FSE2009B澳大利亚化学竞赛试题

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FINAL SELECTION EXAMINATION

for the

2010 AUSTRALIAN CHEMISTRY

OLYMPIAD TEAM

PAPER B

2009

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 answer boxes 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, HIGHLAND PARK QLD 4211.

EXAMINATIONS SHOULD BE RETURNED BY EXPRESS POST ON THURSDAY 11th MARCH

SO THAT THEY ARE RECEIVED BY FRIDAY 12th MARCH 2010.

Final Selection Exam — Paper B

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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 B

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Question 1 (a)

(13 minutes)

Assume that we are in another universe with different physical laws. Electrons in this universe are described by four quantum numbers with meanings similar to those we use. We will call these quantum numbers p, q, r and s. The rules for these quantum numbers are as follows: p = 1, 2, 3, 4, 5, …

q ≤ p but has only positive odd integers.

r takes on all even integer values from –q to +q. Zero is considered an even number. s = +1/2 or –1/2

How many electrons can have p = 4 and q = 3?

(i)

(ii) How many electrons can have p = 3, q = 0 and r = 0?

(iii) How many electrons can have p = 6?

(iv) Sketch what the first four periods of the periodic table would look like in this universe.

(v) What are the atomic numbers of the first four elements you would expect to be the least reactive.

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Final Selection Exam — Paper B

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(b)

In the amide CH3CONH2, the three bonds to nitrogen lie in a plane which is contrary to the predictions of the VSEPR model.

(ii) Suggest a method to synthesise the xenon fluorides mentioned in (b)(i) above. Rationalise why the

noble gases helium, neon, and argon do not form such compounds under similar conditions. (i)

The VSEPR model is a good way to predict the shapes of small molecules and ions.

Using this model draw a Lewis structure (and predict the shape) for each of the following compounds in the gaseous state: XeF2, XeF4, XeO3 and XeO4.

(vi) Give one example, using the numbers of elements in the first four periods drawn in (a)(iv) above, of

an ionic compound with each the formulas XY, XY2, X2Y, XY3 and X2Y3.

(iii) Rationalise why these bonds lie in a plane.

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Final Selection Exam — Paper B

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ClF3 is a highly reactive liquid which is used to produce UF6 in the processing of nuclear fuels.

(iv) Draw a Lewis structure for ClF3, determine the shape of the molecule and suggest why the F–Cl-F

bond angle is not 90º.

(c)

(iii) How many stereoisomers are possible

CH3CH=C=C=CH(CH3)? Explain your answer.

for

the

following

related

compound:

(ii) Do the terminal hydrogen atoms in 1, 2, 3-butatriene (that is, those on carbon 1 and carbon 4), lie in

the same plane as the carbon atoms? Explain your answer. (i)

Consider the compound 1, 2, 3-butatriene which has the following structure: CH2=C=C=CH2. Draw a hybrid orbital diagram of 1, 2, 3-butatriene. Indicate on the diagram the sigma and pi bonds as well as the hybrid state of each of the carbon atoms.

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Final Selection Exam — Paper B

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Question 2

(15 minutes)

The chemistry of ketones and double bonds was investigated independently over summer. However, when an α,β-unsaturated ketone is present the chemistry changes. The β carbon instead becomes electrophilic making it the site of reactivity, called conjugate addition. Through keto/enol tautomerism the ketone is reformed from the enol produced through conjugate addition. Give the mechanism for the following conjugate addition reaction.

OOOO(a)

Base+EtOOEtOOEtOOEt

(b)

Based on the above information, deduce the mechanism for the following substitution reaction.

OOClMeOHMeO

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Final Selection Exam — Paper B

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With a catalytic amount of hydroxide ion and a stoichiometric amount of hydrogen peroxide (H2O2) acting as a nucleophile, an epoxide can be formed from an α,β-unsaturated ketone, utilising conjugate addition as one of the steps.

Propose a mechanism for the following epoxidation reaction.

