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

Question

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

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

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/HP2/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/HP2/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.

State the equilibrium constant expression, Kc.

[1]
d.i.

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

[1]
d.ii.

Determine an approximate order of magnitude for Kc, using sections 1 and 2 of the data booklet. Assume ΔGΘ for the forward reaction is approximately +50 kJ at 298 K.

[2]
d.iii.

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.

Calculate the maximum volume of CO2, in cm3, produced at STP by the combustion of 0.600 g of urea, using sections 2 and 6 of the data booklet.

[1]
g.

Describe the bond formation when urea acts as a ligand in a transition metal complex ion.

[2]
h.

The C–N bonds in urea are shorter than might be expected for a single C–N bond. Suggest, in terms of electrons, how this could occur.

[1]
i.

The mass spectrum of urea is shown below.

M18/4/CHEMI/HP2/ENG/TZ1/01.j_01

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

[2]
j.

The IR spectrum of urea is shown below.

M18/4/CHEMI/HP2/ENG/TZ1/01.k_01

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

[2]
k.

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

[1]
l.i.

Predict the splitting pattern of the 1H NMR spectrum of urea.

[1]
l.ii.

Outline why TMS (tetramethylsilane) may be added to the sample to carry out 1H NMR spectroscopy and why it is particularly suited to this role.

[2]
l.iii.

Markscheme

molar mass of urea «4 \( \times \) 1.01 + 2 \( \times \) 14.01 + 12.01 + 16.00» = 60.07 «g mol-1»

«% nitrogen = \(\frac{{{\text{2}} \times {\text{14.01}}}}{{{\text{60.07}}}}\) \( \times \) 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/HP2/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 \( \times \) 0.100 mol dm–3» = 5.00 \( \times \) 10–3 «mol»

«mass of urea = 5.00 \( \times \) 10–3 mol \( \times \) 60.07 g mol–1» = 0.300 «g»

 

Award [2] for correct final answer.

[2 marks]

c.

\({K_{\text{c}}} = \frac{{[{{({{\text{H}}_2}{\text{N}})}_2}{\text{CO}}] \times [{{\text{H}}_2}{\text{O}}]}}{{{{[{\text{N}}{{\text{H}}_3}]}^2} \times [{\text{C}}{{\text{O}}_2}]}}\)

[1 mark]

d.i.

«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.ii.

ln K « = \(\frac{{ - \Delta {G^\Theta }}}{{RT}} = \frac{{ - 50 \times {{10}^3}{\text{ J}}}}{{8.31{\text{ J }}{{\text{K}}^{ - 1}}{\text{ mo}}{{\text{l}}^{ - 1}} \times 298{\text{ K}}}}\) » = –20

 

«Kc =» 2 \( \times \) 10–9

OR

1.69 \( \times \) 10–9

OR

10–9

 

Accept range of 20-20.2 for M1.

Award [2] for correct final answer.

[2 marks]

d.iii.

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/HP2/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.

«V = \(\frac{{{\text{0.600 g}}}}{{{\text{60.07 g mo}}{{\text{l}}^{ - 1}}}}\) \( \times \) 22700 cm3 mol–1 =» 227 «cm3»

[1 mark]

g.

lone/non-bonding electron pairs «on nitrogen/oxygen/ligand» given to/shared with metal ion

co-ordinate/dative/covalent bonds

[2 marks]

h.

lone pairs on nitrogen atoms can be donated to/shared with C–N bond

OR

C–N bond partial double bond character

OR

delocalization «of electrons occurs across molecule»

OR

slight positive charge on C due to C=O polarity reduces C–N bond length

[1 mark]

i.

60: CON2H4+

44: CONH2+

 

Accept “molecular ion”.

 

 

[2 marks]

j.

3450 cm1: N–H

1700 cm1: C=O

 

Do not accept “OH” for 3450 cm–1.

[2 marks]

k.

1

[2 marks]

l.i.

singlet

 

Accept “no splitting”.

[1 mark]

l.ii.

acts as internal standard

OR

acts as reference point

 

one strong signal

OR

12 H atoms in same environment

OR

signal is well away from other absorptions

 

Accept “inert” or “readily removed” or “non-toxic” for M1.

[2 marks]

l.iii.

Examiners report

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

Syllabus sections

Additional higher level (AHL) » Topic 14: Chemical bonding and structure » 14.1 Covalent bonding and electron domain and molecular geometries
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