OCR A-Level Chemistry AS Depth in chemistry (02), June 2023: Question 2

19 marks · Medium difficulty · Structured Questions

Assess reactions of Group 2 elements with water, calculate percentage composition from a precipitation reaction, and explain lattice structures and bonding properties for Period 3 elements and compounds.

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Question

A three-part structured chemistry question starting with calcium reacting with water, followed by a precipitation titration calculation involving sodium chloride and barium nitrate, and ending with a table and explanation of melting points, electrical conductivities, and lattice structures of Period 3 substances.
Question text

2 This question is about some elements in Period 3 and compounds they form.

(a) A student adds a small piece of calcium to a beaker containing an excess of water.

(i) Construct the equation for the reaction and predict one observation that the student

would make.

Equation …

Observation …

[2]

(ii) Suggest one difference that the student would observe in the reaction of barium with

water compared to the reaction of calcium with water.

… [1]

(b) A student has a 5.00 g mixture of sodium chloride, NaCl(s), and barium nitrate, Ba(NO3)2(s).

The student also has a solution of sodium sulfate, Na2SO4(aq).

The student uses the method below to determine the percentage by mass of NaCl(s) in the

mixture.

Step 1 Dissolve the 5.00 g mixture in distilled water.

Step 2 Add an excess of Na2SO4(aq) to the solution. A precipitate of barium sulfate

forms.

Step 3 Filter off the precipitate, wash with water, and dry.

Step 4 Weigh the dried precipitate.

The molar mass of barium sulfate is 233.4 g mol–1.

(i) Write an equation for the formation of barium sulfate in step 2.

Include state symbols.

… [2]

(ii) The student obtains 3.28 g of precipitate.

Calculate the percentage by mass of NaCl(s) in the 5.00 g mixture.

Give your answer to 3 significant figures.

percentage by mass of NaCl(s) = … % [4]

(iii) The student changes the method in 2(b).

In step 2, the student adds an excess of silver nitrate solution, AgNO3(aq), instead of

Na2SO4(aq).

Explain whether this change would allow the student to determine the percentage by

mass of NaCl(s) in the mixture.

… [2]

(c) The table below shows melting points and electrical conductivities of some elements in

Period 3 and compounds they form.

Magnesium Phosphorus

Substance Aluminium, Al Silicon, Si

sulfide, MgS trichloride, PCl3

Melting

2000 660 1414 –94

point / °C

Electrical

Good Poor

conductivity

Type of Giant …

lattice

structure …

(i) Complete the table above to show the type of lattice structure of each substance. [4]

(ii) Explain the following:

• MgS has a higher melting point than PCl3.

• Al has a greater electrical conductivity than Si.

Melting points …

Conductivities …

[4]

Mark scheme

Show the mark scheme Detailed mark scheme showing required chemical equations, observations, calculation steps for percentage by mass of NaCl, and explanations for lattice types, melting points, and electrical conductivities.

AO

Question Answer Marks Guidance

element

2 (a) (i) Equation 2

Ca + 2H2O → Ca(OH)2 + H2 AO2.6 ALLOW correct multiples including fractions

All formulae and balancing correct ✓ IGNORE state symbols, even if wrong

Observation

Effervescence/fizzing/bubbles AO1.2 IGNORE temperature change, pH change or

OR gas formed i.e. must be an observable

Ca/solid disappears/dissolves change.

OR

Forms a white ppt/solid ✓ IGNORE turns cloudy

DO NOT ALLOW Colour change

(ii) More vigorous effervescence/fizzing/bubbling 1 AO1.2 ORA if clearly references Ca

OR

Ba/solid disappears/dissolves faster ALLOW AW such as stronger/ rapid/ quicker/

OR more quickly/ more violent

White ppt formed less rapidly ✓

ALLOW less or no ppt (as barium hydroxide is

more soluble)

Note: Must reference observation not just

reaction e.g. more vigorous reaction.

IGNORE finishes first

IGNORE more bubbles (need idea of rate)

IGNORE exothermic

(b) (i) Ba(NO3)2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaNO3(aq) 2 AO2.5 ALLOW ionic equation

x2 Ba2+(aq) + SO 2–(aq) → BaSO (s)

Balanced equation

State symbols M2 dependent on M1

IGNORE NaCl balanced on both sides

AO

11 element

AO

element

2 (b) (ii) FIRST CHECK THE ANSWER ON ANSWER LINE 4 ALLOW ECF from incorrect equation in 2(b)(i)

If answer = 26.6 % award 4 marks and throughout

-------------------------------------------------------------------------------------

ALLOW 3SF up to calculated value

throughout

IGNORE rounding errors past 3SF

3.28 AO3.1

n(BaSO4) = 233.4 OR 0.014053… (mol) 3 Calculator: 0.01405312768

mass Ba(NO3)2 = 0.014053… 261.3

OR 3.672 … (g) Calculator: 3.672082262

mass NaCl = 5.00 – 3.672.. OR 1.3279… (g) Calculator: 1.327917738

1.3279 × 100 AO3.2

% NaCl = 5.00 = 26.6(%) 3 SF ALLOW ECF for use of calculated mass NaCl

e.g. 0.014053…x 58.5 = 0.8221….

