Edexcel A-Level Chemistry Paper 1, June 2017: Question 2

6 marks · Medium difficulty · Open Response

Explain why the thermal decomposition of magnesium nitrate is a redox reaction in terms of oxidation numbers, and explain the difference in thermal stability between magnesium and calcium nitrates in terms of cation charge and size.

Practise this question

Question

Question text

2 Magnesium nitrate decomposes on heating as shown by the equation.

2Mg(NO3)2 r 2MgO + 4NO2 + O2

(a) Explain, in terms of all the relevant oxidation numbers, why this is a redox reaction.

(3)

(b) Calcium nitrate decomposes in a similar way to magnesium nitrate, but requires a

higher temperature for decomposition.

Explain this observation in terms of the charge and size of the cations.

(3)

(Total for Question 2 = 6 marks)

Mark scheme

Show the mark scheme

Question

Answer Additional Guidance Mark

Number

2(a) An explanation that makes reference to the following points: These numbers may be written under (3)

the formulae in the equation

N changes from (+)5 to (+)4 (1)

Allow oxidation numbers written as 5+,

O changes from −2 to 0 (in O2) (1) 4+, 2−

Ignore unchanged oxidation numbers of

magnesium and oxygen

so nitrogen / N is reduced (as the oxidation number has Allow this mark if incorrect / missing

decreased) oxidation numbers in M1 and M2

and

oxygen / O (in forming O2) is oxidised (as the oxidation Ignore general statement about redox

number has increased) (1)

Ignore redox explained in terms of

electron gain or loss

Question

Answer Additional Guidance Mark

Number

2(b) An explanation that makes reference to the following Penalise omission of ‘ion’ or just ‘calcium / Ca / (3)

points: magnesium / Mg’ without charge, or

reference to atom or molecule once only

Ignore general references to ionic / covalent

character

Size

calcium ion / Ca2+ has larger (ionic) radius / is Allow ionic radius increases down the group /

larger than magnesium ion decreases up the group

or Allow magnesium ions have a higher charge

magnesium ion / Mg2+ has smaller (ionic) radius / density (than calcium ions)

is smaller than calcium ion (1) Ignore atomic radius

Ignore effective nuclear charge

Polarising power

so calcium ion/ Ca2+ causes less polarisation/ Allow the cation causes less / more polarisation

distortion if it is clear from M1 which cation is involved

magnesium ion/ Mg2+causes more polarisation/

distortion (1)

What is polarised

of the nitrate (ion / electron cloud) / NO − / anion Do not allow this mark for carbonate / C-O

/ negative ion / N―O bonds / N=O bonds / bonds

NO bonds (1) Do not allow mention of bond between cation

and anion

Note

Nitrate ions are less polarised by Ca2+ / more

polarised by Mg2+ scores M2 and M3

(Total for Question 2 = 6 marks)

How to answer it

Thermal Decomposition and Redox of Group 2 Nitrates

What this question tests

This question assesses your understanding of redox reactions using oxidation numbers (identifying species oxidized and reduced) and your ability to apply Inorganic Chemistry periodicity trends (specifically, thermal stability of Group 2 nitrates explained through cation charge, ionic radius, and polarising power).

Question Part (a) - Redox Explanation

Assigning Oxidation Numbers & Identifying Redox Changes

Equation: 2Mg(NO₃)₂ → 2MgO + 4NO₂ + O₂

✅ Correct Answer

Nitrogen changes oxidation number from (+)5 to (+)4 .
Oxygen changes oxidation number from -2 to 0 (in O₂ ).
Nitrogen / N is reduced (oxidation number decreases).
Oxygen / O is oxidised (oxidation number increases).

💡 Key Knowledge

In Mg(NO₃)₂ , Mg is +2, O is -2, meaning N must be calculated as +5.
Uncombined elements like O₂ always have an oxidation number of 0 .
Redox must be justified using changes in oxidation numbers as requested by the command word "in terms of all the relevant oxidation numbers".

🧠 Exam Technique

State the initial and final oxidation numbers clearly for both species.
Explicitly link the change in number to the terms oxidised and reduced to secure the third mark.

❌ Common Errors

Explaining redox in terms of electron gain or loss instead of oxidation numbers (penalised when specific instructions demand oxidation numbers).
Forgetting to state that oxygen is specifically forming O₂ .

Question Part (b) - Thermal Stability Trends

Explaining Thermal Stability via Cation Size and Polarisation

✅ Correct Answer

Size: Calcium ion ( Ca²⁺ ) has a larger ionic radius than the magnesium ion ( Mg²⁺ ).
Polarising Power: The smaller Mg²⁺ ion has a higher charge density / higher polarising power, whereas Ca²⁺ causes less polarisation.
What is polarised: The nitrate ion ( NO₃⁻ electron cloud / N–O bonds) is distorted/polarised.

💡 Key Knowledge

Down Group 2, cations increase in size, decreasing their charge density and polarising ability.
Lower polarisation of the nitrate anion means higher temperatures are required to weaken bonds and cause thermal decomposition. Therefore, stability increases down the group.

🧠 Exam Technique

Always mention ion charges explicitly alongside size comparisons (e.g. Ca²⁺ and Mg²⁺ ). Writing just "calcium" or "magnesium" loses mark credit.
Structure your answer logically: Size comparison → Effect on polarising power → Effect on the nitrate anion/bonds.

❌ Common Errors

Omitting ionic charges (writing "calcium is larger than magnesium" instead of specifying the Ca²⁺ and Mg²⁺ ions).
Referring incorrectly to "atomic radius" instead of ionic radius.
Failing to specify what is being polarised (must mention the nitrate/anion/electron cloud/N-O bonds, not just general "bonds").

Topics

Physical Chemistry · Inorganic Chemistry · Topic 3: Redox I · Topic 4: Inorganic Chemistry and the Periodic Table

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.