OO(c)

HOH2O2O

Question 3

(15 minutes)

2.81 g of an optically active diester A, containing only C, H and O was saponified with 30.00 mL of 1.00 mol L–1 NaOH solution. Following the saponification, the solution required 6.00 mL of a 1. 00 mol L–1 HCl solution to titrate the unused NaOH. The products isolated after workup from the saponification reaction were an optically inactive dicarboxylic acid B, CH3OH and an optically active alcohol C. Alcohol C was mixed with I2/NaOH to give a yellow precipitate and C6H5COONa. The diacid B was allowed to react with Br2 to give a single, optically inactive product (compound D). Ozonolysis of B gave only one product.

Using the titration data shown above determine the molecular mass of compound A.

(a)

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Final Selection Exam — Paper B

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(b) (c) (d)

Question 4

(8 minutes)

Give the structural formula for D labelling any stereocentres.

Give the structural formula for compound C. Include possible stereochemical isomers and label each stereocentre.

Give the structural formula for A and B. Stereochemical information is not required.

Using any reactions that you like, propose a plausible synthesis of the following compound. All carbon atoms present in the final product must have originated from toluene (C7H8).

HO

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Final Selection Exam — Paper B

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Question 5

(13 minutes)

Consider a reaction with the following stoichiometry:

A + B

C + D

Data collected from using the method of initial rates is tabulated below: [A] (mol L–1)

100 200 100

[B] (mol L–1)

100 100 200

[C] (mol L–1)

0 0 0

[D] (mol L–1)

0 0 0

Rate (mol L–1 s–1)

100 200 100

(a) (b)

Determine the rate law, including the value of the rate constant.

Suggest a mechanism consistent with this data.

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Final Selection Exam — Paper B

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The experiment was repeated with C and D present to give the following results: [A] (mol L–1)

100 100 100 100 400 100

(c) (d)

[B] (mol L–1)

100 100 100 100 100 400

[C] (mol L–1)

0 100 400 100 100 100

[D] (mol L–1)

100 0 0 100 0 0

Rate (mol L–1 s–1)

100 50 20 50 200 80

What effect do C and D have on the reaction?

From careful inspection of the data, suggest a rate law and thus mechanism consistent with this data. State what assumptions apply to your mechanism.

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Final Selection Exam — Paper B

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Question 6

(13 minutes)

Rhenium (Re) is a rare heavy transition metal which occurs in a wide variety of oxidation states, and geometries.

Consider the compound (NH4)2[ReCl6]. Name this compound and draw a labelled d-orbital splitting diagram for the Re centre of the anionic complex ion.

(a)

In its other oxidation states, rhenium complexes can take a number of geometric forms. A complex with empirical formula [ReCl4]– is known to exist.

Draw the d-orbital splitting diagrams for the tetrahedral and square planar cases and calculate the CFSE. Which geometry would this complex prefer?

(b)

In actual fact, the [ReCl4]– exists as a dimer ([Re2Cl8]2–), forming a metal-metal bond between the two rhenium centres.

Draw the two conformations the dimer could take.

(c)

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Final Selection Exam — Paper B

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Assuming that the rhenium complex lies along the z-axis (as shown below), the dz2, dxy, dxz and dyz orbitals on both Rhenium centres interact to form σ, π or δ metal-metal bonds.

(Note: the dx2-y2 is used in the sigma framework and the other orbitals are of incorrect energy).

ZClClClClCl

XReClClReYCl

Types of bonding overlap can be classified by the symmetry of the overlapping orbitals. If the orbitals involved have a 2-fold rotation bond axis they are labelled π, a 4-fold rotation bond axis they are labelled δ and if they do not change upon rotation they are labelled σ.

Draw the overlap of the given d-orbitals with the adjacent rhenium orbitals, and label them as σ, π or δ. You may rotate the co-ordinate axes, but remember to also include them in your diagrams.

(d)

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Final Selection Exam — Paper B

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(e) (f)

Hence, what would be the preferred conformation and why would it be favoured?

Consequently, draw the qualitative MO diagram for the Re-Re bonding. What is the order of the metal-metal bond? Assume all d-electrons contribute to this bonding.

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Final Selection Exam — Paper B

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Question 7

(21 minutes)

In an informative lecture demonstration a 5.00 L sample of CO2 at 800 kPa underwent a one-step (irreversible) adiabatic expansion against a constant external pressure of 100 kPa. The word adiabatic means that the system is insulated so that no heat is exchanged at all during the process (i.e. q = 0). The initial temperature of the gas was 300 K.

5nR. Briefly explain why the constant volume heat capacity (cv) of CO2 is 2(a)

! An alternative path between the initial and final states consists of a reversible isothermal expansion

from 5.00 L to the final volume V, followed by (reversible) constant volume cooling to the final temperature T. The two processes are shown on the indicator diagram below in the answer box for (b).

Shade the area of the indicator diagram corresponding to the work done during the one-step adiabatic expansion.

(b)

(c)

More space is available on the next page.

Give equations (in terms of V and T, the final volume and temperature of the gas) for ΔU, q and w

for all three processes.

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Final Selection Exam — Paper B

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(d) (e)

In order to calculate the entropy changes of the system, one must consider a reversible path between the initial and final states.

Do you expect ΔSuniv to be positive or negative or zero for the one-step adiabatic expansion? Hence or otherwise calculate the final volume and temperature of the gas. Briefly state why ΔU is the same for both paths.

(f)

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Final Selection Exam — Paper B

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(g) (h)

Using a reversible process, calculate ΔSsys. Is the process spontaneous? What about ΔSsurr; is it positive, negative or zero?

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Final Selection Exam — Paper B

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Question 8

(17 minutes)

Calcium carbide (CaC2) is an ionic solid that reacts explosively with water to produce acetylene gas. The bonding in the carbide anion (C22–) can be described using molecular orbital theory. Draw a Lewis structure of the carbide anion. How many valence electrons does it have?

(a) (b) (c)

Draw a labelled molecular orbital diagram for the carbide anion. Include the atomic orbitals on the diagram, and show which of them combine to make each molecular orbital.

Based on your diagram, what is the bond order of the molecule, and is it paramagnetic?

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Final Selection Exam — Paper B

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As mentioned previously, calcium carbide reacts with water to form acetylene gas (C2H2). Calcium peroxide (used as a food additive for bleaching flour) reacts in an analogous way with dilute acid to form hydrogen peroxide. We are going to attempt to explain the bonding in acetylene and hydrogen peroxide using molecular orbital theory.

Sketch a molecular orbital diagram for the peroxide anion (O22–). There is no need to draw the atomic orbitals.

(d)

There are essentially three possible arrangements for the two hydrogen atoms in acetylene: linear, syn, or anti. To simplify the problem, we must first take symmetry adapted linear combinations (SALCs) of the two hydrogen 1s orbitals. As the name suggests SALCs are linear combinations which are either symmetric or antisymmetric with respect to the symmetry operations of the molecule.

HHCCHCCHHCCHlinear(e)

synanti

Sketch the 95% inclusion surface for these two SALCs.

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Final Selection Exam — Paper B

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(f) (g)

Hence predict the geometry of acetylene and hydrogen peroxide and explain why they adopt their respective structures. Your answer should include a qualitative molecular orbital diagram. Assume the hydrogen SALCs have energy approximately halfway between the π2p and π*2p orbitals in carbide and peroxide.

Which molecular orbitals in the carbide anion have the correct symmetry to combine with the SALCs in the linear, syn and anti cases?

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Final Selection Exam — Paper B

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Question 9

(17 minutes)

Urease is an enzyme present in most bacteria and plants. It plays a critical part in nitrogen metabolism in these organisms, as it catalyses the hydrolysis of urea to ammonia and carbamic acid, which itself spontaneously decomposes to ammonia and carbon dioxide, as represented in the following diagram:

OCH2NureaNH2+H2OureaseH2NOCOH+NH3carbamic acidOCH2NOHOCO+NH3

(a)

Draw a mechanism for the base-catalysed hydrolysis of urea to ammonia and carbamic acid and for the latter’s subsequent decomposition to carbon dioxide and ammonia.

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Final Selection Exam — Paper B

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Urease is an unusual enzyme in that it is one of very few nickel-containing enzymes. Two Ni(II) ions (denoted NiA2+ and NiB2+) are coordinated by numerous ligands from urease, including a carbamylated lysine sidechain that coordinates both Ni2+ ions, as shown below. The other ligands are represented by a generic “L”.

