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25th Austrian Chemistry Olympiad 1999 National Competition Theoretical part Problem 1
Complex ChemistrySeveral different locations can be imagined for the ligands of a complex of the type MeX3Y2 (Me = centre atom; X, Y = ligands). Draw all possible diastereomeres for the above described complex including contingent optically active isomeres. Additionally state, which of the possible complexes outlined will be the most stable one (assuming that X is a small and Y a large ligand). As an assistant device (so to say as a master copy) you can choose among several different coordination polyhedrons.
25th Austrian Chemistry Olympiad 1999 National Competition Theoretical part Problem 2 10 points
Environmental and Inorganic ChemistryA) Determination of oxygen in a sample of water from the river Schwechat The determination of oxygen in water samples is carried out in the iodometric analysis described below (method by WINKLER) : 1st step: fixation of oxygen. The oxygen in solution oxidizes Mn2+ to Mn(IV) in alkaline medium giving MnO(OH)2 2nd step: By addition of acid the above mentioned manganese compound reacts with a surplus of Mn2+ to give Mn3+-ions (comproportion reaction). 3rd step: The Mn3+-ions developed oxidise added iodide to give iodine and are themselves reduced to Mn2+ . 4th step: The amount of iodine generated in step 3 is titrated with a solution of thiosulfate. Task 1: Write balanced ionic equations for the four reactions described above! 1st step: 2nd step: 3rd step: 4th step: The analysis of water samples from the river Schwechat showed the following data: Standardization of the sodium thiosulfate solution: the standardisation was ensued with KIO3 in acidic medium, whereby iodate was reduced to iodide. 25.00 mL of KIO3-solution ( (KIO3) = 174.8 mg/L) consumed 12.45 mL of thiosulfate. 2. Immediately after sampling, the determination of oxygen was carried out using WINKLERs method described above. 11.80 mL of the standardised sodium thiosulfate solution were consumed for 103.50mL of water sample at 20.0 C. The saturation concentration of O2 in water amounts to 9.08 mg/L at 20.0C. 3. A second sample (V = 102.20 mL, T = 20.0 C) was incubated after sampling for 5 days in a climatic cabinet at 20.0C. The analysis after this incubation gave a consumption of 6.75 mL of thiosulfate.1.
task 2: a) Write balanced ionic equation for the standardisation of the thiosulfate solution!
b) Calculate the concentration of the standardised thiosulfate solution! c(S2O32-)= mol/L
25th Austrian Chemistry Olympiad 1999 National Competition Theoretical part
c) Calculate the oxygen content of the water which was analysed immediately after sampling using the unit mg/L!
d) Calculate the oxygen saturation index for this water sample! OSI= %
e) What is the content of oxygen in the incubate water sample?
Which of the characteristic parameters of water analysis was thus determined? What is the value of it?
B) The peculiar chemistry of a well known elementAn oxygen containing anion of an allotropic element is characteristic for water pollution; the element has a lower electronegativity than oxygen. With halogenes it forms only molecular compounds. Additionally to two monomolecular oxides also high molecular ones are known. Element X is also of great importance in biochemistry. Its p-atomic orbitals are half occupied each. Task 1: What element is talked about? Write down its electron configuration! Element: electron configuration:
Element X also forms numerous covalent compounds with hydrogen with the general formula XaHb. A series of such compounds is analogous to the homologous series of the alkanes. Task 2: Draw the structural (constitutional) formulae of the first four homologous X-Hcompounds!
One of these four substances consists of 3 diastereomeres (similar to tartaric acid). Task 3: Give the number of this compound! compound nr.: Element X forms oxoacids with the general composition H3XOn , where n=2,3, and 4.
25th Austrian Chemistry Olympiad 1999 National Competition Theoretical part
Task 4: Draw the structural (constitutional) formulae of these three acids. Mark the acidic Mark the acidic H-atoms (asterisks or arrows) and give the oxidation numbers of element X in each of these compounds.
A heterocyclic compound of element X with plane structure, which was already synthesised by J. Liebig and F. Whler in 1834, is gained starting with NH 4Cl and the pentachloro compound of X. As by-product a gas evolves, which is readily soluble in water and reacts as strong acid. Task 5: a) Write a balanced equation for the above described reaction!
Draw the structural (constitutional) formula of the compound (NXCl2)3!
The just described inorganic substance shows a peculiar behaviour when heated: it boils at 256C when heated rapidly. If one heats slowly, the substance will start to melt at 250C. Cooling the liquid rapidly, one gains a substance similar to rubber. Task 6: Explain this peculiar behaviour!
