澳大利亚全国化学竞赛试题(1998-2010)-FSE7A

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FINAL SELECTION EXAMINATION

for the

2008 AUSTRALIAN CHEMISTRY

OLYMPIAD TEAM

PAPER A

2007

Please note that this answer book will be photocopied when returned and then

split so that answers are sent to the appropriate markers.

For this reason it is extremely important that you observe instructions 6 to 8.

Instructions to Scholars

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

2. You are not permitted to refer to books or notes but you may use a non programmable electronic

calculator.

3. All questions to be attempted. A guide for time allocation is supplied at the beginning of each question. 4. A periodic table with atomic masses and the values of some physical constants are provided on the

following page. Data are supplied, where necessary, with each question.

5. Answers must provide clearly laid out working and sufficient explanation to show how you reached

your conclusions.

6. Answers must be written in the blank space provided immediately below each question. Rough working

must be on the backs of pages. Only material presented in the answer boxes will be assessed.

7. Ensure that your name is written in the appropriate place on ALL of the pages (even those you may

have left blank) in this examination booklet.

8. Use only black or blue pen for your written answers, pencil or other coloured pens are not acceptable.

Supervisor Declaration

I certify that the final selection examination was carried out under strict examination conditions and that no improper actions occurred during the examination period.

Name of Exam Supervisor: (please print) ……………………………………………………………..…… Signed: ……………………………………………………………… Date: ……………………………

Please use the enclosed pre-addressed Express Post Envelope to return all examination papers to:

Mr R W Switzer, ASO Chemistry Program, 14 Liverpool Street, Golden Crest Manors, NERANG QLD 4211.

EXAMINATIONS SHOULD BE RETURNED BY EXPRESS POST ON WEDNESDAY 12th MARCH SO THAT THEY ARE RECEIVED BY FRIDAY 14th MARCH 2008.

Final Selection Exam — Paper A

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CONSTANTS

speed of light in vacuum c = 2.998 × 108 m s–1 Planck’s constant h = 6.626 × 10–34 J s elementary charge e = 1.602 × 10–19 C electron mass me = 9.109 × 10–31 kg Avogadro constant NA = 6.022 × 1023 mol–1 Faraday constant F = 9.85 × 104 C mol–1 ideal gas constant R = 8.3145 J K–1 mol–1 Boltzmann constant k = 1.381 × 10–23 J K–1 atomic mass unit u = 1.661 × 10-27 kg pi π = 3.14159 Ångström 1 Å = 10–10 m

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Final Selection Exam — Paper A

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Question 1 (a) (i) (b) (i)

(16 minutes)

Write the ground state electron configuration for a chlorine atom, Cl.

(ii) Write an excited state electron configuration for a boron atom, B.

(iii) Draw a Lewis structure for Br2O5 (which contains a Br–O–Br bridge). Describe the shape around

each of the Br atoms.

Consider the molecule, CO2.

Use VSEPR theory to rationalise the shape of CO2.

(ii) Draw resonance structures for CO2 and indicate which structure will make the major contribution to

the bonding.

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Final Selection Exam — Paper A

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(c)

(iii) NOF3 is well established with N–O and N–F single bonds. (ii) N5+ is bent at the central N atom. (i)

Comment on each of the following observations.

XeF6 has a pentagonal pyramidal (distorted octahedral) structure.

(iii) Construct a well-labelled hybrid orbital diagram for CO2 and include a description of the formation

of the sigma and pi bonds.

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Final Selection Exam — Paper A

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(iv) (d)

The dipole moment of (Z)-N2F2 in the gas phase is 0.16 Debyes, but (E)-N2F2 is non-polar. Explain how this difference arises.

BH3 can accept a pair of electrons to form compounds such as H3BN(CH3)3 in which the B atom is tetrahedral.

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Final Selection Exam — Paper A

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Question 2 (a) (i)

(36 minutes)

Data: pKa(CH3COOH)=4.76

Calculate the pH of each of the following solutions:

–1

A. 10.0 mL 1.00 mol L HCl. !

B. 10.0 mL 1.00 mol L–1 CH3COOH

C. 10.0 mL 1.00 mol L–1 CH3COO–

(ii) For each of the solutions above calculate the absolute change in pH when 5.00 mL of 1.00 mol L–1

NaOH is added.

A. B. C.

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Final Selection Exam — Paper A

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A 20.00 mL aliquot of an unknown monoprotic acid of unknown concentration is titrated with 5.00 × 10–1 mol L–1sodium hydroxide solution. The pH was measured after the addition of 10.00 mL and 20.00 mL of base and was found to be 8.50 and 9.50 respectively.

