) = 12, Ar(H) = 1, Ar(O) = 16.) A sample of 4.4 g of propane is burned completely, producing 7.2 g of water alongside the carbon dioxide.

(a)Calculate the relative formula mass (Mr) of propane.(1)

(b)Calculate the mass of carbon dioxide produced when the 4.4 g sample of propane is completely combusted.(3)

(c)Using the law of conservation of mass, calculate the mass of oxygen, O2, that reacted with the 4.4 g sample of propane.(2)

(Total for Question 8 is 6 marks)

Long-chain hydrocarbon fractions from crude oil are cracked to form smaller, more useful molecules.

(a)State two reasons why oil companies crack long-chain hydrocarbon fractions.(2)

(b)Describe the two sets of conditions that can be used to crack hydrocarbons.(2)

(Total for Question 9 is 4 marks)

Cracking is the thermal decomposition of long-chain hydrocarbons into shorter, more useful molecules. In a cracking reaction, atoms are conserved, so the equation must balance.

(a)Complete and balance the following cracking equation: C10H22 -> C8H18 + ____(2)

(b)Higher tier only. A sample of dodecane, C12H26, is cracked to produce propene, C3H6, and one other alkane molecule. Determine the molecular formula of this other product.(2)

(Total for Question 10 is 4 marks)

Required practical: Jamal carries out an experiment in the school laboratory to crack liquid paraffin, a mixture of long-chain alkanes, using a catalyst. Paraffin-soaked mineral wool is placed at one end of a horizontal boiling tube, with the catalyst positioned further along the tube. The tube is heated and the gas produced is collected over water.

(a)Name a suitable catalyst for this experiment.(1)

(b)Describe how Jamal should carry out this experiment, from heating the tube to collecting the gas produced.(3)

(c)Jamal bubbles the collected gas through orange bromine water. State and explain the result he should observe.(2)

(d)Explain why it is important to heat the catalyst before heating the paraffin-soaked wool, rather than the other way round.(3)

(Total for Question 11 is 9 marks)

Alkenes are a homologous series of unsaturated hydrocarbons.

(a)State the general formula of the homologous series of alkenes.(1)

(b)Name the alkene with molecular formula C4H8.(1)

(c)Give the molecular formula of propene.(1)

(Total for Question 12 is 3 marks)

Bromine water is used as a chemical test to distinguish between alkanes and alkenes.

(a)Describe a chemical test, using bromine water, that could be used to distinguish between a sample of hexane (an alkane) and a sample of hexene (an alkene), including the result expected for each.(2)

(b)Explain, in terms of bonding, why alkenes react with bromine water in this way but alkanes do not.(2)

(Total for Question 13 is 4 marks)

Ethene, C2H4, is the simplest alkene.

(a)Draw the displayed structural formula of ethene, showing the double bond between the two carbon atoms and all hydrogen atoms.(2)

(b)Write a balanced symbol equation for the addition reaction between ethene and bromine.(2)

(Total for Question 14 is 4 marks)

Crude oil is a finite resource made up of a mixture of hydrocarbons. It is separated using fractional distillation, but some of the fractions produced do not match the quantities needed by society, so they are processed further by cracking. Explain how fractional distillation separates crude oil into fractions, and explain why cracking is then used to process some of these fractions.

(Total for Question 15 is 6 marks)

The table shows the boiling points of four hydrocarbons.

Hydrocarbon	Carbon atoms	Boiling point (degrees C)
Methane	1	-162
Butane	4	-1
Octane	8	126
Hexadecane	16	287

(a)Describe the trend shown in the table between the number of carbon atoms in a hydrocarbon and its boiling point.(1)

(b)Using the trend shown in the table, predict whether a hydrocarbon with 12 carbon atoms would be a gas, a liquid or a solid at room temperature (approximately 20 degrees C). Justify your answer.(2)

(Total for Question 16 is 3 marks)

Higher tier only. When 2.90 g of a gaseous hydrocarbon, X, undergoes complete combustion, it produces 8.80 g of carbon dioxide and 4.50 g of water. (Relative atomic masses: Ar(C) = 12, Ar(H) = 1; Mr(CO2) = 44, Mr(H2O) = 18.)

(a)Calculate the number of moles of carbon atoms and the number of moles of hydrogen atoms present in the original sample of X.(2)

(b)Deduce the empirical formula of hydrocarbon X.(1)

(c)Given that the relative formula mass of X is 58, determine its molecular formula and name the compound.(2)

(Total for Question 17 is 5 marks)

At a refinery where an engineer named Aisha works, there is a surplus of long-chain fuel oil for which there is limited market demand, but there is high demand for both petrol and for ethene (used to manufacture polymers). Evaluate the economic and environmental case for the refinery investing in a catalytic cracking unit to process this surplus fuel oil.

