INTERNATIONAL CHEMISTRY OLYMPIAD 2010
Final Evaluation
At the end of March, the French Chemistry Olympiad is over and the
laureates are known. Within these laureates begins a new selection in order to
find the two students who will represent the French community of Belgium at
the International Chemistry Olympiad.
Our students are chosen from the secondary school program in
Belgium. It turns out from experience that the chemistry level proposed during
the IChO is by far higher than the one teaching to these students. But In
Belgium the next step of learning is University which means no more possibility
to compete to the IChO. The French laureates are therefore invited to a five
days training at the University of Liège. Several lessons are given to the
students according to the preparation problems published by the organisers
of the IChO. At the end of this training period, two students have been
selected from 6 this year. The final selection these two students have to pass
through consist in two evaluations: one is practical, the other is theoretical.
You will find the questions submitted to the students this year in the
following pages. We hope these questions could help you in anyway of
preparing or selecting your students during the next years.
We would also like to thank all our sponsors for they financial supports
without which it would have been very hard to organise the French chemistry
Olympiad and the selection for the International Chemistry Olympiad.
Belgian French selection questions booklet
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FIRST PART: PRACTICAL EXAMINATION
The students are confronted to two experimental examinations that
have to be performed simultaneously. Students have been evaluated both
from their work and their answers.
Organic Chemistry:
Purification of a mixture of p-hydroxybenzoic acid and
p-hydroxyacetophenone
The aim of this manipulation is to purify a sample of p-hydroxybenzoic
acid A contaminated by p-hydroxyacetophenone B.
A
B
In an Erlenmeyer flask of 100mL, 1 g of the mixture of p-hydroxybenzoic
acid A and p-hydroxyacetophenone B is suspended in 15 mL of water. The
solution is heated to reflux and water is added until complete dissolution of
the solid (max. 10 mL of water added; Vtotal < 25 mL). If the solid is not fully
dissolved, ethanol is added dropwise until complete dissolution.
The hot solution is filtered under gravity. After cooling, the filtrate is kept
at room temperature for 30 to 45 min. The erlenmeyer containing the filtrate is
then cooled down to 0°C in an ice bath until appearance of the crystals of phydroxybenzoic acid.
The solid is isolated by suction filtration, dried under vacuum for 1 hour
and finally weighted.
A tip of a spatula of the solid is dissolved into 1 mL of MeOH. The solution
is spotted on a TLC plate (4 different spots to make, see question 1 below).
After migration, the plate is developed under a UV light.
For information: eluent composition: CHCl3 70v; EtOAc 30v; MeOH 2v; glacial
HAc 2v.
Belgian French selection questions booklet
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1) From each spots recorded on the TLC plate, calculate the Rf value(s).
Spot
Compound
1
p-hydroxybenzoic acid
2
p-Hydroxyacetophenone
3
Initial mixture
4
Obtained Product
Rf
2) Is the recristallized product pure?
3) Determine the recristallization yield.
4) If the product was impure, propose an alternative method to purify the
initial mixture.
Belgian French selection questions booklet
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Analytical Chemistry:
Dosage of a synthetic Al-Zn alloy
This task has been inspired from the preparatory problem number 33
proposed the the 42nd IChO organisers.
Alloys containing aluminium and zinc have been developed in Japan
for use in industry. Recently, such an alloy containing between 7 to 50 wt% of
aluminium exhibits an interesting mechanical property: it is solid at room
temperature but is easily spread like a starch syrup under appropriate
mechanical tension. This property is known as “super-plasticity,” which
facilitates industrial uses of the alloy, including use as a semi-permanent
seismic damper for protecting buildings from earthquakes.
The ethylene-diamine-tetraacetic acid (EDTA, noted Y4-) is a ligand
which forms complexes of 1:1 stoechiometry that are very stable (high value
of the formation constant) with cations like Al3+ and Zn2+.
Complexe 1:1 MY
This ligand will allow you to determine both the concentrations of Zn2+
and Al3+ in acidic solution which simulates the composition of this type of
alloy.
Preparation step
1. Clean the 25 mL burette with desionised water and the zinc etalon solution
provided.
2. Clean the 10 mL pipette with water and the sample solution provided.
3. Clean the 30 mL pipette with water and the EDTA solution provided.
Advice: realise a first titration quickly adding the titration solution millilitre by
millilitre then realise at least two titrations precisely. Note that during a
colorimetric titration like the following one, an experimented student can
perform a precise titration directly by slowing the addition of titrant when the
solution colour begins to turn.
