Edexcel A-Level Chemistry AS Paper 1, June 2016: Question 7

15 marks · Hard difficulty · Extended Writing

Discuss the suggestion that the difference in the rates of reaction of strontium and barium with water is due to the difference in the sum of their first and second ionisation energies, and answer questions on ionisation energy equations, thermal decomposition of nitrates and carbonates.

Practise this question

Question

An exam question split into multiple parts numbered 7(a) through 7(d). Part 7(a) is a 6-mark extended response asking to discuss a student's suggestion about the reaction rates of strontium and barium with water and ionisation energies. Part 7(b) is a multiple-choice question asking for the equation representing the third ionisation energy of barium. Part 7(c) asks to explain why magnesium nitrate decomposes more readily on heating than potassium nitrate. Part 7(d) involves thermal decomposition of metal carbonates, asking to draw apparatus, state a way to ensure a fair test, and state how data would be used to make a comparison.
Question text

7 *(a) A student suggested that the difference in the rates of reaction of strontium and

barium with water is due to the difference in the sum of their first and second

ionisation energies. Discuss this suggestion.

(6)

(b) Which is the equation for the third ionisation energy of barium?

(1)

A Ba(s) r Ba3+(g) + 3e–

B Ba2+(s) r Ba3+(g) + e–

C Ba2+(g) r Ba3+(g) + e–

D Ba3+(g) r Ba4+(g) + e–

(c) Explain why magnesium nitrate, Mg(NO3)2 decomposes more readily on heating

than potassium nitrate, KNO3.

(4)

… 13

… *P49837A01320*

(d) Some metal carbonates also undergo thermal decomposition.

(i) Draw a diagram of the apparatus that could be used to compare the

ease of thermal decomposition of lithium carbonate, Li2CO3, and

magnesium carbonate, MgCO3.

(2)

(ii) State one way in which you would ensure a fair test.

(1)

(iii) State how data obtained in this experiment could be used to make a

comparison.

(1)

(Total for Question 7 = 15 marks)

Mark scheme

Show the mark scheme The mark scheme for question 7 detailing indicative content and marking grids for the 6-mark extended response, the correct multiple-choice option C for part 7(b), marking points for magnesium nitrate thermal decomposition in part 7(c), and diagrams and criteria for the practical apparatus and fair test in part 7(d).

How to answer it

Group 2 Reactions, Ionisation Energies & Thermal Decomposition

What this question tests

This question assesses your understanding of Group 2 trends (reactivity, ionisation energy down the group), definitions and equations for successive ionisation energies, thermal stability trends of Group 1 and 2 nitrates/carbonates in terms of cation size and polarizing power, and practical chemistry skills for comparing rates of thermal decomposition safely and fairly.

Part (a): Rates of Reaction of Strontium and Barium with Water

6-Mark Extended Response

✅ Indicative Scientific Content

  • The sum of the first two ionisation energies for barium is lower than for strontium.
  • Barium has a larger atomic radius / more electron shells than strontium.
  • Barium experiences greater shielding from inner electron shells.
  • Barium has a higher nuclear charge (more protons), but shielding and distance outweigh nuclear charge.
  • Therefore, barium loses its outer electrons more easily, reacting faster and showing higher reactivity.

🧠 Exam Technique & Level of Response

  • QWC (Quality of Written Communication): Evaluated separately for structure and logical reasoning.
  • To achieve top marks (Level 2 reasoning, 4 marks for content + 2 for structure), link atomic properties directly to reactivity using clear causal connectives (e.g., "therefore", "leading to").
  • Avoid disjointed bullet points; write in a logical, sustained narrative flow.

❌ Common Errors & Examiner Warnings

  • Molecules vs Atoms: Never refer to individual Group 2 metal atoms as "molecules". Use "atoms", "elements", or "cations".
  • Incorrect comparisons: Failing to explicitly contrast barium and strontium side-by-side will cap your mark.
Maximum Marks: 6 (4 marks for indicative points + 2 marks for structure and lines of reasoning).

Part (b): Equation for the Third Ionisation Energy of Barium

1-Mark Multiple Choice Question

✅ Correct Answer: C

Ba²⁺(g) → Ba³⁺(g) + e⁻

💡 Key Knowledge

Ionisation energy is defined as the energy required to remove one mole of electrons from one mole of gaseous atoms or ions. The third ionisation energy specifically starts with a 2+ gaseous ion and removes one electron to form a 3+ gaseous ion.

❌ Common Errors

Students often select option A or B by forgetting state symbols (must be g ) or starting with solid metal Ba(s) instead of the gaseous 2+ ion.

Maximum Marks: 1

Part (c): Thermal Decomposition of Magnesium Nitrate vs Potassium Nitrate

4-Mark Explanation

✅ Correct Answer Points

  • The magnesium ion (Mg²⁺) is smaller than the potassium ion (K⁺).
  • Magnesium has a higher charge (2+) compared to potassium (1+), resulting in a higher charge density.
  • The small, highly charged Mg²⁺ ion strongly polarises the nitrate ion (distorts its electron cloud).
  • This weakens the N–O bonds within the nitrate ion, making it decompose more readily upon heating.

💡 Key Knowledge & Polarisation

Thermal stability of Group 1 and Group 2 nitrates and carbonates increases down the group. Smaller cations with higher charges polarise complex anions more effectively, weakening internal bonds and lowering thermal stability.

❌ Common Errors

Penalised if you refer to polarising "magnesium molecule" instead of the magnesium ion or cation. Make sure to explicitly mention the comparison of ionic radii and charge.

Maximum Marks: 4

Part (d): Thermal Decomposition Practical

Sub-parts (i), (ii), and (iii)

✅ (i) Apparatus Diagram (2 Marks)

  • A solid sample of carbonate contained in a side-arm test tube or boiling tube being heated by a Bunsen burner.
  • A delivery tube leading evolved gas into limewater (or connected to a gas syringe).

✅ (ii) Ensuring a Fair Test (1 Mark)

  • Use the same Bunsen burner setting / flame size.
  • Keep the same distance between the flame and the test tube.

✅ (iii) Using Data to Make a Comparison (1 Mark)

Measure and compare the time taken for the limewater to go cloudy, or measure the time taken to produce a fixed volume of gas using a gas syringe.

❌ Examiner Trap

Do not award marks for heating in an open conical flask or crucible where gas escapes freely without collection. Limewater must not be in a sealed airtight container that could cause pressure buildup and glass breakage!

Maximum Marks: 4 total for (d) [(i) = 2 marks, (ii) = 1 mark, (iii) = 1 mark]

Topics

Physical Chemistry · Inorganic Chemistry · Topic 1: Atomic Structure and the Periodic Table · Topic 4: Inorganic Chemistry and the Periodic Table

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2016. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.