Chemistry 2008 Stpm

Time: 1h 45 min

PAPER 1 Section A Four suggested answers labelled A, B, C and D are given for each question. Choose one correct answer.

C Relative intensity 4

1 The percentage of iron in a haemoglobin molecule is 0.335%. If a haemoglobin molecule consists of four iron(II) ions, what is the relative molecular mass of haemoglobin? [Relative atomic mass of Fe is 55.8.] A 6.66 × 102 C 1.67 × 104 3 B 4.16 × 10 D 6.66 × 104 2 Boron has two isotopes with relative isotopic masses of 10 and 11. When 1.00 g of boron reacts completely with hydrogen, 1.278 g of boron hidride, B2H6, is produced. Which graph shows the mass spectrum of boron? A Relative intensity 4

2 1

0

2 1 0 8

9

10

11

12

m/e

B Relative intensity 4 3 2 1 0 8

9

© Majlis Peperiksaan Malaysia 2008

10

11

12 m/e

8

9

10

11

12

8

9

10

11

12

m/e

D Relative intensity 4 3 2 1

0

3

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 1

3

m/e

3 Which equation illustrates the first ionisation energy of the given element? A C(g) → C+(g) + e– B Na(s) → Na+(g) + e– C Cl(g) + e– → Cl–(g) D O(g) + e– → O–(g) 4 Vanadium has the electronic configuration 1s22s22p63s23p63d34s2. Which ion does not exist? A VO42– B VO3– C VO2+ D VO2+ Actual 2008 STPM Chemistry Examination Paper

1

21/01/2009 2:10:14 PM

5 The ionic radii of four ions are as follows: Ion

Ionic radius/nm

+

P Q2+ R– S–

9 The thermochemical equation for the reaction between nitrogen monoxide and ozone is as follows: NO(g) + O3(g) ∆H = –200.0 kJ

0.095 0.031 0.136 0.126

Which ionic compound has bonding with the greatest covalent character? A PR C QR2 B PS D QS2 6 Which species has a trigonal planar shape? A BF3 C ClO3– B PCl3 D SO32– 7 Coordinate bonds do not exist in A CO B BeCl2 C PH4+ D [Fe(CN)6]3– 8 The graphs of initial rate versus concentration of reactants for the reaction X + Y → Z are shown below. Rate

[X]

NO2(g) + O2(g);

If the activation energy of the reaction is 10.5 kJ mol–1, the activation energy of the reverse reaction is A 10.5 kJ mol–1 B 189.5 kJ mol–1 C 200.0 kJ mol–1 D 210.5 kJ mol–1 10 Which underlined molecule reacts as a Lewis base? A K2O + SO3 → K2SO4 B BF3 + NH3 → H3N•BF3 C OH– + CO2 → HCO3– D CH3Cl + FeCl3 → CH3+ + FeCl4– 11 The equilibrium constant, Kc, for a reaction between ethanol and ethanoic acid is 4.0 at 25 °C. If 1.0 dm3 of an aqueous solution containing 92.0 g ethanol and 120.0 g ethanoic acid is left to react until equilibrium is attained at 25 °C, what is the mass of ethanol left? [Relative molecular masses of ethanol and ethanoic acid are 46.0 and 60.0 respectively.] A 18.4 g C 61.3 g B 30.7 g D 73.6 g

Rate

[Y]

What is the order of the reaction with respect to X and Y? X Y A 0 1 B 1 0 C 1 2 D 2 1 2

Actual 2008 STPM Chemistry Examination Paper

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 2

12 In which reaction is the value of Kc equals to the value of Kp? A N2O4(g) 2NO2(g) B CO2(g) + C(s) 2CO(g) C H2O(g) + C(s) H2(g) + CO(g) D 3Fe(s) + 4H2O(g) Fe3O4(s) + 4H2(g) 13 An aqueous solution of a monobasic acid has pH 3.5. A 25.00 cm3 volume of an aqueous solution of the acid is completely neutralised by 27.50 cm3 of a 0.10 mol dm–3 sodium hydroxide © Majlis Peperiksaan Malaysia 2008

