澳大利亚全国化学竞赛试题(1998-2010)-FQEA004

FINAL PAPERPART A1991

AUSTRALIAN CHEMISTRY OLYMPIAD

Instruction to candidates(1)(2)(3)(4)

You are allowed 10 minutes to read this paper, and 3 hours to complete the questions.

You are not permitted to refer to books, notes or periodic tables but you may use a non-programableelectronic calculator and molecular models.You must attempt all questions.

Answers must be written in the blank space provided immediately below each question in the exambooklet. Rough working must be on the backs of pages. Only material presented in the answer boxeswill be assessed.

Answers must provide clearly laid out working and sufficient explanation to show how youreached your conclusions.

Your name must be written in the appropriate place on each page of your answers.

Use only black or blue ball point pen for your written answers, pencil or other coloured pens are notacceptable.

(5)(6)(7)

Question 1(a)In 1962, Neil Bartlett reported the synthesis of the first noble gas compounds, XeF2 and XeF4.

The two fluorides were prepared by the direct reaction of Xe(g), an element of the fifth rowof the periodic table, and F2(g).

(i)

Write ground state valence electron configurations for Xe and F, and describe the valenceorbitals of these two atomic species in terms of the quantum numbers n, l and ml.

(ii)Draw the Lewis structure and predict the electron pair geometry and molecular structure of the

two fluorides. What is the hybridisation of the central atom in each of these two compounds?

Would either of the two species be expected to have a dipole moment?

(b)

Draw clearly labelled molecular orbital energy level diagrams for He2+, OF and O2+. What isthe bond order in each case and which species would be expected to be paramagnetic?

Question 2(a)Sketch and name all the possible stereoisomers of the following coordination compounds.

(i)

[Pt(NH3)2ClBr]

(ii)(NH4)2[Fe(C2O4)2(NH3)Cl]iii)[Co(NH3)3(NCS)3]

(b)

Show how the crystal field theory may be used to account for the diamagnetism of

tris(ethylenediamine)cobalt(III) chloride and the paramagnetism of the hexafluoroferrate(III)anion.

Question 3

(a)Carbonium ions have been postulated to be the reactive intermediate in the Friedel Crafts

alkylation reaction. Frequently they are generated by reacting an alkyl halide withaluminum(III) chloride. However alternative strategies are also available. Thus for examplewhen prop-2-enol is treated with hydrogen fluoride in the presence of benzene two productsare formed: 3-phenyl-1-propene and 1,2-diphenylpropane. Give a clear explanation andmechanism of how these products are formed.

(b)

In addition to substitution reactions carbonium ions are also known to be key intermediates incertain addition and elimination reactions.However another aspect of their chemistry is one in which the carbon skeleton rearranges. Onesuch case is the so called \

CH3

CH3HCH2O3

CCH2Cl

CHS3

CCH2CH3

N1

CH3

conditions

OH

Give a clear mechanism for this reaction, and suggest why this reaction occurs.

(c)

Carbanions are associated frequently with methylene (CH2) or methine (CH) groups adjacent toan electron withdrawing group such as a carbonyl group. Suggest a mechanism for the socalled \

(CH2)n CH2

CO2Et

CH2

CHNaOEt2

(CH2)n

C

O

CO2Et

CHn = 2-4

CO2Et

(d)

Grignard reagents are a well known source of nucleophilic carbon.

Predict the structure of the product of the reaction of excess phenyl magnesium bromide withdimethyl carbonate (1), assuming that the usual acid work up is employed.

CH3OCO

(1)

CH3O

dimethyl carbonate

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(e)

A substance A (C5H10O2) gives a positive test with 2,4-dinitrophenylhydrazine but will notreduce [Ag(NH3)2]2+. Oxidation of A with CrO3 in pyridine affords B which reacts with2,4-dinitrophenylhydrazine and also reduces [Ag(NH3)2]2+.When 1 mole of A is treated with 3 moles of methyl magnesium iodide. One mole of methane isevolved. When the reaction mixture is acidified a second mole of gas is evolved and aproduct C (C6H14O2) isolated.Reaction of C with a trace of sulfuric acid affords D (CF and G.6H10), which upon reaction with ozoneaffords equimolar quantities of E, E is identical to the product obtained from 2-propanol upon oxidation with an acidic aqueoussolution of chromium oxide. Reaction of F with sodium borohydride affords methanol.Oxidation of G gives oxalic acid (ethanedioic acid).Identify compounds A to G.

(f)

The structure of limonene, a terpene found in citrus oils is shown below. Predict the productsformed when limonene reacts with the following reagents

limonene

(i)

excess HBr

(ii)ozone followed by dimethyl sulfide(iii)warm concentrated KMnO4(iv)diborane followed by basic H2O2

Question 4

A volume of a 0.2M potassium chlorosuccinicate (KO2CCH2CH(Cl)CO2K) was hydrolysed byan equal volume of 0.2 M potassium hydroxide. The hydroxide concentration was monitoredwith time at 25°C, the results are listed below.

time[OH–](seconds)(mol L-1)100.085200.074300.065450.056600.049800.042100

0.036

The experiment was repeated at 35°C and it was found that the hydroxide concentration halvedafter 21 minutes.

