Question 18M.2.sl.TZ1.1
| Date | May 2018 | Marks available | [Maximum mark: 19] | Reference code | 18M.2.sl.TZ1.1 |
| Level | sl | Paper | 2 | Time zone | TZ1 |
| Command term | Calculate, Determine, Formulate, Identify, Justify, Predict, Sketch, Suggest | Question number | 1 | Adapted from | N/A |
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]
molar mass of urea «= 4 × 1.01 + 2 × 14.01 + 12.01 + 16.00» = 60.07 «g mol–1»
«% nitrogen = × 100 =» 46.65 «%»
Award [2] for correct final answer.
Award [1 max] for final answer not to two decimal places.
[2 marks]
Suggest how the percentage of nitrogen affects the cost of transport of fertilizers giving a reason.
[1]
«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]
The structural formula of urea is shown.
Predict the electron domain and molecular geometries at the nitrogen and carbon atoms, applying the VSEPR theory.
[3]
Note: Urea’s structure is more complex than that predicted from VSEPR theory.
[3 marks]
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]
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]
Urea can also be made by the direct combination of ammonia and carbon dioxide gases.
2NH3(g) + CO2(g) (H2N)2CO(g) + H2O(g) ΔH < 0
Predict, with a reason, the effect on the equilibrium constant, Kc, when the temperature is increased.
[1]
«Kc» decreases AND reaction is exothermic
OR
«Kc» decreases AND ΔH is negative
OR
«Kc» decreases AND reverse/endothermic reaction is favoured
[1 mark]
Suggest one reason why urea is a solid and ammonia a gas at room temperature.
[1]
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]
Sketch two different hydrogen bonding interactions between ammonia and water.
[2]
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]
The combustion of urea produces water, carbon dioxide and nitrogen.
Formulate a balanced equation for the reaction.
[2]
2(H2N)2CO(s) + 3O2(g) → 4H2O(l) + 2CO2(g) + 2N2(g)
correct coefficients on LHS
correct coefficients on RHS
Accept (H2N)2CO(s) + O2(g) → 2H2O(l) + CO2(g) + N2(g).
Accept any correct ratio.
[2 marks]
The mass spectrum of urea is shown below.
Identify the species responsible for the peaks at m/z = 60 and 44.
[2]
60: CON2H4+
44: CONH2+
Accept “molecular ion”.
[2 marks]
The IR spectrum of urea is shown below.
Identify the bonds causing the absorptions at 3450 cm−1 and 1700 cm−1 using section 26 of the data booklet.
[2]
3450 cm–1: N–H
1700 cm–1: C=O
Do not accept “O–H” for 3450 cm–1.
[2 marks]
Predict the number of signals in the 1H NMR spectrum of urea.
[1]
1
[1 mark]