21/01/2009 2:10:16 PM

solution. The value of the dissociation constant of the acid is A 3.16 × 10–4 mol dm–3 B 3.48 × 10–5 mol dm–3 C 9.08 × 10–7 mol dm–3 D 9.99 × 10–8 mol dm–3 14 When solid ammonium chloride is added in excess to an aqueous solution of ammonia containing phenolphthalein as an indicator, A a buffer solution is formed. B hydrogen choride gas is evolved. C the pink colour of the solution remains unchanged. D a solution with a pH less than 7 is formed. 15 The partition coefficient of a liquid X between ethoxyethane solvent and water is 3.00. What is the mass of X extracted from a 200 cm3 of an aqueous solution containing 6.00 g of X, extracted twice with 100 cm3 of ethoxyethane used in each extraction? A 3.60 g C 5.04 g B 4.50 g D 5.63 g 16 The electrode potential, E, of a half-cell is given by 0.059 [oxidised species] E = E o – ——– log ——–————— , n [reduced species] o

where E is the standard electrode potential and n the number of electrons transferred when the oxidised species changes into the reduced species. Given that E = 0.00 V at equilibrium and Half-cell reaction Zn2+(aq) + 2e– Zn(s) Ag+(aq) + e– Ag(s)

E o /V –0.76 +0.80

What is the equilibrium constant, Kc, for the reaction 2Ag+(aq) + Zn(s) → Zn2+(aq) + 2Ag(s) at 25 °C? A 1.31 × 10–53 dm3 mol–1 B 4.41 × 10–2 dm3 mol–1 C 2.76 × 1026 dm3 mol–1 D 7.61 × 1052 dm3 mol–1 © Majlis Peperiksaan Malaysia 2008

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 3

17 In plating chromium onto plastic, the cathode is a plastic object coated with graphite. The graphite is used to A enable oxidation to occur and chromium to be deposited. B form an alloy with chromium so as to strengthen the coating. C enable the plastic object to act as an electrical conductor. D make the surface of the plastic object coarse so that chromium is adsorbed on it. 18 In industry, ethanoic acid can be prepared from the oxidation of ethanol with oxygen. The standard enthalpies of formation of ethanoic acid, water and ethanol are –487 kJ mol–1, –286 kJ mol–1 and –278 kJ mol–1 respectively. The standard enthalpy change, in kJ mol–1, for the preparation is A –1051 B –495 C –209 D +495 19 The standard enthalpies of neutralisation for the reactions between an aqueous solution of sodium hydroxide and several hydrohalic acids are given below. Hydrohalic acid

Standard enthalpy of neutralisation/kJ mol–1

HF

–68.6

HCl

–57.6

HBr

–57.3

HI

–57.2

The standard enthalpy of neutralisation of HF is the most negative because A HF is the strongest acid. B HF has the strongest covalent bond. C the fluorine atom has the highest electronegativity. D the fluorine ion has the most exothermic enthalpy of hydration. Actual 2008 STPM Chemistry Examination Paper

3

21/01/2009 2:10:16 PM

20 Strontium is an element in Group 2 of the Periodic Table which is situated between calcium and barium. Which statement is true? A The strontium ion has a higher charge density than the calcium ion. B Strontium sulphate is less soluble in water than calcium sulphate. C Strontium carbonate is more difficult to decompose than barium carbonate when heated. D Strontium nitrate is more difficult to decompose than barium nitrate when heated. 21 Which property is true of the elements in Group 14 of the Periodic Table on descending the group? A The electrical conductivity of the elements decreases. B The electronegativity of the elements increases. C The stability of the +4 oxidation state of the elements increases. D The catenation of the elements decreases. 22 SiCl4 compound forms an acidic solution with water while CCl4 compound does not dissolve in water because A silicon atom has empty 3d orbital while carbon atom does not. B the Si Cl covalent bond is weaker than the C Cl covalent bond. C the SiCl4 molecule is polar while the CCl4 molecule is non-polar. D van der Waals forces between SiCl4 molecules are weaker than those between CCl4 molecules. 23 At 500 °C, nitrogen and hydrogen gases react according to the equation N2(g) + 3H2(g) → 2NH3(g). Which statement about the reaction is true? A The reaction is endothermic. B The reaction occurs through three steps. 4

