DP Chemistry (last assessment 2024)

Question 18M.2.hl.TZ1.1

Date	May 2018	Marks available	[Maximum mark: 29]	Reference code	18M.2.hl.TZ1.1
Level	hl	Paper	2	Time zone	TZ1
Command term	Apply, Calculate, Describe, Determine, Formulate, Identify, Justify, Outline, Predict, Sketch, State, Suggest	Question number	1	Adapted from	N/A

[Maximum mark: 29]

18M.2.hl.TZ1.1

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

(a.i)

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

[2]

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.ii)

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

[1]

Markscheme

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

«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]

(b)

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]

Markscheme

M18/4/CHEMI/HP2/ENG/TZ1/01.b/M

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

[3 marks]

(c)

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]

Markscheme

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]

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

(d.i)

State the equilibrium constant expression, K_c.

[1]

Markscheme

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

[1 mark]

(d.ii)

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

[1]

Markscheme

«K_c» decreases AND reaction is exothermic

«K_c» decreases AND ΔH is negative

«K_c» decreases AND reverse/endothermic reaction is favoured

[1 mark]

(d.iii)

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

[2]

Markscheme

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

«K_c =» 2 $\times$ 10^–9

1.69 $\times$ 10^–9

10^–9

Accept range of 20-20.2 for M1.

Award [2] for correct final answer.

[2 marks]

(e.i)

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

[1]

Markscheme

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.ii)

Sketch two different hydrogen bonding interactions between ammonia and water.

[2]

Markscheme

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]

(f)

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

Formulate a balanced equation for the reaction.

[2]

Markscheme

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]

(g)

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]

Markscheme

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

[1 mark]

(h)

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

[2]

Markscheme

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

co-ordinate/dative/covalent bonds

[2 marks]

(i)

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]

Markscheme

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

C–N bond partial double bond character

delocalization «of electrons occurs across molecule»

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

[1 mark]

(j)

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]

Markscheme

60: CON₂H₄⁺

44: CONH₂⁺

Accept “molecular ion”.

[2 marks]

(k)

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]

Markscheme

3450 cm^–1: N–H

1700 cm^–1: C=O

Do not accept “O–H” for 3450 cm^–1.

[2 marks]

(l.i)

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

[1]

Markscheme

[2 marks]

(l.ii)

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

[1]

Markscheme

singlet

Accept “no splitting”.

[1 mark]

(l.iii)

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]

Markscheme

acts as internal standard

acts as reference point

one strong signal

12 H atoms in same environment

signal is well away from other absorptions

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

[2 marks]

Syllabus sections

Core

Core » Topic 1: Stoichiometric relationships » 1.2 The mole concept

Core » Topic 1: Stoichiometric relationships

Core » Topic 4: Chemical bonding and structure » 4.3 Covalent structures

Core » Topic 4: Chemical bonding and structure

Core » Topic 1: Stoichiometric relationships » 1.3 Reacting masses and volumes

Core » Topic 7: Equilibrium » 7.1 Equilibrium

Core » Topic 7: Equilibrium

Additional higher level (AHL)

Additional higher level (AHL) » Topic 17: Equilibrium » 17.1 The equilibrium law

Additional higher level (AHL) » Topic 17: Equilibrium

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

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

Additional higher level (AHL) » Topic 13: The periodic table—the transition metals » 13.1 First-row d-block elements

Additional higher level (AHL) » Topic 13: The periodic table—the transition metals

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

Additional higher level (AHL) » Topic 14: Chemical bonding and structure

Core » Topic 11: Measurement and data processing » 11.3 Spectroscopic identification of organic compounds

Core » Topic 11: Measurement and data processing

Additional higher level (AHL) » Topic 21: Measurement and analysis » 21.1 Spectroscopic identification of organic compounds

Additional higher level (AHL) » Topic 21: Measurement and analysis

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