(iii) Give approximate ranges for reasonable values of original concentration and pKa for the unknown

acid given the information above.

(iv) Calculate the original concentration and pKa of the monoprotic acid.

(v) What will be the pH of the titration at equivalence?

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Final Selection Exam — Paper A

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(b)

Data: E°(VO+/VO2+)=1.00 V

2

E°(VO2+/V3+)=0.34 V E°3+2+=\"0.26 V (V/V)!

!

T=298 K

Batteries use electrochemical reactions to store energy in the form of potential chemical energy. ! Redox batteries are a type of galvanic cell that only utilizes aqueous species. An example is the ! vanadium redox battery (VRB) which only has vanadium species involved in the reaction. In this battery VO2+ is converted to VO2+ while V2+ is oxidised to V3+ in the presence of sulphuric acid. Write half equations and a balanced full equation for the VRB.

(i)

(ii) Calculate E0cell, ΔG0 and K for the main reaction that occurs in a VRB.

(iii) Given that the starting solutions for the battery have a concentration of 2.00 mol L–1with respect to

sulphuric acid and all other species are present at a concentration of 1.00 mol L–1what is the starting voltage, Ecell?

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Final Selection Exam — Paper A

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(iv) What is the voltage, Ecell, of the battery if no sulfuric acid was present initially? (Assume a neutral

solution).

One of the advantages of the VRB over other redox batteries is that if a contamination was to occur the battery could still be used as vanadium is present in both halves of the cell.

2+(v) What would be the resulting concentrations of VO+2 and VO in the cathode solution if 100 mL of

the anode solution was to contaminate 1.00 L of the cathode solution? (assume no change in pH)

! ! (vi) Hence what would be the new Ecell?

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Final Selection Exam — Paper A

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Question 3

(20 minutes)

The hypochlorite anion (ClO–) is found in many strong oxidising agents. NaClO is the active ingredient in bleach, and HClO is the 'chlorine' in salt-based swimming pools. Which element, oxygen or chlorine, is more electronegative?

(a) (i)

(ii) Draw a molecular orbital energy level diagram of the hypochlorite anion, showing how the atomic

orbitals combine.

(iii) Draw a 95% inclusion surface for each of the following MOs:

B) πp C) π*p A) σs

(iv) What is the bond order of ClO–, and is your answer consistent with VB theory?

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Final Selection Exam — Paper A

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(b)

The particle-in-a-box model, despite its simplicity, makes surprisingly accurate predictions of the absorption spectra of conjugated organic molecules. The thiacyanine dyes are one such set of molecules, where n = {0, 1, 2, ...} specifies the length of the conjugated chain. Take β = 140 pm to be the average bond length in the conjugated chain.

SNnSN

(i)

Draw two resonance forms of the dye with n = 0, showing how the electrons are delocalised along the π-system.

(ii) Take the length of the 'box' to be the distance over which the π-electrons are delocalised. What is

the length of the box when n = 0 (ignore any correction due to bond angles)?

(iii) On the same graph, draw the potential energy function V(x) and the second lowest energy

wavefunction Ψ2(x) of the system.

(iv) Derive formulae for L, the length of the box, and p, the number of electrons in the box in terms of

n.

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Final Selection Exam — Paper A

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(v) Draw an energy level diagram for the thiacyanine dye with n = 3. Label the HOMO and LUMO.

(vi) Derive a formula for the energy levels of a particle in a box.

(vii) Calculate the wavelength of light absorbed in the HOMO-LUMO transition of the dye with n = 3.

What colour is the dye?

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Final Selection Exam — Paper A

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Question 4 (a) (i)

(20 minutes)

Name each of the following transition metal complexes: [Rh(en)BrCO]

(ii) K2[Fe(CN)4(H2O)2]

(iii) Na[Co(ox)2(NH3)2]

(iv) Na[Au(CN)4]

Data: en = NH2CH2CH2NH2

ox = C2O42–

(b) (i)

For each of the following compounds, draw all possible isomers, designating each isomer with an appropriate label: [Co(en)2ClBr]+

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Final Selection Exam — Paper A

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(ii) [Pd(NH2CH2CH(CH3)NH2)2]2+ (racemic mixture of ligands)

(iii) [Fe(NH2CH2C(CH3)2NH2)3]2+

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Final Selection Exam — Paper A

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(c)

For each of the following compounds, construct a well-labelled crystal field energy diagram, showing electron occupancy. Also indicate:

• Predicted geometry of the ligands

• Spin-only magnetic momentum (in units of Bohr Magneton (BM)) • Overall crystal field stabilisation energy (CFSE) of the complex

(i)

[Mn(NH3)6]2+

(ii) [Os(CN)6]4–

(iii) [NiBr4]2–

(d)

Explain whether you would expect Jahn-Teller distortion to play a significant role in any of the complexes in part (c).

