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Date May 2021 Marks available 2 Reference code 21M.2.hl.TZ2.5
Level HL Paper 2 Time zone TZ2
Command term Deduce Question number 5 Adapted from N/A

Question

Ethanol and methanoic acid are important industrial products.

Ethanol is used as a fuel.

Write the chemical equation for the complete combustion of ethanol.

[1]
a(i).

Deduce the change in enthalpy, ΔH, in kJ, when 56.00 g of ethanol is burned. Use section 13 in the data booklet.

[2]
a(ii).

Oxidation of ethanol with potassium dichromate, K2Cr2O7, can form two different organic products. Determine the names of the organic products and the methods used to isolate them.

[2]
b.

Write the equation and name the organic product when ethanol reacts with methanoic acid.

[2]
c.

Sketch the titration curve of methanoic acid with sodium hydroxide, showing how you would determine methanoic acid pKa.

[2]
d(i).

Identify an indicator that could be used for the titration in 5(d)(i), using section 22 of the data booklet.

[1]
d(ii).

Determine the concentration of methanoic acid in a solution of pH = 4.12. Use section 21 of the data booklet.

[2]
e.

Identify if aqueous solutions of the following salts are acidic, basic, or neutral.

[2]
f.

Markscheme

CH3CH2OH (l) + 3O2 (g) → 2CO2 (g) + 3H2O (g) ✓

a(i).

«n56.00g46.08gmol-1 =» 1.215 «mol» ✓

«1.215mol × (−1367 kJ mol−1) =» −1661 «kJ» ✓

 

Award [2] for correct final answer.

Award [1 max] for “«+»1661 «kJ»”.

a(ii).

ethanal AND distillation ✓

ethanoic acid AND reflux «followed by distillation» ✓


Award [1] for both products OR both methods.

b.

Equation:
CH3CH2OH + HCOOH HCOOCH2CH3 + H2O ✓

Product name:
ethyl methanoate ✓


Accept equation without equilibrium arrows.

Accept equation with molecular formulas (C2H6O + CH2O2  C3H6O2 + H2O) only if product name is correct.

c.

increasing S-shape pH curve ✓

pKa: pH at half neutralization/equivalence ✓


M1: Titration curve must show buffer region at pH <7 and equivalence at pH >7.

Ignore other parts of the curve, i.e., before buffer region, etc.

Accept curve starting from where two axes meet as pH scale is not specified.

d(i).

phenolphthalein
OR
phenol red ✓

d(ii).

Alternative 1:
KaH+HCOO-HCOOH
OR
[HCOOH] = 10-4.12210-3.75

«[HCOOH] =» 3.24 × 10−5 «mol dm−3» ✓

 

Alternative 2:
«pH = pKa + log HCOO-HCOOH»
4.12 = 3.75 + log10-4.12HCOOH

«[HCOOH] =» 3.24 × 10−5 «mol dm−3» ✓

 

Award [2] for correct final answer.

e.

Sodium methanoate: basic

Ammonium chloride: acidic

Sodium nitrate: neutral ✓ ✓


Award [2] for three correct.

Award [1] for two correct.

f.

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]
e.
[N/A]
f.

Syllabus sections

Additional higher level (AHL) » Topic 15: Energetics/thermochemistry » 15.1 Energy cycles
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Additional higher level (AHL) » Topic 15: Energetics/thermochemistry
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