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Date	May 2022	Marks available	1	Reference code	22M.2.sl.TZ1.1
Level	SL	Paper	2	Time zone	TZ1
Command term	Deduce	Question number	1	Adapted from	N/A

Question

When heated in air, magnesium ribbon reacts with oxygen to form magnesium oxide.

The reaction in (a)(i) was carried out in a crucible with a lid and the following data was recorded:

Mass of crucible and lid = 47.372 ±0.001 g

Mass of crucible, lid and magnesium ribbon before heating = 53.726 ±0.001 g

Mass of crucible, lid and product after heating = 56.941 ±0.001 g

When magnesium is burnt in air, some of it reacts with nitrogen to form magnesium nitride according to the equation:

3 Mg (s) + N₂(g) → Mg₃N₂(s)

The presence of magnesium nitride can be demonstrated by adding water to the product. It is hydrolysed to form magnesium hydroxide and ammonia.

Most nitride ions are ¹⁴N^3–.

Write a balanced equation for the reaction that occurs.

[1]

a(i).

State the block of the periodic table in which magnesium is located.

[1]

a(ii).

Identify a metal, in the same period as magnesium, that does not form a basic oxide.

[1]

a(iii).

Calculate the amount of magnesium, in mol, that was used.

[1]

b(i).

Determine the percentage uncertainty of the mass of product after heating.

[2]

b(ii).

Assume the reaction in (a)(i) is the only one occurring and it goes to completion, but some product has been lost from the crucible. Deduce the percentage yield of magnesium oxide in the crucible.

[2]

b(iii).

Evaluate whether this, rather than the loss of product, could explain the yield found in (b)(iii).

[1]

c(i).

Suggest an explanation, other than product being lost from the crucible or reacting with nitrogen, that could explain the yield found in (b)(iii).

[1]

c(ii).

Calculate coefficients that balance the equation for the following reaction.

__ Mg₃N₂(s) + __ H₂O (l) → __ Mg(OH)₂(s) + __ NH₃(aq)

[1]

d(i).

Determine the oxidation state of nitrogen in Mg₃N₂ and in NH₃.

[1]

d(ii).

Deduce, giving reasons, whether the reaction of magnesium nitride with water is an acid–base reaction, a redox reaction, neither or both.

[2]

d(iii).

State the number of subatomic particles in this ion.

[1]

e(i).

Some nitride ions are ¹⁵N^3–. State the term that describes the relationship between ¹⁴N^3– and ¹⁵N^3–.

[1]

e(ii).

The nitride ion and the magnesium ion are isoelectronic (they have the same electron configuration). Determine, giving a reason, which has the greater ionic radius.

[1]

e(iii).

Suggest two reasons why atoms are no longer regarded as the indivisible units of matter.

[2]

State the types of bonding in magnesium, oxygen and magnesium oxide, and how the valence electrons produce these types of bonding.

[4]

Markscheme

2 Mg(s) + O₂(g) → 2 MgO(s) ✔

Do not accept equilibrium arrows. Ignore state symbols

a(i).

s ✔

Do not allow group 2

a(ii).

aluminium/Al ✔

a(iii).

$⟨ ⟨ \frac{53.726 g - 47.372 g}{244.31 g {mol}^{- 1}} = \frac{6.354 g}{24.31 g {mol}^{- 1}} ⟩ ⟩ = 0.2614$ «mol» ✔

b(i).

mass of product $« = 56.941 g - 47.372 g » = 9.569 « g »$ ✔

$⟨⟨100 × \frac{2 \times 0.001 g}{9.569 g} =0.0209⟩⟩ = 0.02 «%»$ ✔

Award [2] for correct final answer

Accept 0.021%

b(ii).

$⟨ ⟨ 0.2614 mol \times (24.31 g {mol}^{- 1} + 16.00 g {mol}^{- 1}) = 0.2614 mol \times 40.31 g {mol}^{- 1} ⟩ ⟩ = 10.536 « g »$ ✔

$⟨ ⟨ 100 \times \frac{9.569 g}{10.536 g} = 90.822 ⟩ ⟩ = 91 « % »$ ✔

Award «0.2614 mol x 40.31 g mol^–1»

Accept alternative methods to arrive at the correct answer.