(Total for Question 18 is 5 marks)

Mark scheme · C7a Alkanes, Alkenes and Crude Oil

Question 1

(a) B1 a compound (molecule) containing only carbon and hydrogen atoms, oe
(a) Answer: A compound containing only carbon and hydrogen atoms.
(b) B1 a resource that is being used up faster than it can be formed/replaced, so it will eventually run out, oe
(b) Answer: A resource that is being used up faster than it forms, so it will eventually run out.

Question 2

(a) B1 C2H6 cao
(a) Answer: C2H6
(b) B1 propane
(b) Answer: Propane.
(c) B1 C4H10 cao
(c) Answer: C4H10
(d) B1 pentane
(d) Answer: Pentane.

Question 3

(a) B1 C7H16 cao
(a) Answer: C7H16
(b) M1 correct skeleton of 3 carbon atoms joined in a chain by single C-C bonds
(b) A1 all 8 hydrogen atoms correctly shown, each joined to a carbon by a single C-H bond (3 H on each end carbon, 2 H on the middle carbon)
(b) Answer:H H H| | |H-C-C-C-H| | |H H H
(a chain of 3 carbons, each joined by single bonds, with 8 hydrogens shown in total)

Question 4

(a) B1 boiling point, oe
(a) Answer: Boiling point.
(b) M1 longer-chain hydrocarbon molecules have a greater area of contact (more electrons) between neighbouring molecules, oe
(b) M1 this results in stronger intermolecular forces between the molecules, oe
(b) A1 more (thermal) energy is needed to overcome these forces and evaporate/boil them apart, so they have higher boiling points and condense in the hotter, lower part of the column, oe
(b) Answer: Longer-chain molecules have stronger intermolecular forces (greater surface contact), so need more energy to boil apart, giving higher boiling points and causing them to condense lower down the column, where it is hotter.
(c) B1 heat the mixture gently in a flask fitted with a thermometer (bulb level with the side-arm) and a condenser, oe
(c) B1 when the temperature reaches about 40 degrees C, liquid X distils over and is collected in a labelled container; the container is then changed
(c) B1 heating is continued and the collecting container is changed again at about 98 degrees C (collecting Y) and again at about 174 degrees C (collecting Z), so each liquid is collected only within its own boiling temperature range, oe
(c) Answer: Heat the mixture gently using distillation apparatus with a thermometer and condenser; collect the liquid distilling over at about 40 C as X, change the container and collect at about 98 C as Y, then change again and collect at about 174 C as Z.

Question 5

(a) B1 boiling point increases as chain length increases, oe
(a) B1 viscosity increases as chain length increases, oe
(a) Answer: Both boiling point and viscosity increase as chain length increases.
(b) M1 shorter-chain molecules have weaker intermolecular forces, so are more volatile, evaporating more easily, oe
(b) A1 this means they vaporise and mix with oxygen/air more readily, so they ignite more easily, oe
(b) Answer: Shorter-chain molecules have weaker intermolecular forces so evaporate more easily, mixing readily with air/oxygen and igniting more easily.

Question 6

(a) M1 correctly balanced: CH4(g) + 2O2(g) -> CO2(g) + 2H2O(_)
(a) A1 correct state symbol (l) for water cao
(a) Answer: CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)
(b) M1 3CO2 and 4H2O (carbon and hydrogen atoms balanced)
(b) A1 5O2 added so oxygen atoms balance; final equation C3H8 + 5O2 -> 3CO2 + 4H2O cao
(b) Answer: C3H8 + 5O2 -> 3CO2 + 4H2O

Question 7

(a) B1 carbon monoxide (CO)
(a) B1 carbon/soot (particulates)
(a) Answer: Carbon monoxide and carbon (soot).
(b) B1 sulfur impurities react with/burn in oxygen to form sulfur dioxide gas, oe
(b) B1 sulfur dioxide dissolves in atmospheric water droplets/rain, forming a dilute acid, making the rain acidic, oe
(b) Answer: Sulfur impurities burn to form sulfur dioxide, which dissolves in rainwater to form a dilute acid, making the rain acidic.
(c) B1 any one of: damages/kills trees and other plants; acidifies lakes and rivers, harming aquatic life; erodes/damages limestone buildings and statues, oe
(c) Answer: E.g. it damages/kills trees, or acidifies lakes and rivers and harms aquatic life.

