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GCSE · Chemistry

C7 Organic Chemistry

AQA 8461/8462/8463 · Calculator allowed · about 80 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

Crude oil is an important raw material for the petrochemical industry.

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

(b)Describe how crude oil formed over millions of years.(2)

(Total for Question 1 is 3 marks)

Crude oil is separated into fractions using fractional distillation.

(a)Name the physical process used to separate crude oil into fractions.(1)

(b)Explain how fractional distillation separates crude oil into fractions, referring to the fractionating column.(3)

(Total for Question 2 is 4 marks)

Alkanes are a homologous series of saturated hydrocarbons.

(a)State the general formula of an alkane.(1)

(b)Deduce the molecular formula of the alkane that has 8 carbon atoms in its chain.(1)

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

(Total for Question 3 is 3 marks)

The table shows the boiling points of the first four alkanes.
Methane (CH4): -161 C
Ethane (C2H6): -89 C
Propane (C3H8): -42 C
Butane (C4H10): -1 C

(a)Describe the trend shown in the table between chain length and boiling point.(2)

(b)Explain, in terms of intermolecular forces, why boiling point increases with chain length.(2)

(c)Which statement about hexane (C6H14) compared with methane (CH4) is correct?(1)

A) Hexane is more flammable and less viscous than methane
B) Hexane is less flammable and more viscous than methane
C) Hexane and methane have identical viscosity and flammability
D) Hexane is a gas at room temperature, like methane

(Total for Question 4 is 5 marks)

Propane (C3H8) undergoes complete combustion in a plentiful supply of oxygen.

(Total for Question 5 is 3 marks)

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

(a)State two products, other than water, that can form during the incomplete combustion of a hydrocarbon fuel.(2)

(b)Explain why carbon monoxide gas is dangerous to humans.(2)

(c)Suggest one practical measure that reduces the risk of carbon monoxide poisoning in a home with a gas boiler.(1)

(Total for Question 6 is 5 marks)

Long-chain hydrocarbon fractions from crude oil are less useful, and in lower demand, than shorter-chain fractions, so they are processed further by cracking.

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

(b)State the conditions used in (i) catalytic cracking and (ii) steam cracking.(2)

(c)Decane (C10H22) is cracked to produce ethene (C2H4) and one other product. Complete the equation and name the other product: C10H22 -> C2H4 + ____(2)

(d)Explain why cracking is an important process for oil companies.(2)

(Total for Question 7 is 7 marks)

Required practical: A student investigates the cracking of liquid paraffin in the laboratory. Paraffin-soaked mineral wool is placed at one end of a horizontal boiling tube, with a catalyst positioned further along the tube. The tube is heated and the gas produced is collected over water.

(a)State the catalyst commonly used in this experiment and describe how the liquid paraffin is converted into a vapour that passes over it.(3)

(b)Describe and explain the result you would expect from bubbling the gas produced through bromine water, compared with the result you would expect from bubbling an unreacted sample of an alkane, such as hexane vapour, through bromine water.(3)

(c)Describe how the student could safely carry out this experiment and confirm that cracking has produced an alkene, ensuring the comparison with an unreacted alkane is a valid test.(6)

(Total for Question 8 is 12 marks)

Alkenes are a homologous series of unsaturated hydrocarbons.

(a)State the general formula of an alkene.(1)

(b)State the type of bond that makes alkenes unsaturated, and explain what 'unsaturated' means in this context.(2)

(Total for Question 9 is 3 marks)

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

(a)State what you would observe when bromine water is added to and shaken with an alkane, such as hexane.(1)

(b)State what you would observe when bromine water is added to and shaken with an alkene, such as hexene.(1)

(c)Explain, in terms of bonding, why alkenes decolourise bromine water but alkanes do not.(2)

(Total for Question 10 is 4 marks)

But-2-ene is an alkene with four carbon atoms.

(a)Draw (or describe in words) the displayed structural formula of but-2-ene, showing all atoms and bonds.(2)

(b)State the molecular formula of but-2-ene.(1)

(Total for Question 11 is 3 marks)

Ethene molecules can join together to form a polymer.

