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Date May 2018 Marks available 2 Reference code 18M.2.sl.TZ1.1
Level SL Paper 2 Time zone TZ1
Command term Sketch Question number 1 Adapted from N/A

Question

Urea, (H2N)2CO, is excreted by mammals and can be used as a fertilizer.

Calculate the percentage by mass of nitrogen in urea to two decimal places using section 6 of the data booklet.

[2]
a.i.

Suggest how the percentage of nitrogen affects the cost of transport of fertilizers giving a reason.

[1]
a.ii.

The structural formula of urea is shown.

M18/4/CHEMI/SP2/ENG/TZ1/01.b_01

Predict the electron domain and molecular geometries at the nitrogen and carbon atoms, applying the VSEPR theory.

M18/4/CHEMI/SP2/ENG/TZ1/01.b_02

[3]
b.

Urea can be made by reacting potassium cyanate, KNCO, with ammonium chloride, NH4Cl.

                                      KNCO(aq) + NH4Cl(aq) → (H2N)2CO(aq) + KCl(aq)

Determine the maximum mass of urea that could be formed from 50.0 cm3 of 0.100 mol dm−3 potassium cyanate solution.

[2]
c.

Urea can also be made by the direct combination of ammonia and carbon dioxide gases.

                                   2NH3(g) + CO2(g) \( \rightleftharpoons \) (H2N)2CO(g) + H2O(g)     ΔH < 0

Predict, with a reason, the effect on the equilibrium constant, Kc, when the temperature is increased.

[1]
d.

Suggest one reason why urea is a solid and ammonia a gas at room temperature.

[1]
e.i.

Sketch two different hydrogen bonding interactions between ammonia and water.

[2]
e.ii.

The combustion of urea produces water, carbon dioxide and nitrogen.

Formulate a balanced equation for the reaction.

[2]
f.

The mass spectrum of urea is shown below.

M18/4/CHEMI/SP2/ENG/TZ1/01.g_01

Identify the species responsible for the peaks at m/z = 60 and 44.

 

[2]
g.

The IR spectrum of urea is shown below.

M18/4/CHEMI/SP2/ENG/TZ1/01.h_01

Identify the bonds causing the absorptions at 3450 cm−1 and 1700 cm−1 using section 26 of the data booklet.

 

[2]
h.

Predict the number of signals in the 1H NMR spectrum of urea.

[1]
i.

Markscheme

molar mass of urea «= 4 × 1.01 + 2 × 14.01 + 12.01 + 16.00» = 60.07 «g mol–1»

«% nitrogen = \(\frac{{2 \times 14.01}}{{60.07}}\) × 100 =» 46.65 «%»

 

 

Award [2] for correct final answer.

Award [1 max] for final answer not to two decimal places.

[2 marks]

a.i.

«cost» increases AND lower N% «means higher cost of transportation per unit of nitrogen»

OR

«cost» increases AND inefficient/too much/about half mass not nitrogen

 

Accept other reasonable explanations.

Do not accept answers referring to safety/explosions.

[1 mark]

a.ii.

M18/4/CHEMI/SP2/ENG/TZ1/01.b/M

 

Note: Urea’s structure is more complex than that predicted from VSEPR theory.

[3 marks]

b.

n(KNCO) «= 0.0500 dm3 × 0.100 mol dm–3» = 5.00 × 10–3 «mol»

«mass of urea = 5.00 × 10–3 mol × 60.07 g mol–1» = 0.300 «g»

 

 

Award [2] for correct final answer.

[2 marks]

c.

«Kc» decreases AND reaction is exothermic

OR

«Kc» decreases AND ΔH is negative

OR

«Kc» decreases AND reverse/endothermic reaction is favoured

[1 mark]

d.

Any one of:

urea has greater molar mass

urea has greater electron density/greater London/dispersion

urea has more hydrogen bonding

urea is more polar/has greater dipole moment

 

Accept “urea has larger size/greater van der Waals forces”.

Do not accept “urea has greater intermolecular forces/IMF”.

[1 mark]

e.i.

M18/4/CHEMI/SP2/ENG/TZ1/01.e.ii/M

 

Award [1] for each correct interaction.

If lone pairs are shown on N or O, then the lone pair on N or one of the lone pairs on O MUST be involved in the H-bond.

Penalize solid line to represent H-bonding only once.

[2 marks]

e.ii.

2(H2N)2CO(s) + 3O2(g) → 4H2O(l) + 2CO2(g) + 2N2(g)

correct coefficients on LHS

correct coefficients on RHS

 

Accept (H2N)2CO(s) + \(\frac{3}{2}\)O2(g) → 2H2O(l) + CO2(g) + N2(g).

Accept any correct ratio.

[2 marks]

f.

60: CON2H4+

44: CONH2+

 

Accept “molecular ion”.

[2 marks]

g.

3450 cm1: N–H

1700 cm–1: C=O

 

Do not accept “OH” for 3450 cm–1.

[2 marks]

h.

1

[1 mark]

i.

Examiners report

[N/A]
a.i.
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a.ii.
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b.
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c.
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d.
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e.i.
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e.ii.
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f.
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g.
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h.
[N/A]
i.

Syllabus sections

Core » Topic 4: Chemical bonding and structure » 4.4 Intermolecular forces
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