Cambridge O Level

CHEMISTRY 5070/21
Paper 2 Theory October/November 2022
MARK SCHEME
Maximum Mark: 75

Published

This mark scheme is published as an aid to teachers and candidates, to indicate the requirements of the
examination. It shows the basis on which Examiners were instructed to award marks. It does not indicate the
details of the discussions that took place at an Examiners’ meeting before marking began, which would have
considered the acceptability of alternative answers.

Mark schemes should be read in conjunction with the question paper and the Principal Examiner Report for
Teachers.

Cambridge International will not enter into discussions about these mark schemes.

Cambridge International is publishing the mark schemes for the October/November 2022 series for most
Cambridge IGCSE™, Cambridge International A and AS Level components and some Cambridge O Level
components.

This document consists of 12 printed pages.

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5070/21 Cambridge O Level – Mark Scheme October/November 2022
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Generic Marking Principles

These general marking principles must be applied by all examiners when marking candidate answers. They should be applied alongside the
specific content of the mark scheme or generic level descriptors for a question. Each question paper and mark scheme will also comply with these
marking principles.

GENERIC MARKING PRINCIPLE 1:

Marks must be awarded in line with:

• the specific content of the mark scheme or the generic level descriptors for the question
• the specific skills defined in the mark scheme or in the generic level descriptors for the question
• the standard of response required by a candidate as exemplified by the standardisation scripts.

GENERIC MARKING PRINCIPLE 2:

Marks awarded are always whole marks (not half marks, or other fractions).

GENERIC MARKING PRINCIPLE 3:

Marks must be awarded positively:

• marks are awarded for correct/valid answers, as defined in the mark scheme. However, credit is given for valid answers which go beyond the
scope of the syllabus and mark scheme, referring to your Team Leader as appropriate
• marks are awarded when candidates clearly demonstrate what they know and can do
• marks are not deducted for errors
• marks are not deducted for omissions
• answers should only be judged on the quality of spelling, punctuation and grammar when these features are specifically assessed by the
question as indicated by the mark scheme. The meaning, however, should be unambiguous.

GENERIC MARKING PRINCIPLE 4:

Rules must be applied consistently, e.g. in situations where candidates have not followed instructions or in the application of generic level
descriptors.

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GENERIC MARKING PRINCIPLE 5:

Marks should be awarded using the full range of marks defined in the mark scheme for the question (however; the use of the full mark range may
be limited according to the quality of the candidate responses seen).

GENERIC MARKING PRINCIPLE 6:

Marks awarded are based solely on the requirements as defined in the mark scheme. Marks should not be awarded with grade thresholds or
grade descriptors in mind.

Science-Specific Marking Principles

1 Examiners should consider the context and scientific use of any keywords when awarding marks. Although keywords may be present, marks
should not be awarded if the keywords are used incorrectly.

2 The examiner should not choose between contradictory statements given in the same question part, and credit should not be awarded for
any correct statement that is contradicted within the same question part. Wrong science that is irrelevant to the question should be ignored.

3 Although spellings do not have to be correct, spellings of syllabus terms must allow for clear and unambiguous separation from other
syllabus terms with which they may be confused (e.g. ethane / ethene, glucagon / glycogen, refraction / reflection).

4 The error carried forward (ecf) principle should be applied, where appropriate. If an incorrect answer is subsequently used in a scientifically
correct way, the candidate should be awarded these subsequent marking points. Further guidance will be included in the mark scheme
where necessary and any exceptions to this general principle will be noted.

5 ‘List rule’ guidance

For questions that require n responses (e.g. State two reasons …):

• The response should be read as continuous prose, even when numbered answer spaces are provided.
• Any response marked ignore in the mark scheme should not count towards n.
• Incorrect responses should not be awarded credit but will still count towards n.
• Read the entire response to check for any responses that contradict those that would otherwise be credited. Credit should not be
awarded for any responses that are contradicted within the rest of the response. Where two responses contradict one another, this
should be treated as a single incorrect response.
• Non-contradictory responses after the first n responses may be ignored even if they include incorrect science.

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6 Calculation specific guidance

Correct answers to calculations should be given full credit even if there is no working or incorrect working, unless the question states ‘show
your working’.

For questions in which the number of significant figures required is not stated, credit should be awarded for correct answers when rounded
by the examiner to the number of significant figures given in the mark scheme. This may not apply to measured values.

For answers given in standard form (e.g. a  10n) in which the convention of restricting the value of the coefficient (a) to a value between 1
and 10 is not followed, credit may still be awarded if the answer can be converted to the answer given in the mark scheme.

Unless a separate mark is given for a unit, a missing or incorrect unit will normally mean that the final calculation mark is not awarded.
Exceptions to this general principle will be noted in the mark scheme.

7 Guidance for chemical equations

Multiples / fractions of coefficients used in chemical equations are acceptable unless stated otherwise in the mark scheme.

State symbols given in an equation should be ignored unless asked for in the question or stated otherwise in the mark scheme.