(a)Name the polymer formed when many ethene molecules join together.(1)

(b)Describe, with the aid of an equation, how ethene monomers join together to form this polymer.(3)

(c)State the type of polymerisation reaction that forms poly(ethene) from ethene.(1)

(Total for Question 12 is 5 marks)

The repeating unit of a polymer is shown below (n represents a large number): -(-CHCl-CH2-)n-

(Total for Question 13 is 3 marks)

Decane has the molecular formula C10H22. (Relative atomic masses: Ar(C) = 12, Ar(H) = 1)

(a)Calculate the relative formula mass (Mr) of decane.(2)

(b)Calculate the mass, in grams, of 0.5 mol of decane.(2)

(Total for Question 14 is 4 marks)

Propane burns completely in oxygen according to the equation: C3H8 + 5O2 -> 3CO2 + 4H2O (Relative atomic masses: Ar(C) = 12, Ar(H) = 1, Ar(O) = 16. Mr(C3H8) = 44, Mr(CO2) = 44.) A sample of 4.4 g of propane is completely combusted.

(a)Calculate the number of moles of propane in the 4.4 g sample.(2)

(b)Use the balanced equation and your answer to part (a) to calculate the mass of carbon dioxide produced.(3)

(Total for Question 15 is 5 marks)

Cracking is widely used by the petrochemical industry, but it has both benefits and drawbacks for the economy and the environment. Evaluate the economic and environmental issues associated with cracking hydrocarbons and using the alkenes produced to manufacture polymers.

(Total for Question 16 is 6 marks)

Cracking breaks covalent bonds within long-chain hydrocarbon molecules.

(a)Explain why cracking requires a high temperature and/or a catalyst to take place.(2)

(b)A molecule of dodecane, C12H26, is cracked to produce one molecule of octane, C8H18, and one other hydrocarbon molecule. Deduce the molecular formula of this other product.(2)

(Total for Question 17 is 4 marks)

Mark scheme · C7 Organic Chemistry

Question 1

(a) B1 a compound made of hydrogen and carbon atoms only
(a) Answer: A compound containing only hydrogen and carbon atoms.
(b) B1 formed from the remains of plankton/ancient marine organisms that died and sank to the seabed (oe)
(b) B1 buried under layers of mud/sediment and subjected to high temperature and pressure over millions of years (oe)
(b) Answer: Crude oil formed from the remains of ancient plankton/marine organisms that were buried under sediment and transformed by heat and pressure over millions of years.

Question 2

(a) B1 fractional distillation
(a) Answer: Fractional distillation.
(b) M1 crude oil is heated/vaporised and enters a fractionating column that is hotter at the bottom and cooler towards the top (oe)
(b) M1 different hydrocarbons in the mixture have different boiling points, related to their chain length/relative molecular mass
(b) A1 each fraction condenses and is collected at the height in the column where the temperature is at or just below its boiling point
(b) Answer: Vaporised crude oil rises up the column, which has a temperature gradient (hot at the bottom, cool at the top). Each hydrocarbon fraction condenses and is collected at the height where the column temperature matches its boiling point, so fractions with different boiling points are separated.

Question 3

(a) B1 CnH2n+2
(a) Answer: CnH2n+2
(b) B1 C8H18 cao
(b) Answer: C8H18
(c) B1 pentane
(c) Answer: Pentane.

Question 4

(a) M1 as the number of carbon atoms (chain length) increases, the boiling point increases
(a) A1 oe, e.g. reference to the specific values showing the increase, or that longer-chain alkanes are more likely to be liquids at room temperature
(a) Answer: Boiling point increases as chain length increases.
(b) M1 longer chains have a greater surface area/more electrons, so stronger London (intermolecular) forces exist between molecules (oe)
(b) A1 more energy is needed to overcome these forces, so a higher temperature (boiling point) is needed; ignore reference to breaking covalent/C-C bonds
(b) Answer: Longer-chain alkane molecules have stronger intermolecular forces (more surface contact/electrons), so more energy is required to separate the molecules, giving a higher boiling point.
(c) B1 B
(c) Answer: B) Hexane is less flammable and more viscous than methane.