Question 8

(a) B1 44 cao
(a) Answer: 44
(b) M1 moles of propane = mass / Mr = 4.4 / 44 = 0.1 mol
(b) M1 moles of CO2 = 0.1 x 3 = 0.3 mol, using the mole ratio from the balanced equation (ft from their moles of propane)
(b) A1 mass of CO2 = 0.3 x 44 = 13.2 g cao (ft)
(b) Answer: 13.2 g
(c) M1 total mass of products = mass CO2 (ft from b) + mass H2O = 13.2 + 7.2 = 20.4 g
(c) A1 mass O2 = 20.4 - 4.4 = 16.0 g (awrt 16.0, ft)
(c) Answer: 16.0 g

Question 9

(a) B1 there is greater demand for shorter-chain hydrocarbons (e.g. petrol) than crude oil naturally supplies, oe
(a) B1 cracking also produces alkenes, which are used to manufacture polymers and other useful chemicals, oe
(a) Answer: Demand for short-chain fractions (like petrol) exceeds supply; cracking also produces alkenes, used to make polymers.
(b) B1 catalytic cracking uses a catalyst (e.g. aluminium oxide) with a moderately high temperature, oe
(b) B1 steam cracking uses a very high temperature mixed with steam, and no catalyst, oe
(b) Answer: Catalytic cracking uses a catalyst and a moderately high temperature; steam cracking uses a very high temperature with steam and no catalyst.

Question 10

(a) M1 carbon and hydrogen atoms conserved: 10-8=2 carbons, 22-18=4 hydrogens, giving C2H4
(a) A1 named/identified as ethene cao
(a) Answer: C2H4 (ethene)
(b) M1 carbon and hydrogen atoms conserved: 12-3=9 carbons, 26-6=20 hydrogens
(b) A1 C9H20 cao
(b) Answer: C9H20

Question 11

(a) B1 aluminium oxide (accept broken pot/pumice/porous pot)
(a) Answer: Aluminium oxide (or broken pot/pumice).
(b) B1 heat the catalyst strongly first, oe
(b) B1 then heat/move the flame along to heat the paraffin-soaked mineral wool, so that paraffin vapour passes over the hot catalyst, oe
(b) B1 collect the gas produced by displacement of water (delivery tube leading into an inverted, water-filled test tube in a trough of water), oe
(b) Answer: Heat the catalyst strongly first, then heat the paraffin-soaked mineral wool so its vapour passes over the hot catalyst, and collect the gas produced over water.
(c) B1 the bromine water is decolourised, turning from orange(-brown) to colourless
(c) B1 because the gas contains alkenes (unsaturated hydrocarbons with a C=C double bond), which react with the bromine in an addition reaction, oe
(c) Answer: The bromine water turns from orange to colourless, because the cracked gas contains alkenes, which react with bromine by addition.
(d) B1 ensures the catalyst is already at the high temperature needed for cracking by the time the hydrocarbon vapour reaches it, oe
(d) B1 this maximises the amount/rate of cracking, giving a better yield of useful products, oe
(d) B1 reduces the risk of unreacted paraffin vapour condensing and blocking the delivery tube (or another valid safety/efficiency point), oe
(d) Answer: Heating the catalyst first ensures it is hot enough for cracking as soon as paraffin vapour reaches it, maximising the yield of cracked products and reducing the risk of unreacted vapour condensing and blocking the tube.

Question 12

(a) B1 CnH2n
(a) Answer: CnH2n
(b) B1 butene
(b) Answer: Butene.
(c) B1 C3H6 cao
(c) Answer: C3H6

Question 13

(a) B1 add the hydrocarbon to bromine water and shake; the hexene decolourises the bromine water, turning it from orange(-brown) to colourless, oe
(a) B1 the hexane does not decolourise the bromine water; it remains orange(-brown), oe
(a) Answer: Shake each sample with bromine water: hexene decolourises it (orange to colourless); hexane causes no change (stays orange).
(b) B1 alkenes are unsaturated, containing a reactive C=C double bond, which reacts with bromine in an addition reaction, oe
(b) B1 alkanes are saturated, containing only single covalent bonds, so have no double bond available to react in this way, oe
(b) Answer: Alkenes have a reactive C=C double bond that undergoes addition with bromine; alkanes are saturated (only single bonds) so cannot react in this way.

