OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2019: Question 16
15 marks · Hard difficulty · Structured Questions
Assess periodic table properties, relative isotopic mass calculations, reactions of Group 2 oxides, and Born-Haber cycle enthalpy changes including lattice enthalpy calculations and trends in ionization energy and lattice enthalpy.
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Question text
16 Sir Humphry Davy discovered several elements including sodium, potassium, magnesium, calcium
and strontium.
(a) Explain which block in the Periodic Table sodium and magnesium belong to.
… [1]
(b) A sample of magnesium, Ar = 24.305, is found to consist of three isotopes. The accurate
relative isotopic masses and % abundances of two of the isotopes are shown in the table.
Isotope Relative isotopic mass % abundance
24 Mg 23.985 78.99%
25 Mg 24.986 10.00%
Determine the relative isotopic mass of the third isotope of magnesium in the sample.
Give your answer to 5 significant figures.
relative isotopic mass = … [2]
(c) A student adds an excess of calcium oxide to water in a test tube.
In a separate test tube, the student adds an excess of strontium oxide to water.
(i) Write the equation for the reaction of calcium oxide with water.
State symbols are not required.
… [1]
(ii) Suggest the approximate pH of the two solutions formed in the test tubes.
pH with calcium oxide …
pH with strontium oxide …
[1]
(d) The table below shows enthalpy changes involving potassium, oxygen and potassium oxide,
K2O.
Enthalpy change
/ kJ mol−1
formation of potassium oxide –363
1st electron affinity of oxygen –141
2nd electron affinity of oxygen +790
1st ionisation energy of potassium +419
atomisation of oxygen +249
atomisation of potassium +89
(i) The incomplete Born–Haber cycle below can be used to determine the lattice enthalpy of
potassium oxide.
In the boxes, complete the species present in the cycle.
Include state symbols for the species.
2K+(g) + O(g) + 2e–
2K(s)+O(g)
K2O(s)
11 [4]
(ii) Calculate the lattice enthalpy of potassium oxide.
lattice enthalpy = … kJ mol−1 [2]
(e) A similar Born–Haber cycle to potassium oxide in (d) can be constructed for sodium oxide.
(i) The first ionisation energy of sodium is more endothermic than that of potassium.
Explain why.
… [2]
(ii) The lattice enthalpy of sodium oxide is more exothermic than that of potassium oxide.
Explain why.
… [2]
Mark scheme
Show the mark scheme
AO
Question Answer Marks Guidance
element
16 (a) s-block 1 1.1 ALLOW ‘outer’ or ‘valence’ for ‘highest
AND highest energy or outer electron is in a s orbital or s energy’
sub–shell IGNORE electron configurations
DO NOT ALLOW s shell / energy level
(b) FIRST CHECK THE ANSWER ON ANSWER LINE 2 2.2 ×2 ALLOW any correct rearrangement of this
If answer = 25.982 award 2 marks sum for first mark
eg 11.01 x m = 2430.5 – 1894.575 – 249.86
78.99 × 23.985 + 10.00 × 24.986 + 11.01 × m
100 = 24.305 ALLOW ecf for transcription errors in first
Relative isotopic mass = 25.982 (must be 5 SF) sum but answer must be 5 sf
(c) (i) CaO + H2O → Ca(OH)2 1 2.8 ALLOW multiples
IGNORE state symbols
ALLOW CaO + 2H2O → Ca(OH)2 + H2O
AND CaO + H O Ca2++ 2OH-
(ii) both pH values > 7 AND ≤ 14 1 1.2 ALLOW ranges within these values but
AND ranges must not overlap
pH with SrO > pH with CaO
H432/01 Mark Schemes June 2019
AO
element
16 (d) (i) 4 1.2 ×4
. 2K+(g) + O2–(g) Mark each marking point independently
Correct species AND state symbols required
for each mark
For e–, ALLOW e
For e– only, IGNORE any state symbols
added
2K+(g) + O–(g) + e–
2K(g) + O(g)
2K(s) + ½O2 (g)
16 (ii) FIRST CHECK THE ANSWER ON ANSWER LINE 2 2.2 ×2 IF there is an alternative answer, check to
If answer = –2277 (kJ mol-1) award 2 marks see if there is any ECF credit possible using
H432/01 Mark Schemes June 2019
6 AO
element
working below
–363 – (2 × +89 +249 + 2 × 419 – 141 + 790)
See list below for marking of answers
–363 – 1914 from common errors
= –2277 (kJ mol–1)
ALLOW for 1 mark ONE mistake with sign
OR use of 2 ×:
+2277 (wrong sign)
–601 (2 × –419 instead of 2 × +419)
–697 (–790 instead of +790)
–1551 (+363 instead of –363)
–1858 (2 × +419 not used for K)
–1921 (2 × –89 instead of 2 × +89)
–2152.5 or –2153 (+249 ÷ 2)
–2188 (2 × +89 not used for K)
–2280 (rounded to 3SF)
–2559 (+141 instead of –141)
For other answers, check for a single
transcription error or calculator error which
could merit 1 mark
16 (e) (i) For sodium 2 1.1 ×2 ALLOW ‘Na/sodium is smaller’
atomic radius smaller IGNORE smaller radius / fewer shells / less
H432/01 Mark Schemes June 2019
AO
Question Answer 7 Marks Guidance
element
OR shielding if applied to ions but