(b)

In addition to the ligands mentioned above, the NiB2+ in urease is coordinated to a water molecule, as shown below. The carbamoylated lysine is represented by a curve underneath the Ni2+ ions; all other ligands are omitted for clarity.

HONiA2+NiB2+HL:L:L:NiA:L2+:LNiBONHLys2+:L:LO-Give two examples of ligands from urease that could co-ordinate with the Ni(II) ions.

(c) (d)

Explain why the binding is likely to decrease the pKa of the coordinated water molecule.

The NiA2+ ion is involved in the coordination of the urea oxygen. What effect is this likely to have on the electrophilicity of the urea carbon?

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Final Selection Exam — Paper B

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(e)

Taking into account the information in (d), draw the structure of urea bound to the active site of urease (represented in 2D below). Mark with dotted lines any interactions, (hydrogen bonding or otherwise) that urea makes with the enzyme.

AlaOHSCysAlaHisN+HOO-OHNiA2+NiB2+ Ala

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Final Selection Exam — Paper B

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(f) HisN+HPropose a mechanism for the urease-catalysed hydrolysis of urea to carbamic acid and ammonia. The important parts of the active site have been copied out for you.

-OHNiA2+NiB2+

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Final Selection Exam — Paper B

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Question 10

(10 minutes)

Five elements have had their symbols disguised as A, B, C, D and E. The formulas are written in the usual order for the compound, e.g. if X=Si and Y=Cl then SiCl4 would be represented as XY4. • Elements B, C, D and E all exist as diatomic molecules at room temperature and pressure. • A reacts with C2 to form AC2, which forms a weak acid in aqueous solution.

• AC3, a white solid, can also be prepared but it is unstable above 165°C, decomposing to AC2 and C2. • Direct reaction of A with D2 gives either AD4 or AD6 as stable products depending on the conditions. AD4 reacts with AC2 to form ACD2, which contains 59.1% A by mass. • E forms many compounds with D, including ED, ED3, ED5 and ED7. • C forms only two compounds with D: CD2 and C2D2.

• B2A reacts with C2D2 to form AD6, C2 and BD, which is a weak acid in aqueous solution. Determine the identity of the five elements A to E, showing all reasoning.

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Final Selection Exam — Paper B

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Question 11

(38 minutes)

Data: Oxygen content of air is 20.95% by volume.

pKb1(en) = 7.56 pKb2(en) = 10.72 logKstab{Cu(en)3} = 18.70 Cu+ + e– → Cu Cu2+ + 2e– → Cu

Ksp(Cu(OH)2) = 2.20 × 10–20

logK2 = 5.60

logK3 = 2.50

logK1 = 10.60 E0 = 0.520 V E0 = 0.340 V E0 = 1.23 V E0 = 0.687 V E0 = 0.401 V

O2 + 4H+ + 4e– → 2H2O O2 + 2H+ + 2e– → H2O2 2H2O + O2 + 4e– → 4OH–

Copper(II) is able to form a tris-complex with the bidentate ligand ethylenediamine (en).

ethylenediamine (en)

We will consider the dissolution of metallic copper in a 0.100 mol L–1 en solution that is exposed to air.

Write out the two half equations and the overall equation for the dissolution of metallic copper and calculate E0 for the reaction.

(a) (b)

Calculate the pH of a 0.10 mol L–1 solution of en.

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Final Selection Exam — Paper B

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(c)

Metallic copper was allowed to dissolve in the en solution until saturation. At this stage, the pH of the solution was measured to be 7.5 and the concentration of free copper(II) ions was known to be approximately 1 × 10–7 mol L–1.

Calculate the electrochemical potential for the dissolution of metallic copper at saturation. Why does dissolution cease?

Develop an equation that relates the electrochemical potential of the system to the concentration of hydroxide ions and free copper(II) ions.

(d) (i) (e) (i)

(ii) Calculate a more accurate value for the concentration of free copper(II) ions in solution.

Develop a mass balance equation for en.

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Final Selection Exam — Paper B

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(f)

What is the solubility of copper(II) hydroxide in a 0.100 mol L–1 solution of en? (ii) Find the final concentration of uncomplexed en.

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