Oxford Revise AQA GCSE Chemistry | Chapter 20 answers

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Mark

AO / Specification reference

01.1

pure metal has atoms/ions arranged in layers

these layers can slide, thus it is soft

alloys have distorted layers

these cannot slide, so alloys are hard

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AO1

4.10.3.2

01.2

copper, tin

copper, zinc

accept either order

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AO1

4.10.3.2

01.3

rarer

ignore references to reactivity (in the example given, copper is also not reactive)

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AO3

4.10.3.2

01.4

ignore any recurring sign shown

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AO2

4.10.3.2

01.5

high carbon steel

stainless steel

low carbon steel

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4.10.3.2

02.1

4 H atoms, 2 C atoms and 1 O atom is drawn

one C atom is joined to 2 H atoms by – and joined to 1 C atom by =

one C atom is joined to 1 O atom and 1 H atom by = and joined to 1 C atom by =

one O atom is joined to 1 C atom and 1 H atom by –

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AO2

4.10.3.3

02.2

addition

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AO2

4.7.3.1

02.3

property: more viscous/thicker/solid

explanation: borax introduces cross links between polymer chains

prevent chains from across each other

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4.10.3.3

03.1

N₂ + 3H₂ → 2NH₃

one mark for formulae

one mark for reactants

ignore state symbols

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AO2

4.10.4.1

03.2

fertilisers

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AO1

4.10.4.1

03.3

air

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AO1

4.10.4.1

03.4

six from:

• 450°C
• high temperature increases rate of reaction
• forward reaction is exothermic so high temperature shifts equilibrium/decreases yield
• 200 atm pressure
• high pressure favours the forward reaction/increases yield
• can’t be too high or it will be dangerous/expensive
• iron catalyst
• lowers rate of reaction so a lower temperature can be used and still have a reasonable rate

one mark for each correct point up to a maximum of six marks

6

AO2

4.10.4.1

04.1

nitrogen, phosphorous and potassium

all three needed for the mark

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AO1

4.10.4.2

disadvantages:

• costs money to buy
• takes time to apply to crops
• can go into water ways/cause pollution

advantages:

• increase yield of crops
• faster growing crops
• healthier crops

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AO3

4.10.4.2

04.3

2NH₄Cl + Ca(OH)₂ → 2NH₃ + CaCl₂ + 2H₂O

04.4

four from:

• in industrial process/Haber process, nitrogen from air and hydrogen from natural gas react together
• 450°C, 200 atm, iron catalyst
• ammonia is cooled, condenses and is tapped off
• ammonia in laboratory is produced as a gas
• in industry, process is continuous/in laboratory, it is produced in small batches

one point for each correct answer up to a maximum of four marks

4

05.1

The destruction of materials by chemical reactions with substances in the environment.

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05.2

hard/shiny

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AO1

4.10.3.2

05.3

iron painted/greased/electroplated

barrier protection and water accessing iron

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4.10.3.1

05.4

aluminium reacts with oxygen in the air

forms an oxide coating

that prevents further aluminium reacting with oxygen

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4.10.3.1

06.1

4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃

one mark for oxygen

one mark for balanced

2

AO1

4.10.3.1

06.2

Level 3: Three valid issues with the method are identified and improvements are clearly made. The writing is clear, coherent and logical.

5-6

AO3

4.10.3.1

Level 2: Less than three issues/ improvements are made. The writing is mainly clear, although the structure may lack logic and coherence.

3-4

Level 1: An issue is identified with an improvement made. The writing lacks clarity, coherence and logic.

1-2

No relevant content.

0

Indicative content

half fill three with water:

• this is not an accurate value of measurement, this is a control variable
• accurately measure the volume of water

put lids on A and B:

• this does not exclude air
• add in dehumidifying crystals for and boil for B

put a spoon full of salt:

• this is not an accurate measurement
• weigh salt before

add in nails:

• this does not specify what type or size of nails
• should be ungalvanized nails of a set diameter and length

no marks for any section of the answer using the word ‘amount’

06.3

E

water and air both needed for rusting

salt dissolved in water will allow electrons to move more freely than pure water

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AO2

AO3

4.10.4.1

06.4

zinc is a stronger reducing agent than iron

so electrons from zinc are lost/zinc is oxidised in place of iron

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AO1

4.10.3.1

07.1

sand and boron trioxide

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AO1

4.10.3.3

07.2

borosilicate glass has higher melting point

accept reverse answer

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AO1

4.10.3.3

07.3

Na₂CO₃

one mark for correct positive ions

one mark for correct negative ions

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AO2

4.10.3.3

07.4

fibre optic cables: germanium-oxide glass

decorative vase: crystal glass

windows: aluminosilicate glass

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AO3

4.10.3.3

08.1

2 C atoms and 4 H atoms are drawn

each C atom is joined to 1 C atom and 2 H atom by –

all atoms are inside square brackets with – extending beyond brackets

subscript n to the right of the brackets

one mark for inside brackets

one mark for brackets, line outside the brackets and n

2

AO1

4.7.3.1

08.2

2 C atoms and 2 H atoms are drawn

each carbon atom is joined to 2 H atoms by – and joined to 1 C atom by =

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AO1

4.7.3.1

08.3

LD poly(ethene):