Actual 2008 STPM Chemistry Examination Paper

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 4

C The value of the equilibrium constant is lower at a temperature more than 500 °C. D The percentage yield of ammonia increases when the pressure is reduced. 24 Ammonia gas can be prepared in a laboratory by heating an ammonium salt with A Devarda’s alloy B hydrochloric acid C calcium hydroxide solution D acidified solution of potassium manganate(VII) 25 The reactivity of the halogens Cl2, Br2 and I2 decreases down Group 17 of the Periodic Table. Which property does not influence the trend in the reactivity of the halogens? A Atomic size B Electron affinity C Ionisation energy D Bond energy 26 The oxidation states of several d-block elements represented by letters which are not the usual symbols for the elements concerned in the Periodic Table are as follows: Element

Oxidation state

X

–

–

3

–

–

–

–

Y

–

2

3

4

–

–

–

Z

–

2

3

4

5

6

7

Which statement is not true? A X forms colourless hydrated ions. B Y forms a compound with the molecular formula K2YO4. C Z2O is an oxidising agent. D X, Y and Z form octahedral complexes. 27 Zn2+ ions form a white precipitate, Zn(OH)2, with an aqueous solution of ammonia. The white precipitate © Majlis Peperiksaan Malaysia 2008

21/01/2009 2:10:17 PM

dissolves when a NH4Cl solution is added to it. Which statement explains the observation? A A soluble ZnCl2 compound is formed. B NH3 molecule is a stronger ligand than OH–. C NH4+ ions increase the solubility product of Zn(OH)2. D NH4+ ions decrease the concentration of OH– ions in the solution. 28 Menthol is one of the substances added into a cough mixture. The structure of its molecule is as follows: CH3

C CH=CHCH2CH3

D

CH2CHCH2CH3

31 When a compound is heated with an acidified solution of potassium manganate(VII), a white precipitate is formed and a gas which turns lime water chalky is evolved. The possible structural formula of the compound is A CH2CH3 C CH2OH

OH CH(CH3)2

Which statement is true of menthol? A It has two chiral centres. B It has two functional groups. C It reacts with an aqueous solution of sodium hydroxide. D It decolourises an acidified solution of potassium manganate(VII). 29 Which compound reacts with chlorine following the mechanisms for free radicals and electrophilic substitution? A CH3CH3 B CH2Cl2 C CH3CH=CH2 D C6H5CH3 30 Which compound is formed when benzene reacts with 1-butene in the presence of aluminium chloride? A CH2CH2CH2CH3

B CH(CH3)CH2CH3

© Majlis Peperiksaan Malaysia 2008

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 5

CH2Cl

B

CHO

D

32 Each of the following compounds is put into a test tube and an aqueous solution of silver nitrate is added. The three test tubes are then placed in a hot water bath. Cl CH3COCl

I

ClCH2COOH

II

III

Which sequence of the compounds according to the ascending order of their reactivity is correct? A I, III, II B II, I, III C II, III, I D III, II, I 33 Which pair of compounds corresponds to the reagent used to differentiate the two compounds? Actual 2008 STPM Chemistry Examination Paper

5

21/01/2009 2:10:17 PM

A B C D

Pair of compounds (CH3)3COH and (CH3)2CHCH2OH CH3CH2OH and C6H5OH CH3CH2OH and CH3COOH C6H5OH and CH3COOH

Reagent Sodium metal Bromine water Phosphorus(V) chloride Aqueous solution of sodium hydroxide

34 An organic compound X, with molecular formula C5H11Cl, is converted to compound Z according to the following reaction scheme. NaOH(aq) KMnO4/H+ C5H11Cl ