Question 22M.2.SL.TZ1.1
| Date | May 2022 | Marks available | [Maximum mark: 23] | Reference code | 22M.2.SL.TZ1.1 |
| Level | SL | Paper | 2 | Time zone | TZ1 |
| Command term | Calculate, Deduce, Determine, Evaluate, Identify, Justify, Outline, State, Suggest | Question number | 1 | Adapted from | N/A |
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 14N3–.
Write a balanced equation for the reaction that occurs.
[1]
2 Mg(s) + O2(g) → 2 MgO(s) ✔
Do not accept equilibrium arrows. Ignore state symbols
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.
State the block of the periodic table in which magnesium is located.
[1]
s ✔
Do not allow group 2
Many students did not know what "block" meant, and often guessed group 2 etc.
Identify a metal, in the same period as magnesium, that does not form a basic oxide.
[1]
aluminium/Al ✔
Many students confused "period" and "group" and also many did not read metal, so aluminium was not chosen by the majority.
Calculate the amount of magnesium, in mol, that was used.
[1]
«mol» ✔
A number of students were not able to interpret the results and hence find the gain in mass and calculate the moles correctly.
Determine the percentage uncertainty of the mass of product after heating.
[2]
mass of product ✔
✔
Award [2] for correct final answer
Accept 0.021%
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".
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]
✔
✔
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.
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.
Evaluate whether this, rather than the loss of product, could explain the yield found in (b)(iii).
[1]
yes
AND
«each Mg combines with N, so» mass increase would be 14x 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”.
This proved to be a very difficult question to answer in the quantitative manner required, with hardly any correct responses.
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]
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.
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".
Calculate coefficients that balance the equation for the following reaction.
__ Mg3N2 (s) + __ H2O (l) → __ Mg(OH)2 (s) + __ NH3 (aq)
[1]
«1» Mg3N2 (s) + 6 H2O (l) → 3 Mg(OH)2 (s) + 2 NH3 (aq)
This was generally very well done with almost all candidates being able to determine the correct coefficients.
Determine the oxidation state of nitrogen in Mg3N2 and in NH3.
[1]
Mg3N2: -3
AND
NH3: -3 ✔
Do not accept 3 or 3-
About 40% of students managed to correctly determine both the oxidation states, as -3, with errors being about equally divided between the two compounds.
Deduce, giving reasons, whether the reaction of magnesium nitride with water is an acid–base reaction, a redox reaction, neither or both.
[2]
Acid–base:
yes AND N3- 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
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.
State the number of subatomic particles in this ion.
[1]
Protons: 7 AND Neutrons: 7 AND Electrons: 10 ✔
Most candidates could answer the question about subatomic particles correctly.
Some nitride ions are 15N3–. State the term that describes the relationship between 14N3– and 15N3–.
[1]
isotope«s» ✔
Identification of isotopes was answered correctly by most students.
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]
nitride AND smaller nuclear charge/number of protons/atomic number ✔
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.
Suggest two reasons why atoms are no longer regarded as the indivisible units of matter.
[2]
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.
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.
State the types of bonding in magnesium, oxygen and magnesium oxide, and how the valence electrons produce these types of bonding.
[4]
Award [1] for all bonding types correct.
Award [1] for each correct description.
Apply ECF for M2 only once.
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.