fewer shells DO NOT ALLOW responses which refer to
ions losing electrons
DO NOT ALLOW molecules
ALLOW energy levels for shells
IGNORE fewer orbitals OR fewer sub–shells
ALLOW less (electron) shielding OR electron
repulsion between shells
IGNORE just ‘shielding’
nuclear attraction increases ALLOW more/stronger/bigger nuclear
OR attraction etc
(outer) electron(s) experience more attraction
IGNORE ‘pull’ for attraction
IGNORE electrons more tightly held
IGNORE ‘nuclear charge’ for ‘nuclear
attraction’
IGNORE more energy (in question)
ALLOW reverse argument for potassium
throughout
16 (ii) Comparison of size of cations 2 1.2 ×2 comparison of IONS is essential
For sodium ions
H432/01 Mark Schemes June 2019
8 AO
element
ionic radius of sodium / Na+ is smaller ALLOW Na+ has a larger charge density
IGNORE ‘Na has smaller atomic radius’ but
DO NOT ALLOW contradictory sentences eg
‘Na+ ions have smaller atomic radius’
Comparison of attraction of cation and anion IGNORE pull for attraction
Na+ has stronger attraction to O2-
ALLOW ‘sodium ion’ and ‘oxygen ion’
IGNORE just ‘oxygen’ or just ‘O’ for oxygen
ion
ALLOW stronger attraction between
oppositely charged ions
Total 15
How to answer it
Group 2 Chemistry & Born-Haber Cycles Study Guide
What this question tests
This comprehensive multi-part question assesses your understanding of Periodicity (s-block classification), Mass Spectrometry (relative isotopic mass calculations), Group 2 oxides and water reactions, Enthalpy changes & Born-Haber cycles, and periodic trends in ionization energy and lattice enthalpy.
Periodic Table Blocks
✅ Correct Answer
s-block because the highest energy electron (or outer electron) is in an s-orbital / s-sub-shell.
🧠 Exam Technique
Keep your justification concise. Mention both the block name and the orbital designation of the differentiating electron to secure the mark reliably.
Relative Isotopic Mass Calculation
📐 Step-by-Step Calculation
- Identify that the sum of all percentage abundances must equal 100%. Calculate the third abundance: 100 - (78.99 + 10.00) = 11.01%
- Set up the Ar formula: Ar = [(78.99 × 23.985) + (10.00 × 24.986) + (11.01 × m)] / 100 = 24.305
- Rearrange to solve for mass ( m ): 2430.5 - 1894.575 - 249.86 = 11.01m → 286.065 / 11.01 = 25.9818...
- Round to the required 5 significant figures: 25.982
❌ Common Errors & Traps
- Forgetting to calculate the correct percentage for the final isotope (using 1 instead of 11.01).
- Failing to round to the specified 5 significant figures, resulting in the loss of the final accuracy mark.
Reactions of Group 2 Oxides with Water & pH
✅ Correct Answers
(i) Equation: CaO + H₂O → Ca(OH)₂ (State symbols not required).
(ii) pH values: Both pH values must be > 7 AND ≤ 14 , with Strontium oxide having a higher pH than Calcium oxide due to increased solubility down the group.
💡 Key Knowledge
Group 2 oxides react with water to form alkaline metal hydroxides. Solubility of Group 2 hydroxides increases down the group, resulting in higher concentrations of OH⁻ ions and higher pH values.
Born-Haber Cycle for Potassium Oxide (K₂O)
✅ Correct Answer: Cycle Completion ((i))
- Top right box: 2K⁺(g) + O²⁻(g)
- Middle right box: 2K⁺(g) + O⁻(g) + e⁻
- Middle left box: 2K(g) + O(g)
- Bottom left box: 2K(s) + 0.5O₂(g)
📐 Lattice Enthalpy Calculation ((ii))
- Apply Hess's Law or rearrange energy levels: ΔH_lattice = ΔH_f(K₂O) - [ΔH_atom(K) × 2 + ΔH_atom(O) + ΔH_IE₁(K) × 2 + ΔH_EA₁(O) + ΔH_EA₂(O)]
- Substitute values: -363 - (2(+89) + 249 + 2(+419) + (-141) + (+790))
- Calculate intermediate sum: -363 - 1914
- Final Answer: -2277 kJ mol⁻¹
❌ Common Errors
Forgetting to multiply the enthalpy of atomisation and first ionisation energy of potassium by 2 (since the formula is K₂O, requiring 2 moles of K atoms/ions).
Trends in Ionisation Energy & Lattice Enthalpy
✅ Correct Answers
(i) Ionisation Energy: Sodium has a smaller atomic radius / fewer shells, meaning greater nuclear attraction on the outer electron.
(ii) Lattice Enthalpy: Na⁺ has a smaller ionic radius than K⁺, leading to stronger electrostatic attraction to the O²⁻ ion.
🧠 Top-Level Exam Strategy
When comparing lattice enthalpies involving ions, you must explicitly compare the sizes of the cations and state the effect on electrostatic attraction between oppositely charged ions. Avoid vague references to "nuclear charge" when discussing ionic radii.
Topics
Module 3: Periodic table and energy · Module 5: Physical chemistry and transition elements · Module 2: Foundations in chemistry · 5.2 Energy · 3.1 The periodic table · 2.1 Atoms and reactions · 2.2 Electrons, bonding and structure
Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.