25th Austrian Chemistry Olympiad 1999 National Competition Theoretical part Problem 3 5 points
Cerimetric AnalysisIn cerimetric analysis the oxidising agent are Ce4+-ions, which are easily reduced to Ce3+-ions. The value of the standard redox potential of these ions depends on the anions present. As an original titer for standardisation of Ce4+-solutions As2O3 is used, which gives with sodium hydroxide and subsequent acidification arsenite (AsO33-), which the is oxidised by Ce4+ to form arsenate (AsO43-). As a catalyst one uses small amounts of OxO 4. Ferroin acts as redox indicator. Write a balanced ionic equation for the described redox titration of arsenite with Ce4+ and calculate the equivalent potential when working at pH = 1! E1(AsO43-/AsO33-) = 0.56 V E2(Ce4+/Ce3+; HClO4) = 1.70 V
25th Austrian Chemistry Olympiad 1999 National Competition Theoretical part Problem 4 7 points
Physical ChemistryA. Vapour pressure of a mixture
The vapour pressure of pure ethylethanoate amounts to 9706 Pa at 20C, the one of ethylpropanoate to 3693 Pa at the same temperature. Due to the similarity of these two compounds the mixture shows an ideal behaviour. Calculate the vapour pressure of a mixture 25 g of ethylethanoate and 50 g of ethylpropanoate at 20C.
A kinetic problem
Bromomethane may react with OH- according to a SN-mechanism. a) Write a balanced equation for this substitution. The following table shows the initial rates of the reaction together with the corresponding initial concentrations of CH3Br and KOH. All experiments were carried out at 25C. c(CH3Br) 1st experiment 2nd experiment 3rd experiment 0.10 mol/L 0.10 mol/L 0.033 mol/l c(KOH) 0.10 mol/L 0.17 mol/L 0.20 mol/L vo (mol/L*s) 2.80*10-6 4.76*10-6 1.85*10-6
b) Determine the reaction order relative to the individual starting substances and also the overall reaction order using a calculation. c) Calculate the velocity constant of the reaction. d) How long will it last to reach a concentration of KOH of 0,05 mol/L in the first experiment? e) Give the more accurate name of the mechanism which may possibly reaction. apply to the
25th Austrian Chemistry Olympiad 1999 National Competition Theoretical part Problem 5 7 points
ThermochemistryThe following table shows a series of reactions and reaction enthalpies at standard conditions: # 1 2 3 4 Reaction 2 NH3 + 3 N2O N2O + 3 H2 2 NH3 + 0,5 O2 H2 + 0,5 O2 4 N2 + 3 H2O N2H4 + H2O N2H4 + H2O H2O H 298
- 1011 - 317 -143 -286
Additionally the entropies of the listed substances are given at standard conditions: S 298 298
(N2H4) = 240 J/mol.K (H2O) = 66.6 J/mol.K298
(N2) = 191 J/mol.K (O2) = 205 J/mol.K
a) Calculate the formation enthalpies H and ammonia.
of the compounds hydrazine, dinitrogenoxide
b) Write a balanced equation for the combustion of hydrazine to give water and nitrogen. c) Calculate the reaction heat of the above combustion at constant pressure and at 298 K. Also calculate G 298 and Kth. d) What will be the reaction heat of reaction #3 at constant volume, if one starts with 2 mol ammonia and 0,5 mol oxygen?
25th Austrian Chemistry Olympiad 1999 National Competition Theoretical part Problem 6 6 points
0.85 g of AgNO3 are solved in water to give 500 mL of solution. A second solution of 500 mL of NH3(aq) with c0(NH3) = 2.0 mol/L is at your hand. a) Calculate the concentration of Ag+. Now the two solutions are mixed. In this case the complex [Ag(NH3)2]+ is generated primarily, the dissociation constant of it amounts to KD = 5.9*10-8 mol2/L2 . b) Write a balanced equation for the formation of the complex. c) Calculate the equilibrium concentration of free silver. The formation constant of [Ag(NH3)]+ is K1 = 1.8*103 L/mol. d) Calculate the equilibrium concentration of [Ag(NH3)]+ using the data from c). e) What is the value for K2 of the reaction: [Ag(NH3)]+ + NH3 Hint: Use in your calculations appropriate simplifications.
25th Austrian Chemistry Olympiad 1999 National Competition Theoretical part Problem 7 11 points
Organic ChemistryA. Spectra of an antifungal agent:
A fungicide and oxidizing substance A is described by an 13C-NMR-spectrum (containing a solvent peak) and by the mass spectrum. The infrared spectrum shows a distinct peak at 1685 cm-1 and another peak at 1570 cm-1. The 1H-NMR-spectrum gives no signal.
The production of A starts from C6H4O2. The cycle addition / rearrangement formi