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Question Answer Marks

1(a) Ni 1

1(b) Al 1

1(c) Ag 1

1(d) O 1

1(e) Cl 1

Question Answer Marks

2(a)(i) 2 bonding pairs between the C atom and each O atom (1) 2

4 non-bonded electrons on each O atom (1)

2(a)(ii) motion: 2
random / fast / rapid (1)

separation:
far apart (1)

2(b)(i) glucose AND oxygen 1

2(b)(ii) biological catalyst / catalyst in organisms 1

2(c)(i) water 1

2(c)(ii) (fuel) for heating / for cooking / for aircraft engines / lighting / lamps 1

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Question Answer Marks

3(a) CnH2n 1

3(b)(i) butene 1

3(b)(ii) unsaturated: 2
has C=C bond / has carbon-carbon double bond (1)

hydrocarbon:
contains only hydrogen and carbon / contains hydrogen and carbon and no other element (1)

3(c) breakdown / decomposition (1) 2

idea of long chain hydrocarbons to short chain hydrocarbons / larger hydrocarbons to smaller hydrocarbons (1)

3(d) poisonous / toxic 1

3(e)(i) division by correct relative atomic mass 2

e.g.
22.2 3.7 74.1
C= H= Br =
12 1 80
OR
1.85 3.7 0.93 (1)

division by lowest value to get correct answer

1.85 3.7 0.93
0.93 0.93 0.93

C2H4Br (1)

3(e)(ii) C6H4Br2 1

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Question Answer Marks

4(a)(i) iron 1

4(a)(ii) lowers the activation energy (of the reaction) 1

4(b) relative molecular mass of ammonium phosphate = 149 (1) 3

42
 100 OR 28% / 28.18(791946)% (1)
149

28.2% (1)

4(c)(i) calcium hydroxide / calcium oxide 1

4(c)(ii) neutralises (the acid) / removes hydrogen ions 1

4(d) reactants on the left and products on the right and reactant line above product line (1) 2

enthalpy change shown by downward arrow and labelled enthalpy change (1)

4(e) 3CuO + 2NH3 → 3Cu + N2 + 3H2O 2

correct formulae (1)

correct balance (1)

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Question Answer Marks

5(a) magnesium loses electrons and is oxidation (1) 2

iron(II) ions gain electrons and is reduction (1)

5(b) (some of the) electrons move (throughout the structure) 1

5(c) magnesium is too reactive / magnesium is very reactive / magnesium is high in the reactivity series 1

5(d) add (aqueous) sodium hydroxide / (aqueous) ammonia (1) 3

iron(II) ions give green precipitate (1)

iron(III) ions give red-brown precipitate (1)

5(e)(i) the ions can move 1

5(e)(ii) anode: chlorine / Cl2 (1) 2

cathode: magnesium / Mg (1)

5(f) does not corrode in water / does not corrode in air / unreactive 1

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Question Answer Marks

6(a)(i) position of equilibrium moves to the right (1) 2

fewer moles of gas on left of the equation / lower volume of gas on left of the equation (1)

6(a)(ii) the reaction is endothermic (1) 2

because there is a higher concentration of (NO2) at higher temperatures / there is a lower concentration of N2O4 at lower
temperatures (1)

6(b)(i) 2Pb(NO3)2 → 2PbO + 4NO2 + O2 2

correct formulae (1)

correct balance (1)

6(b)(ii) damage to tree leaves (in conifers) / kills organisms in lakes 1

6(c)(i) dissociates completely (in solution) / ionises completely (in solution) 1

6(c)(ii) any value from 0 to 2 (inclusive of these values) 1

6(c)(iii) OH– 1

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Question Answer Marks

7(a) C4H8O3 1

7(b)(i) compounds with the same molecular formula but with different structures / compounds with the same molecular formula but 1
with different arrangement of atoms (1)

7(b)(ii) structure of any isomer of butan-1-ol correctly drawn 1

7(b)(iii) butyl ethanoate (1) 2

H O H H H H
H C C O C C C C H
H H H H H (1)

7(c) HO(CH2)6OH (1) 2

HOOC(CH2)6COOH (1)

7(d)(i) clothing 1

7(d)(ii) fat 1

7(e) breakdown of a substance by water 1

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Question Answer Marks

8(a) in pure metal the layers of atoms/ions can slide (1) 3

in alloys there are different sized atoms/ions (1)

which stop the layers from sliding (1)

8(b) mercury < chromium < uranium < potassium 1

8(c) number of electrons: 78 (1) 2

number of neutrons: 119 (1)

8(d) (water of) crystallisation 1

8(e)(i) Br2(aq) + 2I–(aq) → I2(aq) + 2Br-(aq) 2

balanced equation (1)

correct state symbols dependent on correct formulae (1)

8(e)(ii) bromine is more reactive than iodine / iodine is less reactive than bromine 1

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Question Answer Marks

9(a)(i) mol H2 = 60/24 000 OR 2.5  10–3 mol (1) 3

mol HCl = M1  2 OR 5  10–3 mol (1)

concentration of HCl = 0.25 mol / dm3 (1)

9(a)(ii) rate decreases (no mark) 2

fewer particles exposed on surface (1)

frequency of collisions decreases (1)

9(b) 2H+ + 2e– → H2 1

9(c)(i) water AND oxygen 1

9(c)(ii) zinc is more reactive than iron / zinc is higher in the reactivity series than iron (1) 2

zinc corrodes in preference to iron (1)

9(c)(iii) (stops) underground pipes (corroding) / (stop) ships hulls (corroding) 1

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