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Final Selection Exam — Paper A

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Question 5

(20 minutes)

0° C = 273.15 K

Data: Gas Constant R = 8.3145 J K–1 mol–1

Sulfonamides are an important group of drugs known for their effect on inhibiting bacterial growth, amongst other things. Because cell membranes consist mainly of lipid, the lipid/water partition coefficient of these drugs is a major determinant of properties of the drug, such as its absorption and penetration into bacteria.

We will study the thermochemistry of the n-octanol/water partition coefficients of three different sulfonamides as a close approximation of how sulfonamide drugs behave.

Partition coefficient KD is defined as the equilibrium constant of the partition process (assume the drug is not ionizable in aqueous phase):

[drug]aq sulfamethazine sulfamethoxazole sulfachloropyrazine

[drug]oct

KD (at 333.15 K) 0.73 0.5320 0.3121

KD (at 298.15 K) 1.8423 7.5057 1.7927

(a) (b)

Write an equation for the molar standard ΔG of the partition process from water to n-octanol, in terms of the partition coefficient KD and other relevant variables/constants.

Hence, calculate the molar standard ΔG of the partition process of the three given drugs (from water to n-octanol) at the two temperatures 298.15 K and 333.15 K.

ΔG (at 298.15 K) ΔG (at 333.15 K) sulfamethazine sulfamethoxazole sulfachloropyrazine

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Final Selection Exam — Paper A

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(c) Hence, calculate the molar ΔH and ΔS of the partition process (from water to n-octanol) of the three sulfonamides given.

ΔH ΔS sulfamethazine sulfamethoxazole sulfachloropyrazine (d)

What does the sign of ΔS and ΔH tell us about what drives the solvation of sulfonamides from water to n-octanol? Hence explain, using the Second Law of Thermodynamics, why the partition coefficients of all three drugs decrease with increased temperature.

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Final Selection Exam — Paper A

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(e)

Calculate the concentration of each drug in the n-octanol phase in equilibrium with an aqueous phase concentration of 5.00 × 10–5 mol L–1 of each sulfonamide at body temperature (310.15 K). Which drug would be expected to have the best penetration into cell membrane lipids at body temperature?

[drug]aq (mol L–1) 5.00 × 10–5 5.00 × 10–5 5.00 × 10–5 [drug]oct (mol L–1) sulfamethazine sulfamethoxazole sulfachloropyrazine Drug with best penetration into cell membrane lipids: (f)

Derive an expression that relates the ratio of partition coefficients at different temperatures

!KD(2)\" solely in terms of ΔH, T1 and T2. #$#K$%D(1)&

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Final Selection Exam — Paper A

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Question 6

(10 minutes)

Draw and name the products when (Z)-2-butene is treated with the following reagents. Indicate stereochemical and regiochemical outcomes where appropriate. O3 followed by Zn/H+ or DMS

(a) (b) (c) (d)

hot, concentrated KMnO4

cold, dilute KMnO4

gaseous HBr

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Final Selection Exam — Paper A

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(e) (f)

liquid Br2

aqueous Br2

Question 7 (a)

O(10 minutes)

Draw and name the products of the following reactions. Indicate stereochemical and regiochemical outcomes where appropriate.

1. Me-MgBr2. H+ (b)

O1. Me-MgBrOEt2. H+

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Final Selection Exam — Paper A

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(c)

O1. LiAlH42. H+ (d)

NaOCMe3HBrDMSO (e)

NaCNHBrHMPA (f)

ZnCl2OHconc. HCl (aq) Question 8

(6 minutes)

A single product forms when sodium ethoxide in ethanol is added to benzaldehyde and then acetone added dropwise to the mixture. Draw the structure of this product.

(a) (b) (c)

Heating the initial product results in a product of molecular formula C10H10O. Draw its structure and explain its formation.

If the acetone is not added dropwise another product competes. Draw the structure of this competing product and explain why the acetone must be added dropwise.

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Final Selection Exam — Paper A

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Question 9

(10 minutes)

p-aminobenzoic acid (PABA) is a common additive to sunscreens. When PABA is treated with bromine in the presence of iron(III) bromide, a single isomer of formula C7H6NO2Br is produced. Draw a mechanism including Whelan intermediates for attack at two different positions to predict the structure of this isomer. Indicate any particularly good or bad resonance contributors in your answer.