Question 14

(a) M1 correct skeleton showing a C=C double bond between the two carbon atoms
(a) A1 all four hydrogen atoms correctly shown, two joined to each carbon
(a) Answer: H H
\ /
C=C
/ \
H H
(two carbons joined by a double bond, each carbon also joined to two hydrogen atoms)
(b) M1 correct reactant formulae: C2H4 and Br2
(b) A1 correct product and balancing: C2H4 + Br2 -> C2H4Br2 cao (accept displayed formula showing 1,2-dibromoethane)
(b) Answer: C2H4 + Br2 -> C2H4Br2

Question 15

Level 1 (1-2): Simple, isolated statement(s), e.g. that crude oil is a mixture separated by fractional distillation, and/or that cracking happens, with little or no linking of ideas.
Level 2 (3-4): A clear description of fractional distillation OR a clear explanation of why cracking is used, with some correct detail, but the two processes are not both explained and linked together.
Level 3 (5-6): A detailed, logically linked explanation covering both how fractional distillation separates crude oil (evaporation, temperature gradient in the column, condensing according to boiling point/chain length) and why cracking is needed (mismatch between the fractions supplied and the quantities of products in demand, so long-chain fractions are cracked to produce more useful shorter alkanes and alkenes).
Indicative content:
- crude oil is a mixture of hydrocarbons, mostly alkanes, with a range of chain lengths and boiling points
- the crude oil is heated so that it vaporises and enters the fractionating column as a vapour
- the column has a temperature gradient: hot at the bottom, cooler towards the top
- vapours rise up the column and condense when they cool to below their boiling point
- shorter-chain hydrocarbons have lower boiling points (weaker intermolecular forces), so condense higher up, further from the bottom
- longer-chain hydrocarbons have higher boiling points (stronger intermolecular forces), so condense lower down, nearer the bottom
- fractions are collected at different heights, each fraction being a mixture of hydrocarbons with similar chain lengths/boiling points
- the quantities of each fraction produced do not match the quantities that society demands, e.g. there is much greater demand for petrol/diesel and for alkenes (used to make polymers) than for some long-chain fractions
- cracking is the thermal decomposition of larger, less useful hydrocarbon molecules into smaller, more useful alkanes (e.g. for fuels) and alkenes (e.g. for making polymers)
- cracking conditions: catalytic cracking uses a catalyst and a moderately high temperature; steam cracking uses a very high temperature with steam
- cracking allows the oil industry to better match the supply of fractions to the demand from consumers and industry

Question 16

(a) B1 as the number of carbon atoms increases, the boiling point increases, oe
(a) Answer: As the number of carbon atoms increases, the boiling point increases.
(b) B1 liquid
(b) B1 justification, e.g. its boiling point would be expected to lie between that of octane (126 degrees C) and hexadecane (287 degrees C), well above room temperature, so it is not a gas; nothing in the trend suggests it would be a solid at room temperature, oe (accept other reasoned justification consistent with the trend)
(b) Answer: Liquid - its predicted boiling point (between octane's 126 degrees C and hexadecane's 287 degrees C) is well above room temperature, so it is not a gas, but the trend gives no reason to expect it is a solid.

Question 17

(a) M1 moles CO2 = 8.80 / 44 = 0.2 mol, so moles C atoms = 0.2 mol
(a) A1 moles H2O = 4.50 / 18 = 0.25 mol, so moles H atoms = 0.25 x 2 = 0.5 mol
(a) Answer: 0.2 mol carbon atoms; 0.5 mol hydrogen atoms
(b) B1 ratio C : H = 0.2 : 0.5 = 2 : 5, giving empirical formula C2H5 cao
(b) Answer: C2H5
(c) M1 mass of empirical formula unit C2H5 = (2 x 12) + (5 x 1) = 29; 58 / 29 = 2
(c) A1 molecular formula C4H10, named butane cao
(c) Answer: C4H10 (butane)

Question 18

B1 cracking would convert the low-value, low-demand fuel oil into higher-value, high-demand products such as petrol and ethene, increasing income (economic benefit), oe
B1 selling cracked products (fuels and ethene as polymer feedstock) generates more revenue than selling the fuel oil alone, which could offset the cost of the cracking unit over time (economic benefit), oe
B1 installing and running a catalytic cracking unit has significant capital and running costs, e.g. the catalyst, heating and equipment/maintenance (economic drawback), oe
B1 making better use of crude oil already extracted reduces waste and the need to extract/process more crude oil (environmental benefit), oe
B1 cracking requires large amounts of additional energy input to heat the reaction, increasing fossil fuel use and carbon dioxide emissions (environmental drawback), oe
B1 a suitably justified overall judgement, e.g. despite the extra costs and energy use, cracking is likely worthwhile as it better matches supply to demand and produces valuable feedstock for the polymer industry, oe
Answer: Award up to 5 marks for any valid points, e.g.: converts low-demand fuel oil into higher-value petrol and ethene, increasing revenue (economic benefit); has significant capital/running costs (economic drawback); reduces waste of extracted crude oil (environmental benefit); needs extra energy input, increasing emissions (environmental drawback); a supported overall judgement.

Alkanes, Alkenes and Crude Oil - Worksheets, Questions and Revision