(a) What is the overall order of the reaction?(b)

Calculate the activation energy for the reaction.

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Question 5(a)The solubility product for mercuric sulfide, HgS, is 1.6 x 10-54 at 25°C. Calculate the solubility of

mercuric sulfide in water at this temperature, and express your answer in terms of thenumber of Hg2+ and S2- ions per litre. Does this seem reasonable? Discuss.Avogadro constant is 6.02 x 1023.(b)

A buffer solution is made by mixing 50 mL of 0.200 M acetic acid with 50 mL of 0.200 M sodiumacetate. The pKa for acetic acid is 4.8.(i)

Calculate the pH of this buffer solution.

(ii)How much does the pH change if 3 mL of 0.100 M HCl is added to the buffer solution?(c)(i)

If 10.0 mL of 0.050 M AgNO3 solution are added to 20.0 mL of 0.030 M NaCl solution, what arethe final concentrations of all species in solution at equilibrium? Assume additive volumes.Ksp for AgCl is 1.7 x 10-10

(ii)In concentrated chloride solutions the reaction

AgCl (s) + Cl- (aq) AgCl-

2 (aq)

Keq = 1.0 x 10-2occurs. What mass of solid NaCl must be added to the solution in part (i) in order tocompletely redissolve all the silver chloride? Assume no volume changes occur.

(d)

Ethylenediaminetetraacetic acid, EDTA, is a chelating agent useful for the titration of calciumion, and is used for determining water hardness by titration. It has four ionisable protons andcan be represented by H4Y.

The complex formed with calcium ion is given by:

Ca2+ + Y4- CaY2-with a formation constant of:

K2f = [CaY-]

[Ca2+] [Y4-]

At a given pH only a fraction, α, of the EDTA exists as Y4-, and so the concentration of Y4- in

equilibrium is αC

H4

Y,where CH4

Y is the analytical concentration of EDTA ( the total of allforms existing at a given pH). Hence, [Y4-] = αC

H4

Y.

The formation constant for CaY2- is 5.0 x 1010. At pH 10, α is 0.35.

Calculate [Ca2+] and pCa (-log[Ca2+]) in 100mL of a solution of 0.100 M Ca2+ at pH 10 afteraddition of

(i)0 mL(ii)50 mL(iii)

150 mL

of a 0.100 M EDTA solution.

Question 6

Plant and animal physiology involve a complicated and very interesting branch of biology andchemistry termed biochemistry characterised into what are termed “pathways’. Each pathwayinvolves the synthesis or consumption of one or more important chemical compounds forbiochemical ends. One such pathway in the glycolytic pathway, in which glucose isconsumed releasing energy for the organism to utilise. Another pathway, and the one ofimportance here is the electron transport pathway termed respiration. One series of steps inthis pathway involves a group of enzymes (an enzyme is a biological catalyst) andcomplexes termed the cytochromes. The family of cytochromes are haem based units withan iron atom situated at the active site of the complex, as indicated in figure 1.

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Figure 1

CH3CH2CH2SCH2PrHCCHotNH3CCHeNFeN3iCH2CHSCHnSFeNN2CH22CH2HCCHCO2-Haem groupCH2CH3The Active SiteCH2Cytochrome cCO2

-As the Fe atom cycles between the 2+ and 3+ oxidation states, an electron is transported.Altering the ligation of the central Fe atom affects its standard reduction potential, and natureutilises this to allow a transfer of electrons between a series of complexes at differentpotentials, as indicated in figure 2.Figure 2

NADP+

cytochrome b

cytochrome c

cytochrome a

oxygen

The role of the cytochrome is central to both the electron transport and biochemical processessuch as biosynthesis and (barbituate) detoxification. For example, cytochrome P450 acts asindicated in figure 3 to both reduce oxygen to water and allow incorporation of oxygen to abiological molecule, altering its biochemical activity.In an effort to understand the processes involved, Professor Richard Holm modelled thecytochrome complexes, successfully contributing to our understanding of the factorsaffecting the reduction potentials of the Fe II/III . He took Fe substituted haem and changedthe ligands in the remaining two positions (recall Fe is 6 coordinate!!) and measured theresulting reduction potentials. This data is presented is the data table, with the cytochromemodel, its measured Fe II/III potential and ligation pattern. These complexes react inbiological conditions, at a temperature of 36.9 °C, and normal pH buffered to 7.6. However aproton source is present as indicated in figure 3, and the local pH in the complexes localecan be considered to be 7. This proton source is NADPH (or nicotinamide adeninedinucleotide phosphate).

Figure 3

NADPH

RHO2cytochrome P450NADP+

ROHHOHThe following data and information is made available for you.

Data :

Equations:

?G = ?H - T?S

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