Question 5

M1 3CO2 (carbons balanced)
M1 4H2O (hydrogens balanced)
A1 5O2 added so oxygen atoms balance on both sides; final equation C3H8 + 5O2 -> 3CO2 + 4H2O cao
Answer: C3H8 + 5O2 -> 3CO2 + 4H2O

Question 6

(a) B1 carbon monoxide (CO)
(a) B1 carbon/soot (C)
(a) Answer: Carbon monoxide and carbon (soot).
(b) M1 carbon monoxide binds to haemoglobin in red blood cells in place of oxygen (oe)
(b) A1 this reduces the amount of oxygen that can be carried/transported around the body, starving cells of oxygen (oe)
(b) Answer: Carbon monoxide binds to haemoglobin instead of oxygen, reducing the oxygen carried in the blood, which can starve cells of oxygen and cause suffocation.
(c) B1 install a carbon monoxide alarm/detector (oe, e.g. ensure good ventilation, have the boiler serviced regularly)
(c) Answer: Fit a carbon monoxide alarm (or ensure adequate ventilation/regular appliance servicing).

Question 7

(a) B1 the thermal decomposition of long-chain (saturated) hydrocarbons into shorter, more useful hydrocarbon molecules, including alkenes
(a) Answer: Cracking is the thermal decomposition of long-chain hydrocarbons into smaller, more useful molecules.
(b) B1 catalytic cracking: a high temperature (about 550-600 C) and a catalyst (e.g. aluminium oxide/zeolite)
(b) B1 steam cracking: a very high temperature (about 800-900 C) mixed with steam, with no catalyst
(b) Answer: Catalytic cracking uses a catalyst at about 550-600 C; steam cracking uses very high temperatures (about 800-900 C) with steam and no catalyst.
(c) M1 C8H18 (atoms conserved: 10-2=8 carbons, 22-4=18 hydrogens)
(c) A1 named as octane
(c) Answer: C8H18 (octane).
(d) M1 the demand for shorter-chain hydrocarbons (e.g. petrol) is greater than the supply obtained directly from fractional distillation, while demand for longer-chain fractions is lower than the supply (oe)
(d) A1 cracking converts less useful long-chain hydrocarbons into more useful shorter-chain hydrocarbons and alkenes (e.g. for making polymers), matching supply to demand (oe)
(d) Answer: Cracking matches the supply of hydrocarbon fractions to consumer demand by converting excess long-chain, low-demand fractions into more useful short-chain fractions and alkenes.

Question 8

(a) B1 aluminium oxide (or broken pot/pumice) used as the catalyst
(a) B1 the mineral wool is soaked in liquid paraffin
(a) B1 the mineral wool is heated (strongly), producing paraffin vapour that passes over the hot catalyst
(a) Answer: Aluminium oxide catalyst; the paraffin-soaked mineral wool is heated to produce a vapour that passes over the hot catalyst.
(b) B1 the gas produced from cracking decolourises the bromine water (orange-brown to colourless)
(b) B1 the alkane (hexane) vapour would not decolourise the bromine water; it stays orange-brown
(b) M1 the cracked product contains alkene(s), which have a C=C double bond and react with bromine by addition; the alkane is saturated (no C=C double bond) so does not react (oe)
(b) Answer: The cracked gas decolourises bromine water because it contains alkenes (the C=C double bond reacts with bromine by addition); an alkane such as hexane would not decolourise it, as alkanes are saturated.
(c) L1 (1-2): Basic, isolated statements about the experiment or safety, with little logical structure.
(c) L2 (3-4): A method with some logical steps and at least one relevant safety precaution or reference to a valid comparison, but with gaps.
(c) L3 (5-6): A clear, logically ordered method covering set-up, heating order, gas collection, safety and how the comparison is made valid.
(c) Answer: Levels of response question - no single correct answer; credit a logically ordered method covering set-up, correct heating order, gas collection over water, relevant safety precautions and a fair comparison against an unreacted alkane control. See indicative content.

Question 9

(a) B1 CnH2n
(a) Answer: CnH2n
(b) B1 a carbon-carbon double bond (C=C)
(b) B1 unsaturated means the molecule does not contain the maximum possible number of hydrogen atoms/contains at least one C=C double bond (oe)
(b) Answer: Alkenes contain a C=C double bond; unsaturated means they do not have the maximum number of hydrogens possible for that number of carbons.

Question 10

(a) B1 no colour change / bromine water stays orange(-brown)
(a) Answer: No colour change (stays orange-brown).
(b) B1 bromine water is decolourised / turns from orange(-brown) to colourless
(b) Answer: The bromine water turns from orange-brown to colourless.
(c) M1 alkenes contain a C=C double bond, which can undergo an addition reaction with bromine (oe)
(c) A1 alkanes are saturated (contain only single C-C and C-H bonds) and have no double bond available to react, so no reaction occurs (oe)
(c) Answer: Alkenes have a reactive C=C double bond that reacts with bromine by addition; alkanes are saturated with no double bond, so they do not react.