to give final % 16.4 to 3SF

-----------------------------------------------

Alternative approach for last 2 marks

3.672 × 100

% Ba(NO3)2 = 5.00 = 73.44 …

% NaCl = 100 – 73.44 = 26.6 %

(iii) Silver chloride/AgCl would be produced (as a precipitate) 2 AO3.4 ALLOW Chloride reacts to give (white) ppt

2 IGNORE incorrect formula of silver chloride

ALLOW equation showing formation of

AgCl(s)

(Mass of NaCl) can be calculated from the mass/moles of AgCl

ALLOW Weigh AgCl and use to calculate

%/mass/moles

AO

element

2 (c) (i) Magnesium Phosphorus

Substance Aluminium Silicon

sulfide trichloride 4 AO1.1 ALLOW Simple covalent instead of simple

Melting 2 molecular

o 2000 660 1414 –94

point/ C

Electrical AO2.1

Good Poor

conductivity

Type of 2

Giant Giant Giant Simple

lattice

Ionic Metallic Covalent Molecular

structure

(ii) Melting points 4

AO1.1

MgS: ionic bonds (between oppositely charged ions)

ALLOW London forces or permanent

PCl3: intermolecular forces AO2.1 dipole dipole interactions

More energy needed (to separate ions in MgS) ORA answer must be comparative

OR AO3.1

Strong ionic bonds AND weak intermolecular forces 2 ALLOW ECF from incorrect type of

bonding i.e. stronger attraction/more

energy

Conductivity .

Al: mobile/delocalised electrons IGNORE ‘free electrons’ for mobile/delocalised

AND electrons

Si: no mobile/delocalised electrons OR no charge carriers OR no

mobile ions

How to answer it

Period 3 Elements and Compounds Study Guide

What this question tests

This question assesses your knowledge of Group 2 chemistry (trends down the group, reactions with water), quantitative chemistry (gravimetric analysis, percentage mass, stoichiometry, and state symbols), and bonding and structure across Period 3 (lattice types, melting points linked to bonding types, and electrical conductivity mechanisms).

Question 2(a) — Group 2 Reactions with Water

Reactions of Calcium and Barium

✅ Correct Answers

  • (i) Equation: Ca + 2H₂O → Ca(OH)₂ + H₂ (multiples/fractions allowed).
  • (i) Observation: Effervescence / fizzing / bubbles, solid disappears, or white precipitate forms.
  • (ii) Barium difference: More vigorous effervescence, solid disappears faster, or white precipitate forms less rapidly (due to lower solubility of barium hydroxide).

❌ Common Errors

  • Stating "turns cloudy" for calcium — note that the mark scheme ignores this as calcium hydroxide has limited solubility.
  • Giving colour changes (e.g. "turns green") which score zero since no transition metals are involved.
  • Describing the rate qualitatively without a clear comparative observation for barium part (ii).
Question 2(b) — Gravimetric Analysis & Stoichiometry

Purity Determination and Precipitation Calculations

✅ Correct Answers (Part i & iii)

  • (i) Equation: Ba(NO₃)₂ (aq) + Na₂SO₄ (aq) → BaSO₄ (s) + 2NaNO₃ (aq) with correct state symbols. Ionic equations ( Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) ) also accepted.
  • (iii) Silver nitrate change: Yes, it works because AgCl (silver chloride) would precipitate out, allowing mass/moles to be calculated.

📐 Calculation Steps (Part ii) - Target: 26.6%

  1. Moles of BaSO₄: 3.28 g ÷ 233.4 g mol⁻¹ = 0.014053 mol
  2. Moles of Ba(NO₃)₂: Ratio is 1:1, so moles = 0.014053 mol
  3. Mass of Ba(NO₃)₂: 0.014053 mol × 261.3 g mol⁻¹ = 3.672 g
  4. Mass of NaCl: 5.00 g total - 3.672 g = 1.3279 g
  5. Percentage NaCl: (1.3279 g ÷ 5.00 g) × 100 = 26.6% (to 3 sig fig).

🧠 Exam Technique & Traps

Always check if your final answer matches the requested significant figures (3 sig fig here). If you make an error in earlier steps, follow-through marks (ECF) protect you as long as the mathematical operations are sound.

Question 2(c) — Period 3 Bonding, Structure & Properties

Lattice Types and Physical Properties

✅ Table Completions (Part i)

  • MgS: Giant Ionic
  • Al: Giant Metallic
  • Si: Giant Covalent
  • PCl₃: Simple Molecular

💡 Key Knowledge (Part ii Explanations)

  • Melting Points (MgS vs PCl₃): MgS has strong ionic bonds (electrostatic attraction between oppositely charged ions Mg²⁺ and S²⁻ ) requiring a lot of energy to break. PCl₃ has weak intermolecular forces (London forces/dipole-dipole) requiring little energy to overcome.
  • Electrical Conductivity (Al vs Si): Aluminium has mobile/delocalised electrons that carry a charge. Silicon has no mobile/delocalised electrons or mobile ions.
Examiner Tip: When explaining melting point differences, always explicitly link the type of bonding/forces present to the relative amount of energy required to overcome them. Never state that bonds are "melted" — intermolecular forces or crystal lattices are overcome/broken!

Topics

Module 2: Foundations in chemistry · Module 3: Periodic table and energy · 2.1 Atoms and reactions · 2.2 Electrons, bonding and structure · 3.1 The periodic table

Question and mark scheme from the OCR A-Level Chemistry examination, AS Depth in chemistry (02), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.