• polymer chains are branched
• spaces between polymers

HD poly(ethene):

• fewer branches on polymer chains
• chains sit closely together

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AO1

4.10.3.3

08.4

polymer chains are not linked together by crosslinks/strong covalent bonds

so chains are relatively easy to separate

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AO2

4.10.3.3

09.1

calcium nitrate

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AO1

4.10.4.1

4.10.4.2

09.2

Ca₃(PO₄)₂

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AO2

4.10.4.1

4.10.4.2

09.3

sulfuric acid

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AO2

4.10.4.1

4.10.4.2

09.4

ammonia produced in Haber process

nitrogen from air and hydrogen from natural gas react together

iron catalyst, 450°C and 200 atm pressure

ammonia then reacted with nitric acid to form ammonium nitrate

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AO1

4.10.4.1

4.10.4.2

10.1

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AO2

10.2

+2

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AO1

10.3

calcium nitrate

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AO1

4.10.4.2

10.4

(NH₄)₃PO₄

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AO2

4.10.4.2

11.1

N₂(g) + 2H₂(g) ⇌ 2NH₃(g)

one mark for reactants

one mark for products

one mark for state symbols

3

AO2

4.10.4.1

11.2

data line at 450°C roughly halfway between 400 and 500

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AO2

4.10.4.1

11.3

45%

accept between 45 and 55%

allow error carried forward from 12.2, as long as data point comes from 200atm

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AO3

4.10.4.1

11.4

Level 3: All three conditions discussed and why it is a compromise is explained fully. The writing is clear, coherent and logical.

5-6

AO2

4.10.4.1

Level 2: Two conditions discussed fully. An attempt is made to explain why it is a compromise. The writing is mainly clear, although the structure may lack logic.

3-4

Level 1: One condition discussed fully or description of conditions, but no attempt made at explaining why it is a compromise. The writing lacks clarity, coherence and logic.

1-2

No relevant content.

0

Indicative content

• high pressure gives higher yield of ammonia favours the forward reaction
• as fewer molecules on the product side of reaction
• however, too high a pressure is expensive/dangerous
• increase in pressure from 200 to 400 (at 450°C) does not produce significant/large increase in yield
• lower temperature gives higher yield of ammonia
• because reverse reaction is endothermic
• however low temperature would have too slow a rate of reaction
• as increasing the temperature increases the rate of reaction
• iron catalyst
• increases the rate of reaction so lower temperatures can be used

11.5

ammonia is used in the production of fertilisers

fertilisers are used to improve agricultural productivity/ increase production of food/biofuels

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12.1

high-yield ores are gone/used up/hard to find

growing need in modern society for metals

traditional mining techniques will not ore on low-yield ores

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AO1

4.10.1.4

12.2

Level 3: Advantages and disadvantages are described. A judgement of the use of phytomining is given with a detailed justification made. The writing is clear, coherent and logical.

5-6

AO1

AO3

4.10.1.4

Level 2: An advantage and a disadvantage are described. A judgement of the use of phytomining is given with some justification made. The writing is mainly clear, although the structure may lack logic.

3-4

Level 1: An advantage or disadvantage is phytomining is described. The writing lacks clarity, coherence and logic.

1-2

No relevant content.

0

advantages:

• recovers valuable ore from land
• energy from burning plants can be used
• plants remove carbon dioxide from atmosphere
• can use low grade ores

disadvantages:

• burning crop release carbon dioxide into the atmosphere
• high energy use in electrolysis/smelting
• plant could be used as food
• land could be used for growing food/housing
• fertilisation of soil may lead to environmental issues (eutrophication)

answer must have an opinion and a justification

12.3

C

even though it has the lowest mass of copper

preferred climate matches the UK, so will grow quickly

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AO3

4.10.1.4

12.4

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AO2

4.10.1.4

13.1

pen ink would separate in chromatography experiment/results would not be visible/clear

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AO1

4.8.1.3

13.2

so the sample doesn’t dissolve in the solvent without moving through the paper

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AO1

4.8.1.3

13.3

to stop evaporation of the solvent or to stop contaminant falling in

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AO1

4.8.1.3

13.4

<chromatogram>

one mark for each correct substance

3

AO3

4.8.1.3

13.6

methyl red

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AO3

4.8.1.3

14.1

D

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AO1

4.7.2.1

14.2

B

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AO1

4.7.2.1

14.3

C and D

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AO1

4.7.3.2

14.4

B

poly(ethene)

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AO1

4.7.3.1

14.5

-(-CH₂-OOC-CH₂-COO-)_n–

one mark for correct use of brackets and one mark for correct inside of brackets

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AO2

4.7.3.2