OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2020: Question 16

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Question Answer Marks Guidance

element

16 (a) (The mean/average mass) taking into account the 1 1.1 ALLOW

relative abundancies of the isotopes sum of (isotopic mass × %abundance)

sum of (isotopic mass × abundance) / total

abundance

DO NOT ALLOW average mass of the isotopes

(i) 2+ – 2 ALLOW 8 electrons in Mg2+ BUT ‘extra’ electron

Mg x Br in Br– must match symbol for electrons in Mg2+

– IGNORE inner shells and circles

x Br

ALLOW 1 mark if both electron arrangements

Mg with no (or 8) outer electrons and charges are correct but only one Br is

AND drawn.

2 × Br with ‘dot-and-cross’ outer octet 1.2

ALLOW 2[Br–], 2[Br]– (brackets not required)

Correct charges 2.5

(ii) FIRST CHECK THE ANSWER ON ANSWER LINE 3 2.2×3 ALLOW ECF

If answer = 1.71 × 1022 award 3 marks

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1.74 Calculator answer = 9.451385117 × 10–3

n(MgBr2) = = 0.00945….. mol

184.1

ALLOW ECF from incorrect moles of ions.

Moles of ions = 0.00945…. × 3 = 0.0283…. mol e.g. 0.00945

Common error

23 22 5.69 × 1021 no × 3 2 marks

Number of ions = 0.0283... × 6.02 × 10 = 1.71 × 10

3SF required

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element

(c)* Refer to marking instructions on page 5 of mark scheme for 6 1.1×3 Indicative scientific points may include:

guidance on marking this question. 2.1×3

Structure and bonding

Level 3 (5–6 marks) Magnesium

Explains all three melting point values and conductivities in • Structure: giant lattice

terms of structure, bonding, particles and relative strengths of • Metallic bonding

the f orces. • Delocalised electrons

There is a well-developed line of reasoning which is clear and Bromine

logically structured. The information presented is relevant and • Structure: simple molecular

substantiated.

• induced dipole dipole forces (London forces)

Level 2 (3–4 marks) • (Between) molecules

Attempts to explain all three melting point values and DO NOT ALLOW (between) atoms

conductivities in terms of the structure, bonding, particles of all

three substances, but explanations may be incomplete or may Magnesium bromide

contain only some correct statements or comparisons. • Structure: giant lattice

OR • Ionic bonding

Correctly explains two of the melting point values and

conductivities in terms of the structure, bonding, particles. • (Between) oppositely charged ions

Comparison of bond strengths

There is a line of reasoning presented with some structure.

The information presented is relevant and supported by some • Metallic and ionic bonds are stronger than

evidence. London forces

OR Metallic and Ionic bonds need more energy to

Level 1 (1–2 marks) break than London forces

Identif ies only some of the structures, forces and particles

AND Conductivity

Attempts to explain the melting point values OR conductivities • Magnesium: conducts due to delocalised

in terms of the structure, bonding, particles electrons can move/mobile.

IGNORE ‘Carry’ charge for movement

There is an attempt at a logical structure with a line of

reasoning. The information is in the most part relevant. • Magnesium bromide: In solid IONS cannot move;

in solution IONS can move.

0 marks DO NOT ALLOW electrons.

No response or no response worthy of credit. • Bromine: Does not conduct as no mobile charge

carriers.

7 AO

element

(d) (i) Mg2+(g) + 2Br(g) + 2e– 2 1.2× 2 State symbols required.

Mg(s) + Br2(l) CARE: Liquid state symbol for Br2

(ii) FIRST CHECK THE ANSWER ON ANSWER LINE 2 2.2×2 ALLOW -347 (kJ mol–1) for 2 marks.

If answer = –346.5 award 2 marks

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2∆H hyd = ALLOW f or 1 mark ONE error with sign OR use of 2:

– 525 – 186 – (2 x 112) – 148 – 736 – 1450 + (2 x -325) –693 (not divided by 2 at the end)

346.5 (wrong sign on answer)

+ 1926

OR Common errors for 1 mark

– 525 – 186 – 224 – 148 – 736 – 1450 + 650 + 1926 –2272.5 (–1926 instead of 1926)

OR -1386 (2 x -693 instead of -693)

= – 693 –996.5 (–650 instead of 650)

–509 (2 × 325 not used)

∆H hyd = –346.5 (kJ mol–1) –290.5 (2 × 112 not used)

–198.5 (148 instead of –148)

–160.5 (186 instead of –186)

–122.5 (224 instead of –224)

178.5 (525 instead of –525)

389.5 (736 instead of –736)

1103.5 (1450 instead of –1450)

For other answers, check for a single transcription

error or calculation error which could merit 1 mark

DO NOT ALLOW any answer which involves two

errors

e.g. –453 (2 × 325 not used AND 2 x 112 not used)

AO

Question Answer 8 Marks Guidance

element

(iii) Equation: Mg2+(g) + 2Br –(g) → MgBr2(s) 3 1.2 State symbols required

For other answers, check for a single transcription

CHECK THE ANSWER ON ANSWER LINE error or calculation error which could merit 1 mark

If answer = –2433 award 2 marks 2.2 x 2

DO NOT ALLOW any answer which involves two

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Lattice enthalpy =

∆hyH(Mg2+) + 2 × ∆hyH(Br–) – ∆solH(MgBr2) OR ALLOW ECF f rom incorrect answer to d(ii)

-1926 + (2 x -346.5) – (-186)

OR

∆fH(MgBr2) – 2∆atH(Br) – ∆atH(Mg)

– 1st IE(Mg) – 2nd IE(Mg) – 2∆eaH(Br) OR

-525 – (2 x 112) – 148 – 736 – 1450 – (2 x -325)

Lattice enthalpy = –2433 kJ mol–1

Total 18