[Missing in source] Data: Thermodynamic data (at 298 K): CO2(g) Glucose(s) Ethanol(l) ΔfH° (kJ mol‒1 — Physical Chemistry — Thermodynamics Chemistry Question
Which wine is it? Blind tasting challenge
[Missing in source]
Data:
Thermodynamic data (at 298 K):
CO2(g) Glucose(s) Ethanol(l)
ΔfH° (kJ mol‒1) ‒393.5 ‒1274 ‒277.0
S°m (J mol‒1K‒1) 213.6 212.1 160.7
Cr2O7 2–/Cr3+ CH3COOH/CH3CH2OH
E° (V) 1.33 0.19
Density of ethanol at 293 K: 0.79 g cm–3
[Missing in source]
Model Answer
Half-reactions:
C6H12O6(s) + 12 H+(aq) + 12 e− = 3 C2H6O(l) + 3 H2O(l)
C2H6O(l) + 3 H2O(l) = 2 CO2(g) + 12 H+(aq) + 12 e−
Hence:
C6H12O6(s) = 2 C2H6O(l) + 2 CO2(g)
No, this reaction does not require the presence of dioxygen.
[Missing in source]
Model Answer
ΔrH°(298 K) = 2 ΔfH°(C2H6O(l)) + 2 ΔfH°(CO2(g)) – ΔfH°(C6H12O6(s)) =
Write a balanced equation for the transformation of glucose into carbon dioxide and water. Does this reaction require the presence of dioxygen? (Yes/No)
Model Answer
C6H12O6(s) + 6 O2(g) = 6 CO2(g) + 6 H2O(l)
Yes, this reaction requires the presence of dioxygen.
Write a balanced equation for the oxidation reaction of ethanol by dichromate anions.
Model Answer
Half-reactions:
(1) C2H6O(l) + H2O(l) = C2H4O2(l) + 4 H+(aq) + 4 e−
(2) Cr2O72–(aq) + 14 H+(aq) + 6 e− = 2 Cr3+(aq) + 7 H2O(l)
Hence, 2×(2) + 3×(1):
2 Cr2O72–(aq) + 3 C2H6O(l) + 16 H+(aq) = 4 Cr3+(aq) + 3 C2H4O2(l) + 11 H2O(l)
...concentration of ethanol in wine? (Yes/No)
Calculate the pH of the solution of potassium dichromate and sulfuric acid before starting the titration. Here, sulfuric acid can be treated as a strong monoacid.
Model Answer
Hence, pH = –log[H3O+] = 1.3 taking into account the dilution.
Calculate the pH of the solution of potassium dichromate and sulfuric acid at the equivalence point (sulfuric acid is still considered to be a strong monoacid). Is it possible to determine the equivalence point using the pH change of the solution? (Yes/No)
Model Answer
No, because the pH does not change significantly along the reaction course.
Calculate the concentration (in % vol) of ethanol contained in wine X. Is this wine a German Riesling or a French Châteauneuf du Pape?
Model Answer
At the equivalence point,
where c(C2H6O) is the concentration of alcohol in the diluted wine.
c(C2H6O) = 0.05 mol dm-3 so the concentration of ethanol in the non-diluted wine is 2.5 mol dm-3.
In terms of mass concentration: cm(C2H6O) = c(C2H6O) × M(C2H6O) = 115 g dm-3.
The percentage of alcohol per volume of this wine is thus:
Wine X is thus a Châteauneuf du Pape.