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Final Selection Exam — Paper A

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Question 10

(8 minutes)

The stoichiometric equation of the thermal decomposition of azomethane (CH3NNCH3) is

H3CNNCH3 (g)

N2 (g) + C2H6 (g)

The total pressure of the reaction mixture was measured at different times and at different temperatures as follows.

T = 571.6 K

T (min) P (torr)

10.0 491.9

20.0 8.0 T = 593.6 K

T (min) P (torr)

33.0 609.7

Infinity 861.6

9.1 318.5

18.2 371.5

Infinity 424.6

Determine the reaction order, rate constant k and the parameters of Arrhenius equation for this reaction.

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Final Selection Exam — Paper A

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Question 11

(8 minutes)

The measurement of 14C disintegrations is a powerful and widely used technique for determining the age of carbon-containing specimens.

Obtain the law of radioactive disintegrations, assuming it follows first order kinetics.

(a) (b)

The specific activity of 14C, in equilibrium with the atmosphere and the earth-skin, is 18.3 disintegations min–1 g–1. The minimum measurable value is 0.03. The half-life of 14C is 5720 years. Calculate the oldest measurable age.

(c)

In Zagreb, an Egyptian mummy, enveloped in a cloth with Etruscan inscriptions, was found. In order to ascertain the Etruscan origin of the cloth, the 14C method was used. By combustion of a splinter a gas was obtained which contained CO2. An activity of 12.9 disintegrations min–1 was counted in 2 dm3 of purified CO2 at 1 atmosphere and 25˚C. Calculate the age of the specimen. (1 dm = 10 cm)

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Final Selection Exam — Paper A

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Question 12

(6 minutes)

Insulin isolated from meat has been found to have molecular weight 6000 and to contain two peptide chains bound together by disulfide bridges. The two chains were separated after oxidation of the disulfide bridges. One of the chains, A-chain, was found to contain 21 amino acids units in the following proportions:

Gly, Ala, 2 x Val, 2 x Leu, Ile, 4 x Cys, 2 x Asp, 4 x Glu, 2 x Ser and 2 x Tyr

N-terminal assay by the dinitrofluorobenzene method (DNFB) gave DNP-Gly, and C-terminal assay using carboxypeptidase liberated Asp.

After partial acid hydrolysis the following peptides were isolated and determined using the method of Edman Degradation. 1. Cys-Asp 2. Tyr-Cys 3. Cys-Cys-Ala 4. Glu-Asp-Tyr

5. Glu-Cys-Cys 6. Glu-Glu-Cys 7. Glu-Leu-Glu 8. Leu-Tyr-Glu

9. Ser-Leu-Tyr 10. Ser-Val-Cys 11. Gly-Ile-Val-Glu-Glu

Furthermore, it was found that the A chain in its native state contained an intramolecular disulfide bridge giving a dodeca-ring.

Give the amino acid sequence for the A chain.

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Final Selection Exam — Paper A

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Question 13

(10 minutes)

Isoelectric point (pI) is defined as the pH at which a particular molecule or surface carries no net electrical charge. Consider a generic naturally occurring amino acid whose side group R is neutral. Draw the structure in Fischer projection form for such an amino acid.

(a) (b)

By considering the two equilibria associated with the C terminus and N terminus of the amino acids, and where the respective equilibrium constants are ka1 and ka2, derive an expression for pI.

Glutamatic acid (Glu) is very important in cellular metabolism. Its other important function is to serve as the precursor for the synthesis of GABA (an inhibitory neurotransmitter) in a reaction that involves glutamic acid decarboxylase (GAD), which is very abundant in the cerebellum and pancreas.

What class of biomolecules does GAD belong to? What is the function of GAD?

(c) (d) (e)

Where would you expect to find glutamic acid in the 3D structure of GAD in an aqueous environment?

Calculate the pI for glutamic acid given its pKa (N-terminus) = 9.67, pKa (C-terminus) = 2.19, pKa (side group R-COOH) = 4.25.

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Final Selection Exam — Paper A

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(f)

Lysine is a basic amino acid with side group of (CH)4NH3+ at pH 7. It has pKa (C-terminus) = 2.18, pKa (N-terminus) = 9.04 and pKa (side group R'–NH3+) = 8.95.

Write down (in short hand) all possible dipeptides that can be formed from Lys and Glu. Calculate the pH at which two thirds of glutamic acid’s γ-COOH group has been deprotonated.

(g) (h)

If we extend the idea of isoelectric point to peptides, calculate the isoelectric point of the dipeptide Lys-Glu. [Assume that there is no difference in the pKa of each amino acid functional group before and after they form peptides.]

(i)

Draw the dipeptide Lys-Glu at pI. You may represent the respective side groups as R–COOH/R–COO– and R'–NH2/R'–NH3+

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