AL Chemistry 2008 Paper I Solution

8/8/2019 AL Chemistry 2008 Paper I Solution

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SECTION A

Answer ALL questions. Write your answers in the spaces provided.

1. (a) (i) Draw a dot-and-cross diagram to show the arrangement of the

outermost electrons in the species NH 2-(g), and predict the shape of

this species.

1

V-shape (Bent) 1 *According to VSEPR, 2lone pair + 2bond pair = bent

(ii) Arrange the H-N-H bond angles in descending order for the three

species: NH 2-(g), NH 3(g) and NH 4+(g). Explain your ordering.

NH2- has lp-lp repulsion, NH3 has lp-bp repulsion, NH4+ has bp-bp repulsion only

As lp-lp repulsion lp-bp bp-bp

greater repulsion = smaller H-N-H bond angles NH 4+ NH 3 NH 2-

(iii) Excess NH 3(g) reacts with Cl 2(g) in two steps to give N 2(g) and

NH 4Cl (s). It is known that NH 3(g) functions as a reducing agent and as

a base in the reaction. For each step, write the chemical equation and

state the function of NH 3(g) .

3Cl 2 + 2NH 3 6HCl + N 2 (Reducing Agent) 1 + HCl + NH 3 NH 4Cl (Base) 1 +

Overall equation: 3Cl 2 + 8NH 3 6NH 4Cl + N 2

*This question is a bit difficult, esp. when you are nervous during

exam, just try your best.

(7 marks)

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1. (b) Both nickel(II) bromide and cobalt(II) bromide react with triethylphosphate,

PEt 2 (Et represents C 2H5), forming metal complexes with the general

formula M(PEt 2)2Br 2; where M is either Ni or Co.

(i) The nickel complex can occur in two forms, one of which has a net

dipole moment whereas the other does not. The cobalt complex,

however, occurs in only one form.

Account for the above results.

Cobalt complex occurs in tetrahedral form Nickel complex exists in both cis and trans sq. planar complex

Reason: Ni is d 8 configuration (8 d e-) Co is d 7 configuration _______________________________________________________

The actual approach to explain is out of syllabus, just remember, d 8

has high tendency to form sq. planar complex

Information:

1. The complexs formula is ML4 (L = ligand), therefore, it must be

sq. planar for d8

2. Nickel occurs in 2 forms (1 with net dipole moment)3. Cobalt only occurs in 1 form

_______________________________________________________

Trans-sq. planar complex can cancel out dipole moment of Br no net dipole moment

Cis- sq.planar complex is not symmetrical has dipole moment

d7 doesnt favor the formation of sq.planar complex tetrahedral only

(ii) Will the cobalt complex have a net dipole moment? Explain.

Yes the dipole moment cannot cancel out in tetrahedral complex

*Dont panic when you see an unknown ligand PEt 2(5 marks)

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2. (a) Three reagent bottles each containing 0.5 M KI (aq) , 14 M HNO 3(aq) and 0.02

M KMnO4 (aq) have been kept in the laboratory for a long time. The table

below lists the observation for each of the bottles.

Solution Observation

0.5 M KI (aq) The liquid is pale yellow.

14 M HNO 3(aq) There are brown fumes above the yellow

liquid.

0.02 M KMnO 4(aq) There are brown stains on the interior wall of

the bottle.

In each case, account for the observation and write the relevant chemical

equation(s).

0.5 M KI (aq)

2I - I2 + 2e - , I - + I 2 I3-

Iodide combines with elemental iodine to form triiodide *If you know triiodide is yellow, this shouldnt be difficult

14 M HNO 3(aq)

4HNO 3 4NO 2 + 2H 2O + O 2

Photodecomposition of conc. HNO 3, brown NO 2(g) is formed

* CE level, remember it is conc. (14M)

0.02 M KMnO 4(aq)

2KMnO 4 K 2MnO 4 + MnO 2 + O 2

Photo/thermal decomposition, MnO 2 (brown solid) is formed

* This is actually a combination of 2 half equations:

MnO 4- + 4H + + 3e - MnO 2(s) + 2H 2O

MnO 4- + 4OH - MnO 42- + 2H 2O + O 2 + 3e -

(6 marks)

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2. (b) The diagram below shows the design of a hydrogen-oxygen fuel cell:

(i) Describe the working principle of the fuel cell.