Belgian French selection questions booklet
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Dosage of the alloy
4. Pipette 10.00 mL of the sample solution and place it within a 250 mL
Erlenmeyer flask. Stir the solution with a magnetic stirrer and add a few
drops of methylorange indicator (MO). Add 30.00 mL of the 0.01402 mol L-1
EDTA standard solution provided. Adjust the pH around 3.5 by adding
dropwise a 10% (w/v) solution of hexamine until the indicator turns from red
to orange.
5. Boil the mixture for around 5 minutes and then let the solution cool in an
ice-bath. Add a few drops of xylenol orange indicator (XO) after cooling.
6. Adjust the pH to around 5.5 by adding the hexamine solution dropwise
until the solution colour changes from yellow to slightly purple. Add an
acetic acid 0.1 mol L-1 solution dropwise until a clear yellow colour
reappears.
7. Titrate the mixture by adding the standard solution of Zn2+ 0.01002 mol L-1
provided until the solution turns purple. Be careful, the colour transition
could be hard to detect. When you think to be at the equivalent point,
note the volume and add one drop more. If the colour changes, this
means that you were not at the equivalent point!
Caution: keep the sample for the following titration!
1) Write the correct equations for the reaction between each cations and
the EDTA.
2) Report the volumes of the standard Zn2+ solution added to the mixture
to reach the equivalent point.
8. Add 1g of NaF to the solution resulting from the precedent titration. Heat
the mixture until boiling. Cool down the solution again. The solution must
have turns back yellow. If it does not the case add some acetic acid
dropwise until the yellow colour reappears.
3) Why adding the NaF to the mixture?
9. Titrate the mixture by the standard EDTA solution.
4) Report the volumes of the standard Zn2+ solution added to the mixture
to reach the second equivalent point.
5) Calculate the concentration of both Al3+ and Zn2+ in the sample
solution.
10. Do points 4 to 9 again in order to get reproducible values.
Belgian French selection questions booklet
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SECOND PART: THEORETICAL EXAMINATION
During the training, different lessons are organised for the students. Each
teacher they met has been invited to write a question in order to select the
two students for the IChO.
The students are provided with a non programmable calculator and a
periodic table in order to answer the questions.
Electrochemistry
The following electrochemical cell
(-) Pt(s)| H2(g) (P = 1 atm) || HBr (10-4 mol/L) | CuBr(s) | Cu(s) (+)
displays an electrochemical potential of 0.559 V at 25°C.
1) Balance the reduction, oxidation and overall reactions for this galvanic
cell.
2) Determine the standard electrode potential for the CuBr/Cu couple at
25°C.
3) What is the value of the equilibrium constant for the overall reaction
when the cell is exhausted?
4) Determine the standard Gibbs free energy for the overall reaction, at
25°C.
5) Determine the solubility product of CuBr at 25°C knowing that the
change in standard Gibbs free energy for the CuBr dissociation
amounts to 42,1 kJ/mol.
6) Indicate the change in electrochemical potential of the cell (amplitude
and direction of change) when the dihydrogen pressure is increased by
a factor of 2?
Belgian French selection questions booklet
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Organic Chemistry
Consider the following reaction scheme.
1) How many stereoisomers are possible for compound A?
2) Represent, in space, compound A in his most stable conformation.
3) Draw compound A’ which, once mixed with an equimolar amount of
A, optical rotation is on principle zero degree.
4) Draw an epimer of compound A.
5) Draw compounds B and E.
6) Show chemical reagents necessary for the transformation B → C?
7) Draw compounds D, D’, G and G’.
Sugars chemistry
D-erythrose and D-threose are two aldotetroses belonging to the
natural carbohydrates family.
1) Draw Fisher projections of the open-form for these two
monosaccharides.
2) Draw Fisher projections of their L enantiomeric forms.
3) Write the conversion reaction of the D forms into ketotetrose and
indicate the nature of the reaction intermediate in the aldose ⇌ ketose
interconversion.
4) How many enantiomeric forms are there for ketotetrose? Draw their
Fisher projections.
5) What is the optical property which most easily allows following the
processes of anomerisation, interconversion and epimerization? Explain
the physical basis for this optical property.
Belgian French selection questions booklet
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Thermodynamic
Here is the production reaction of methanol by hydrogenation of
carbon monoxide. Consider it occurring at 25 °C.
CO (g) + 2 H2 (g) → CH3OH(l)
You may enjoy referring to the thermodynamic values hereunder to answer
the following questions. Note that these values are given under the standard
conditions and at a temperature of 25°C.
‐ Standard formation enthalpy: – 110.53 J mol-1 for gaseous CO and –
238.66 J mol-1 for liquid CH3OH.
‐ Standard variation of entropy for the reaction : – 300.29 J K-1
1) For the reaction above, calculate :
a) the variation of enthalpy;
b) the variation of free enthalpy ;
c) the equilibrium constant Kp.
2) To your point of view, would the methanol quantity produced be higher
or reduced if :
a) the temperature is increased, the pressure remaining constant?
b) the pressure is increased, the temperature remaining constant?
Spectroscopy: electronic levels
The energy of a hydrogen atom is quantized. The energies of the
successive allowed levels are given by the following formula:
En = −
me e 4 ⎛ 1
⎜
8ε 20h2 ⎝ n 2
R
⎞
⎟ = − H2
n
⎠
n = 1,2,3....
where me is the electron mass, e is the elementary charge, ε 0 is the
permittivity of the vacuum and h is Planck constant.
1) Calculate, in Joule (J) and in electron-volt (eV), the value of the RH
constant.
Belgian French selection questions booklet
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In the emission spectrum of the hydrogen atom, the so-called
Balmer series corresponds to n' ≥ 3 → n = 2 transitions, where n' is the quantum
number of the upper level and n is the quantum number of the lower level.
2) To which n' value does the transition observed at a wavelength of
434.047 nm in the Balmer series correspond? (1 nm = 10–9 m)
The muon (denoted as µ) is a particle which belongs to the same family
as the electron, i.e. the lepton family. Its charge is the same as that of the
electron: q = – e = – 1.6022×10–19 C but its mass is much larger : mµ =
1.8835×10–28 kg. A muonic hydrogen atom is a hydrogen atom in which the
electron has been replaced by a muon: it consists therefore of the assembly
of a proton and a muon.
3) Evaluate the ionization energy (in eV) of a muonic hydrogen atom,
that is, the energy input required to separate the proton and the
muon.
The dissociation energy of a diatomic molecule is the energy which
needs to be absorbed by this molecule in order to fully separate its two
atoms. The He2 molecule is not stable but its molecular ion He2+ is stable, and
its dissociation energy is equal to 2.47 eV. The dissociation energy of the H2
molecule amounts to 4.52 eV whereas that of H2+ is equal to 2.65 eV.
4) Explain why He2+ is stable whereas neutral He2 is not.
5) Why does the ionization of H2 lead to a weakening of the
dissociation energy?
6) Based on the fact that the interatomic distance (Re) in He2+ is equal
to 0.108 nm, draw a schematic representation of the potential
energy of He2+ as a function of the interatomic distance (R). Please
respect the orders of magnitude and mention a few numerical
values as references on your drawing.
Permittivity of vacuum
Speed of light in vacuum
Planck constant
Elementary charge
Electron mass
Proton mass
Gas constant
Boltzmann constant
Avogadro constant
Belgian French selection questions booklet
ε0
c
h
e
me
mp
R
k
NA
8.8542×10-12 F m–1
2.9979×108 m s–1
6.6261×10-34 J s
1.6022×10-19 C
9.1094×10-31 kg
1.6726×10-27 kg
8.3145 J mol–1 K–1
1.3807×10-23 J K–1
6.0221×1023 mol–1
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Spectroscopy: vibration levels
The fundamental and harmonic transitions of vibration of the CO
molecule are observed at the wavenumber given in the following table.
v
Ev (cm-1)
0
2050.43
1
6151.29
2
10252.10
3
14353.00
4
?
The energy of every vibrating level can be calculated from the well known
formula:
Ev =
h k ⎛
1⎞
⎜v + ⎟
2π μ ⎝
2⎠
Where h is the Planck constant, k the force constant of the C=O bond, µ the
reduced mass of CO and v the quantum number of vibration.
1) Calculate the reduced mass of the CO molecule (expressed your
answer in kilogram).
2) Estimate the force constant of the CO bond (expressed your answer in
Newton per meter).
3) The force constant of the H-Cl bond has been determined to be equal
to 516.5 N m. How could you explain the difference between this value
and the one calculated at the second point?
4) Calculate the energy of the v = 4 level of vibration (express your answer
in joule).
5) Calculate the energy required to process the transition from the
vibration level v = 4 to the vibration level v = 2. What is the wavelength
of the wave corresponding to this transition?
6) Calculate the energy required to process the transition from the
vibration level v = 0 to the vibration level v = 3.
7) Without any calculation, do you think the energy of the transition from
vibration level v = 6 to vibration level v = 9 will be by far different from
the one you have calculated at the fifth point if the same hypotheses
are applied?
Notes:
You can refer to the table in the preceding question.
1 eV = 1.6022 10-19 J
Belgian French selection questions booklet
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