Question 11

(a) M1 correct skeleton of 4 carbon atoms joined in a chain (C-C=C-C)
(a) A1 double bond correctly placed between the second and third carbon atoms, with the correct number of hydrogen atoms shown on each carbon (CH3-CH=CH-CH3)
(a) Answer: CH3-CH=CH-CH3 (a chain of 4 carbons with a C=C double bond between carbons 2 and 3, and 3, 1, 1, 3 hydrogens on carbons 1 to 4 respectively).
(b) B1 C4H8
(b) Answer: C4H8

Question 12

(a) B1 poly(ethene)/polyethene
(a) Answer: Poly(ethene) (polyethene).
(b) M1 n ethene (CH2=CH2) molecules are represented reacting together (oe)
(b) M1 the C=C double bond in each monomer opens up and becomes a single C-C bond joining the monomers together in a long chain (oe)
(b) A1 correct repeat unit shown in brackets with n outside, e.g. n(CH2=CH2) -> -(CH2-CH2)n-
(b) Answer: n CH2=CH2 -> -(CH2-CH2)n- ; the C=C double bonds open and join the monomers into a long chain.
(c) B1 addition polymerisation
(c) Answer: Addition polymerisation.

Question 13

M1 recognise that forming the monomer reverses addition polymerisation: the single C-C bond in the repeat unit re-forms as a C=C double bond (oe)
M1 correct monomer structure drawn/described: CH2=CHCl (a C=C double bond with one Cl and one H on one carbon, and two H on the other)
A1 correctly named chloroethene (accept vinyl chloride)
Answer: Chloroethene, CH2=CHCl.

Question 14

(a) M1 correct substitution: (10 x 12) + (22 x 1)
(a) A1 = 142 cao
(a) Answer: 142
(b) M1 mass = moles x Mr = 0.5 x 142
(b) A1 = 71 g cao
(b) Answer: 71 g

Question 15

(a) M1 moles = mass / Mr = 4.4 / 44
(a) A1 = 0.1 mol cao
(a) Answer: 0.1 mol
(b) M1 correct mole ratio applied: moles CO2 = 0.1 x 3 = 0.3 mol
(b) M1 mass = moles x Mr = 0.3 x 44
(b) A1 = 13.2 g cao; ft from their moles of propane in (a)
(b) Answer: 13.2 g

Question 16

Level 1 (1-2): Simple statement(s) of an advantage or disadvantage, with little or no linking of ideas.
Level 2 (3-4): A number of relevant points made, showing some balance between economic and environmental issues, with some linking of ideas.
Level 3 (5-6): A well-balanced, logically structured evaluation covering both economic and environmental issues, leading to a supported overall judgement.
Indicative content:
- cracking allows oil companies to convert lower-demand, long-chain fractions into higher-demand, shorter-chain fractions, matching supply to demand and reducing waste
- cracking also produces alkenes, which are valuable feedstock for the polymer/plastics industry, creating additional economic value and jobs
- cracking requires high temperatures and/or catalysts, which uses significant energy, often from burning fossil fuels, contributing to carbon dioxide emissions and climate change
- crude oil is a finite (non-renewable) resource, so relying on cracking to supply useful hydrocarbons does not solve the long-term problem of crude oil running out
- polymers made from cracked alkenes (e.g. poly(ethene)) are often not biodegradable, so their disposal, e.g. in landfill, causes long-term environmental pollution
- burning waste polymers can release toxic gases, and recycling plastics can be costly and energy-intensive
- an overall judgement should weigh the economic benefits of a versatile, marketable range of products against the environmental costs of energy use, emissions and plastic waste

Question 17

(a) M1 covalent bonds between carbon atoms (C-C bonds) in the long-chain hydrocarbon must be broken (oe)
(a) A1 breaking bonds requires energy, which is supplied by heating; a catalyst provides an alternative route/lowers the energy needed for this to happen (oe)
(a) Answer: Breaking the C-C covalent bonds in the hydrocarbon chain requires energy; high temperature supplies this energy, and a catalyst lowers the amount of energy needed.
(b) M1 carbon and hydrogen atoms conserved: 12-8=4 carbons, 26-18=8 hydrogens
(b) A1 C4H8 cao
(b) Answer: C4H8 (e.g. but-1-ene, or another isomer of butene)

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