O2 to cathode (reduced to O 2-)

H2 to anode (oxidized to H +) H2 + 2OH - 2H 2O + 2e - O2 + 4H 2O + 4e - 4OH - 2H 2 + O 2 2H 2O NaOH acts as electrolyte *H 2 2H

+ + 2e - is WRONG as the electrolyte is basic

(ii) One advantage of using hydrogen-oxygen fuel cells is that they do not

emit air pollutants. Suggest ONE other advantage of usinghydrogen-oxygen fuel cells.

Select one of the followings: + elaborate

Quiet (less moving parts compared with conventional combustionengines)

Effective (combustion of fossil fuels produce many by-products,which reduces efficiency, hydrogen fuel cell only produces water

by redox, much higher efficiency)

Sustainable energy alternative(H 2 and O 2 are abundant in nature,while fossil fuels are non-regenerable fuels)

*Not difficult if you read beyond textbooks (esp. new way)

(4 marks)

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Answer the following multiple-choice questions.3. (a)

(i) Which one of the following ions is most ready to donate electrons?

A. Al 3+

B. Fe 2+

C. Cu 2+

D. Zn 2+

Answer: B

(ii) Which one of the following molecules has a zero dipole moment?

A. BF 3B. PH 3C. SO 2D. HCl

Answer: A(iii) Which one of the following graphs represents the distribution of

molecular speeds of one mole of a gas at two different temperaturesT 1 and T 2, where T 2 > T 1?

A.

C.

B.

D.

Answer: CExplanation: (i) Have you ever heard about Al 4+, Cu 3+ or Zn 3+?

(ii)

(iii) maxwells distribution, T2 must be flatter + higher average

molecular speed, both are from the origin (there must be molecules

with speed = 0)

(iv) by definition

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3. (a) (iv) Which one of the following equations represents the change

corresponding to the standard enthalpy change of atomization of

iodine?

A. I 2(s) I(g)B. I 2(l) 2 I (g)C. I 2(s) 2 I (g)D. I 2(g) 2 I (g)

Answer: A

(4 marks)

(b) Account for the following observations:

(i) The boiling point of magnesium bromide is lower than that of sodium

bromide.

Ionic bond strength of MgBr2 < NaBr Since MgBr2 carries more covalent character Weakens ionic bond strength Less energy is required to vaporize MgBr2 b.p. of MgBr2 < NaBr

(ii) The first ionization enthalpy of aluminium is smaller than that of

magnesium.

Al: [Ne]3s 2 3p1 Mg: [Ne]3s 2 Al adopts extra stability when lose outermost e- Higher tendency (lower energy is needed) to lose outermost e- Lower first I.E.

*straightforward question

(4 marks)

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4. (a) Identify A, B, D, E , F and G in the following reactions:

(i)

A: 1 (ii)

B: 1

(iii)

D: 1

(iv)

E : 1

(v)

F : HCN 1 G : 1(6 marks)

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4. (b) Consider the substitution reaction shown below:

(i) Outline a mechanism of the reaction.

Initiation

***

Propagation

***

***

Termination

***

Fishhook arrow 3 steps 4 key steps (***) x 4

(ii) Suggest why the hydrogen atoms of the benzene ring will not be

substituted.

Delocalized electrons stabilize the benzene ring For Benzylic radical (R-CH2 ), it is stabilized by donating

electron density through resonance by benzene ring, so it is

attacked by free radicals readily

However, phenyl radical is not stabilized by this effect It does not form radical readily, therefore is not susceptible to

substitution

(5 marks)

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5. (c) Answer the following multiple-choice questions.

(i) Which one of the following compounds is most reactive towards Br 2?

A. B.

C. D.

Answer: B

(ii) Which one of the following compounds is most reactive towards

substitution with NaOH (aq)?

A. B. CH 3(CH 2)3Br

C. D.

Answer: A

(iii) Which one of the following compounds is most reactive towardssubstitution with Cl 2?

A. B.

C. CH 3CH 2CH 2CH 3 D.

Answer: B

(3 marks)

Explanation: (i) D is aromatic, B is tri-substituted (more stable intermediate)

A,C are di-substituted

(ii) C,D are stable aryl halide, A has more stable intermediate due to

resonance

(iii) Benzylic radicals are stabilized by resonance

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6. Read the passage below and answer the questions that follow:

Silicates and aluminosllicatesSilicates occur in a wide variety of structures. Orthosilicates contain discrete

tetrahedral SiO 42- anions with O atoms at the corners. Other silicates can be

considered as formed from the tetrahedral by sharing oxygen atoms, e.g.disilicates contain Si 2O76- ions and cyclic silicates contain (SiO 2)n2n+ ions, where nis an integer greater than 2. Polymers of silicates also exist includingone-dimensional chain silicates, two-dimensional sheet silicates andthree-dimensional network silicates.

A type of aluminosilicate is formed when some of the Si(IV) in athree-dimensional network silicate is replaced by Al(III) and another metal cation.Figure 6.1 shows a part of the structure of such type of aluminosilicate, andFigure 6.2 shows the framework representation of this part of the structure witheach vertex representation a Si or Al atom.

Zeolites belong to one of the above type of aluminosilicate. They have aregular and opoen structure with cavities of a very precise size. As such, they canfunction as molecular sieves ( ). Sodium zeolite A, one of the zeolites, iscommonly used as a water softener for reducing water hardness. The framework representation of a part of the structure of sodium zeolite A is shown in Figure6.3.

(a) Suggest TWO reasons for the occurrence of silicates in a wide variety of structures.

ANY TWO, each 1

Si-O bond is exceptionally strong SiO 4 tetrahedra similar to diamond structure stable Silicates adopt giant covalent tetrahedral network stable

(2 marks)

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6. (b) Some liquid detergents contain sodium orthosilicate, Na 4SiO 4, to maintain a

moderately high pH.

(i) Suggest why detergents need to have a moderately high pH.

Detergents work as hydrophilic head Low pH remove hydrophilic head No longer works

(ii) With the help of a chemical equation, explain how Na 4SiO 4 functions.

Na 4SiO 4 + 4H + Si(OH) 4 + 4Na + Reacts with H +, so less acidic

(2 marks)

(c) Draw the structure of each of the following anions:(i) Si 2O76- (ii)(SiO 3)612-

Represent SiO 44- =

Si2O76- = 1 (SiO 3)612- = 1

*Remember to mention SiO 44- = the triangle

(2 marks)(d) Suggest why sodium zeolite A can function as a molecular sieve.

Its cavity has specific size only ions with suitable size and charge can fix into it

Too large blocked out, Too small pass through it -ve charge from O can only hold cations

(2 marks)

(e) Sodium zeolite A is commonly used as a water softener:

(i) Suggest why sodium zeolite A has a high affinity for Ca 2+(aq) and

Mg 2+(aq) ions.

Size and distribution of negative charge are suitable for these ions 1

(ii) Suggest ONE way to regenerate exhausted sodium zeolite A.

Wash with NaCl(brine) solution 1

(2 marks)

END OF SECTION A

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SECTION B

Answer ALL questions. Write your answers in the spaces provided.

7. (a) The diagram below shows the essential components of an instrument for

studying the kinetics of the reaction:

(i) What is this instrument?

Colorimeter 1

(ii) What physical parameter of the reaction mixture is measured by this

instrument?

Transmittance (or Absorbance) 1

(iii) Sketch a graph to show the variation of the measured physical

parameter with time.

OR

naming x y axis suitable shape

(3 marks)

(b) (i) Hydride ion, H -, is a strong Bronsted base. Write the chemical

equation for the reaction of H - with water.

H - + H 2O -> OH- + H 2 1

*Full equation: LiAlH 4 + 4H 2O LiOH + Al(OH) 3 + 4H 2* LiAlH 4 should not contact with water

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7. (b) (ii) Suggest why the following hazard warning labels should be displayed

on a bottle of LiAlH 4(s) .

