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Date May 2017 Marks available 2 Reference code 17M.2.hl.TZ1.5
Level HL Paper 2 Time zone TZ1
Command term Explain Question number 5 Adapted from N/A

Question

Two hydrides of nitrogen are ammonia and hydrazine, N2H4. One derivative of ammonia is methanamine whose molecular structure is shown below.

M17/4/CHEMI/HP2/ENG/TZ1/05

Hydrazine is used to remove oxygen from water used to generate steam or hot water.

N2H4(aq)+O2(aq)N2(g)+2H2O(l)

The concentration of dissolved oxygen in a sample of water is 8.0×103 gdm3.

Estimate the H−N−H bond angle in methanamine using VSEPR theory.

[1]
a.

State the electron domain geometry around the nitrogen atom and its hybridization in methanamine.

 

[2]
b.

Ammonia reacts reversibly with water.
NH3(g)+H2O(l)NH+4(aq)+OH(aq)
Explain the effect of adding H+(aq) ions on the position of the equilibrium.

[2]
c.

Hydrazine reacts with water in a similar way to ammonia. (The association of a molecule of hydrazine with a second H+ is so small it can be neglected.)

N2H4(aq)+H2O(l)N2H+5(aq)+OH(aq)

pKb (hydrazine)=5.77

Calculate the pH of a 0.0100 moldm3 solution of hydrazine.

[3]
d.i.

Suggest a suitable indicator for the titration of hydrazine solution with dilute sulfuric acid using section 22 of the data booklet.

[1]
d.ii.

Outline, using an ionic equation, what is observed when magnesium powder is added to a solution of ammonium chloride.

[2]
e.

Determine the enthalpy change of reaction, ΔH, in kJ, when 1.00 mol of gaseous hydrazine decomposes to its elements. Use bond enthalpy values in section 11 of the data booklet.

N2H4(g)N2(g)+2H2(g)

[3]
f.

The standard enthalpy of formation of N2H4(l) is +50.6 kJmol1. Calculate the enthalpy of vaporization, ΔHvap, of hydrazine in kJmol1. N2H4(l)N2H4(g) (If you did not get an answer to (f), use 85 kJ but this is not the correct answer.)

[2]
g.

Calculate, showing your working, the mass of hydrazine needed to remove all the dissolved oxygen from 1000 dm3 of the sample.

[3]
h.i.

Calculate the volume, in dm3, of nitrogen formed under SATP conditions. (The volume of 1 mol of gas = 24.8 dm3 at SATP.)

[1]
h.ii.

Markscheme

107°

 

Accept 100° to < 109.5°.

Literature value = 105.8°

[1 mark]

a.

tetrahedral

sp3

 

 

No ECF allowed.

[2 marks]

b.

removes/reacts with OH

moves to the right/products «to replace OH ions»

 

Accept ionic equation for M1.

[2 marks]

c.

Kb = 10–5.77 / 1.698 x 10–6
OR
Kb=[N2H+5]×[OH][N2H4]

 [OH]2 «= 1.698 × 10–6 × 0.0100» = 1.698 × 10–8

OR

[OH] «=1.698×108» = 1.303 × 10–4 «mol dm–3»

pH «=log101×10141.3×104» = 10.1

 

Award [3] for correct final answer.

Give appropriate credit for other methods containing errors that do not yield correct final answer.

[3 marks]

d.i.

methyl red

OR

bromocresol green

OR

bromophenol blue

OR

methyl orange

[1 mark]

d.ii.

bubbles

OR

gas

OR

magnesium disappears

2NH+4(aq)+Mg(s)Mg2+(aq)+2NH3(aq)+H2(g)

 

Do not accept “hydrogen” without reference to observed changes.

Accept "smell of ammonia".

Accept 2H+(aq) + Mg(s)  Mg2+(aq) + H2(g)

Equation must be ionic.

[2 marks]

e.

bonds broken:

E(N–N) + 4E(N–H)

OR

158 kJmol1+4×391 kJmol1/1722 kJ

bonds formed:

E(NN) + 2E(H–H)

OR

945 kJmol1+2×436 kJmol1/1817 kJ

ΔH=bonds brokenbonds formed=17221817=≫95 kJ

 

Award [3] for correct final answer.

Award [2 max] for +95 «kJ».

[3 marks]

 

f.

M17/4/CHEMI/HP2/ENG/TZ1/05.g/M

OR

ΔHvap=50.6 kJmol1(95 kJmol1)

ΔHvap=≫+44 kJmol1

 

Award [2] for correct final answer. Award [1 max] for 44 «kJ mol1».

Award [2] for:

ΔHvap = –50.6 kJ mol1 – (–85 J mol1) = 34 «kJ mol1».

Award [1 max] for –34 «kJ mol1».

[2 marks]

g.

total mass of oxygen ≪=

n( O 2 ≪= 8.0  g 32.00  g mo l 1 =≫   0.25   mol

OR

n( N 2 H 4 ) = n( O 2 )

mass of hydrazine = 0.25  mol × 32.06  g mo l 1 =≫   8.0   g

 

Award [3] for correct final answer.

[3 marks]

h.i.

n( N 2 H 4 ) = n( O 2 ) = 8.0  g 32.00  g mo l 1 =≫   0.25   mol

volume of nitrogen = 0.25  mol × 24.8  d m 3 mo l 1 ≫= 6.2   d m 3

 

Award [1] for correct final answer.

[1 mark]

h.ii.

Examiners report

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

Syllabus sections

Additional higher level (AHL) » Topic 14: Chemical bonding and structure » 14.2 Hybridization
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Core » Topic 4: Chemical bonding and structure » 4.3 Covalent structures
Additional higher level (AHL) » Topic 14: Chemical bonding and structure
Core » Topic 4: Chemical bonding and structure
Core
Additional higher level (AHL)

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