Accept final answers in the range 91-92%

[2] for correct final answer.

b(iii).

yes
AND
«each Mg combines with $\frac{2}{3}$ N, so» mass increase would be 14x $\frac{2}{3}$ which is less than expected increase of 16x
OR
3 mol Mg would form 101g of Mg₃N₂ but would form 3 x MgO = 121 g of MgO
OR
0.2614 mol forms 10.536 g of MgO, but would form 8.796 g of Mg₃N₂ ✔

Accept Yes AND “the mass of N/N₂ that combines with each g/mole of Mg is lower than that of O/O₂”

Accept YES AND “molar mass of nitrogen less than of oxygen”.

c(i).

incomplete reaction
OR
Mg was partially oxidised already
OR
impurity present that evaporated/did not react ✔

Accept “crucible weighed before fully cooled”.

Accept answers relating to a higher atomic mass impurity consuming less O/O₂.

Accept “non-stoichiometric compounds formed”.

Do not accept "human error", "wrongly calibrated balance" or other non-chemical reasons.

If answer to (b)(iii) is >100%, accept appropriate reasons, such as product absorbed moisture before being weighed.

c(ii).

«1» Mg₃N₂(s) + 6 H₂O (l) → 3 Mg(OH)₂(s) + 2 NH₃(aq)

d(i).

Mg₃N₂: -3
AND
NH₃: -3 ✔

Do not accept 3 or 3-

d(ii).

Acid–base:
yes AND N^3- accepts H⁺/donates electron pair«s»
OR
yes AND H₂O loses H⁺ «to form OH^-»/accepts electron pair«s» ✔

Redox:
no AND no oxidation states change ✔

Accept “yes AND proton transfer takes place”

Accept reference to the oxidation state of specific elements not changing.

Accept “not redox as no electrons gained/lost”.

Award [1 max] for Acid–base: yes AND Redox: no without correct reasons, if no other mark has been awarded

d(iii).

Protons: 7 AND Neutrons: 7 AND Electrons: 10 ✔

e(i).

isotope«s» ✔

e(ii).

nitride AND smaller nuclear charge/number of protons/atomic number ✔

e(iii).

Any two of:

subatomic particles «discovered»
OR
particles smaller/with masses less than atoms «discovered»
OR
«existence of» isotopes «same number of protons, different number of neutrons» ✔

charged particles obtained from «neutral» atoms
OR
atoms can gain or lose electrons «and become charged» ✔

atom «discovered» to have structure ✔

fission
OR
atoms can be split ✔

Accept atoms can undergo fusion «to produce heavier atoms»

Accept specific examples of particles.

Award [2] for “atom shown to have a nucleus with electrons around it” as both M1 and M3.

Award [1] for all bonding types correct.

Award [1] for each correct description.

Apply ECF for M2 only once.

Examiners report

This was not as well done as one might have expected with the most common errors being O instead of O₂ oxygen and MgO rather than MgO₂.

a(i).

Many students did not know what "block" meant, and often guessed group 2 etc.

a(ii).

Many students confused "period" and "group" and also many did not read metal, so aluminium was not chosen by the majority.

a(iii).

A number of students were not able to interpret the results and hence find the gain in mass and calculate the moles correctly.

b(i).

Only a handful could work out the correct answer. Most had no real idea and quite a lot of blank responses. There also seems to be significant confusion between "percent uncertainty" and "percent error".

b(ii).

This was not well answered, but definitely better than the previous question with quite a few gaining some credit for correctly determining the theoretical yield.

b(iii).

This proved to be a very difficult question to answer in the quantitative manner required, with hardly any correct responses.

c(i).

Quite a few students realised that incomplete reaction would lead to this, but only 30% of students gave a correct answer rather than a non-specific guess, such as "misread balance" or "impurities".

c(ii).

This was generally very well done with almost all candidates being able to determine the correct coefficients.

d(i).

About 40% of students managed to correctly determine both the oxidation states, as -3, with errors being about equally divided between the two compounds.

d(ii).

Probably only about 10% could explain why this was an acid-base reaction. Rather more made valid deductions about redox, based on their answer to the previous question.

d(iii).

Most candidates could answer the question about subatomic particles correctly.

e(i).

Identification of isotopes was answered correctly by most students.

e(ii).

In spite of being given the meaning of "isoelectronic", many candidates talked about the differing number of electrons and only about 30% could correctly analyse the situation in terms of nuclear charge.

e(iii).

The question was marked quite leniently so that the majority of candidates gained at least one of the marks by mentioning a subatomic particle. A significant number read "indivisible" as "invisible" however.

About a quarter of the students gained full marks and probably a similar number gained no marks. Metallic bonding was the type that seemed least easily recognised and least easily described. Another common error was to explain ionic bonding in terms of attraction of ions rather than describing electron transfer.

Syllabus sections

Core » Topic 1: Stoichiometric relationships » 1.1 Introduction to the particulate nature of matter and chemical change

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