*From the equation mentioned above:

LiAlH 4 + 4H 2O LiOH + Al(OH) 3 + 4H 2 LiAlH4 decomposes upon contact with water Forms corrosive LiOH 1

Explosive H 2 1 *Difficult if you dont know the full equation

(3 marks)

(c) (i) Outline how you would prepare a sample of dry CuSO 45H 2O crystals

from copper turnings in a laboratory.

Dissolve Cu turnings in minimum amount of conc. H2SO4 Stir until fully dissolved + filtrate to obtain CuSO 4(aq)

Evaporate at room temperature (form crystals) Filtrate to obtain crystals Rinse with little cold water Air-dry the crystals

(ii) Suggest suitable reagent(s) for the determination of the purity of a

sample of the dry CuSO 45H 2O crystals by titrimetric analysis.

Standardized Na 2S2O3 solution Excess but known amount of KI solution Starch Indicator (better write it out)

*Redox + Back titration

*Cu(OH) 2 does not work as the end pt is hard to determine

(4 marks)

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8. (a) Super glue works as an adhesive by addition polymerization as shown below:

(i) The progress of this polymerization was monitored by infra-redspectroscopy. Figure 8.1 shows the change of transmittance with time(t ) for the reaction system. Assign the functional group correspondingto the absorption at 1614cm -1. Account for the change of transmittance of this peak with time.

C=C double bond C=C undergoes addition as the polymerization

proceeds [C=C] decreases The intensity of characteristic peak decrease

(ii) The experiments were carried out to study the effects of NaOH(aq)and H 2O(l) on the polymerization. The conditions of the experiments

were the same except that one was conducted in the presence of NaOH(aq) and the other in the presence of H 2O(l). Figure 8.2 showsthe change of temperature of the two reaction mixtures with time.

(I) Account for the increase and decrease in temperature of thereaction mixtures.

NaOH the reaction rate and therefore the reaction is moreexothermic , rate of change of temperature

[polymer] rate rate of change of T as reaction proceeds(II) Suggest a reason for the significant difference in the two curves.

OH - is a stronger nucleophile than H 2O 1 Increases rate of reaction***The mechanism is NOT free radical polymerization!!! *** (5 marks)

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(b) Propenamide, the monomer of polypropenamide (also known as polyacrylamide) is a potential carcinogen. The melting point of propenamide is 84C and its solubility in water is 2.16gcm -3 at 30C.

(i) Polyacrylamide gel (PAAG) is polyacrylamide saturated with water. Asample of PAAG for breast augmentation is suspected to containabout 1% propenamide. Suggest a chemical test to show the presenceof propenamide in the sample.

Test for C=C: ANY ONE, test + observation

Br 2 decolorization OR Cold acidified KMnO4 decolorization

Test for amide: ANY ONE, test + observation

NaOH gives off ammonia (bubbles) on heating OR Br 2/NaOH gives off CO2 (bubbles) (hofmann degradation)

(ii) Propenamide can be identified by converting it to a solid derivativeand determining the melting point of the derivate. With the help of achemical equation, suggest ONE solid derivative of propenamidesuitable for this purpose.

2,3-dibromopropanamide

+ Br 2

(iii) Suggest an instrumental method for the further characterization of the

derivative suggested in (ii). State clearly the expected results.

Mass Spectroscopy

Isotopic abundance of 79Br : 81Br = 1:1 Relative intensity of e/m M:M+2:M+4 = 1:2:1 Name a few other peaks (M=molecular mass)

(5 marks)END OF SECTION B

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SECTION C

Answer ONE question only, and write your answer in the AL(D) Answer Book.

Marks will be allocated approximately as follows:

Chemical knowledge 50%

Organization 30%

Presentation (including use of language) 20%

Use equations, diagrams and examples where appropriate.

The examiners are looking for the ability to analyse, to evaluate and to express ideas.

9. Write an essay on the chemistry of chlorine including its industrial manufactureand applications in daily life.

(20 marks)

10. Write an essay on the application of the principle of green chemistry in industry.

(20 marks)

* everyone writes 12 principles + 2 applications in Q.10

if you are familiar with green chem. + only wants C-E grades, choose Q.10

if you aim higher grades + not bad in chlorine, better choose Q.9 as it is easier toshow how extraordinary you are

END OF PAPER

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AL Chemistry 2008 Paper I Solution

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