A-level Chemistry Specimen mark scheme Paper 1

AQA Qualifications

A-level Chemistry

Paper 1 (7405/1): Inorganic and Physical Chemistry Mark scheme

7405 Specimen paper

Version 0.5

MARK SCHEME ? A-level Chemistry ? Specimen paper 1

Question

Marking guidance

Mark AO

Comments

01.1 This question is marked using levels of response. Refer to the Mark

Indicative chemistry content

Scheme Instructions for Examiners for guidance on how to mark this question.

6 2 AO1a

Level 3 All stages are covered and the explanation of each stage

2 AO2a Stage 1: Electrons round P

5?6 marks is generally correct and virtually complete.

2 AO2b ? P has 5 electrons in the outside shell

? With 3 electrons from 3 fluorine, there are a total

Answer is communicated coherently and shows a logical

of 8 electrons in outside shell

progression from stage 1 to stage 2 then stage 3.

? so 3 bond pairs, 1 non-bond pair

Level 2 3?4 marks

All stages are covered but the explanation of each stage may be incomplete or may contain inaccuracies OR two stages are covered and the explanations are generally correct and virtually complete.

Stage 2: Electron pair repulsion theory

? Electron pairs repel as far as possible ? Lone pair repels more than bonding pairs Stage 3: Conclusions

Answer is mainly coherent and shows progression from stage 1 to stage 3.

? Therefore, tetrahedral / trigonal pyramidal shape ? With angle of 109(.5)? decreased to 107?

Level 1 1?2 marks

Two stages are covered but the explanation of each stage may be incomplete or may contain inaccuracies, OR only one stage is covered but the explanation is generally correct and virtually complete.

Level 0 0 marks

Answer includes isolated statements but these are not presented in a logical order or show confused reasoning.

Insufficient correct chemistry to gain a mark.

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01.2 01.3

1s22s22p63s23p63d7 Too many electrons in d sub-shell / orbitals

01.4 Tetrahedral (shape) 109.5?

MARK SCHEME ? A-level Chemistry ? Specimen paper 1

1

AO1a Allow correct numbers that are not superscripted

1 AO3 1b

1

AO2a

1

AO2a Allow 109?

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MARK SCHEME ? A-level Chemistry ? Specimen paper 1

Question

Marking guidance

Mark AO

Comments

02.1 The number of protons increases (across the period) / nuclear charge

1

increases

Therefore, the attraction between the nucleus and electrons increases 1

AO1a AO1a Can only score M2 if M1 is correct

02.2 S8 molecules are bigger than P4 molecules

Therefore, van der Waals / dispersion / London forces between molecules are stronger in sulfur

02.3

Sodium oxide contains O2? ions These O2? ions react with water forming OH? ions

02.4

P4O10 + 12OH?

4PO43? + 6H2O

1

AO1a Allow sulfur molecules have bigger surface area and

1

AO1a sulfur molecules have bigger Mr

1

AO2c

1

AO2c O2? + H2O

1

AO2d

2OH? scores M1 and M2

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MARK SCHEME ? A-level Chemistry ? Specimen paper 1

Question

Marking Guidance

Mark AO

Comments

03.1

03.2 03.3

The ions in the ionic substance in the salt bridge move through the salt bridge To maintain charge balance / complete the circuit

F?

E SO42?/ SO2 < E Br2/Br?

1

AO1b

1

AO1b

1 AO3 1a

1 AO3 1a Allow correct answer expressed in words, eg electrode potential for sulfate ions / sulfur dioxide is less than that for bromine / bromide

03.4 1.23 (V)

1

AO2d

03.5 A fuel cell converts more of the available energy from combustion of

1 AO3 1b

hydrogen into kinetic energy of the car / an internal combustion engine

wastes more (heat) energy

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MARK SCHEME ? A-level Chemistry ? Specimen paper 1

Question

Marking guidance

Mark AO

04.1 Bonds broken = 2(C=O) + 3(H?H) = 2 ? 743 + 3 ? H?H Bonds formed = 3(C?H) +(C?O) + 3(O?H) = 3 ? 412 + 360 + 3 ? 463 1

?49 = [2 ? 743 + 3 ? (H?H)] ? [3 ? 412 + 360 + 3 ? 463]

3(H?H) = ?49 ? 2 ? 743 + [3 ? 412 + 360 + 3 ? 463] = 1450

1

H?H = 483 (kJ mol?1)

1

AO1b Both required

AO1b Both required AO1b Allow 483.3(3)

Comments

04.2 Mean bond enthalpies are not the same as the actual bond enthalpies

1

AO1b

in CO2 (and/or methanol and/or water)

04.3 The carbon dioxide (produced on burning methanol) is used up in this reaction

1 AO3 1b

04.4 4 mol of gas form 2 mol

1

AO2f

At high pressure the position of equilibrium moves to the right to lower the pressure / oppose the high pressure

1 AO3 1b

This increases the yield of methanol

1 AO3 1b

04.5 Impurities (or sulfur compounds) block the active sites

1

AO1b Allow catalyst poisoned

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MARK SCHEME ? A-level Chemistry ? Specimen paper 1

04.6

Stage 1: moles of components in the equilibrium mixture CO2(g) + 3H2(g) CH3OH(g) + H2O(g)

Initial

1.0

3.0

0

0

moles

Eqm moles

(1?0.86)

= 0.14

(3-3?0.86)

= 0.42

0.86

Stage 2: Partial pressure calculations

Total moles of gas = 2.28

0.86

Extended response question

1

AO2f

1

AO2f

Partial pressures = mol fraction ? ptotal pCO2 = mol fraction ? ptotal = 0.14 ? 500/2.28 = 30.7 kPa pH2 = mol fraction ? ptotal = 0.42 ? 500/2.28 = 92.1 kPa

pCH3OH = mol fraction ? ptotal = 0.86 ? 500/2.28 = 188.6 kPa pH2O = mol fraction ? ptotal = 0.86 ? 500/2.28 = 188.6 kPa

1

AO2f M3 is for partial pressures of both reactants

Alternative M3 =

ppCO2 = 0.0614 ? 500 ppH2 = 0.1842 ? 500

1

AO2f M4 is for partial pressures of both products

Alternative M4 =

Stage 3: Equilibrium constant calculation Kp = pCH3OH ? pH2O / pCO2 ? (pH2)3

ppCH3OH = 0.3772 ? 500 ppH2O = 0.3772 ? 500

1

AO2f

Hence Kp = 188.6 ? 188.6 / 30.7 ? (92.1)3 = 1.483 ? 10?3 = 1.5 ? 10?3

1

AO1b Answer must be to 2 significant figures

Units = kPa?2

1

AO2f

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Question

Marking guidance

05.1

S = 238 + 189 ? 214 ? 3 ? 131 = ?180 J K?1 mol?1 G = H ? TS = ?49 ? 523 ? (?180)

1000

= +45.1 kJ mol?1

05.2 When G = 0, H = TS therefore T = H/S = ?49 ? 1000/?180 = 272 (K)

MARK SCHEME ? A-level Chemistry ? Specimen paper 1

Mark AO

1

AO1b

1

AO1a

1

AO1b

Comments

1

AO1b Units essential

1

AO1b

1

AO1b Mark consequentially to S in 5.1

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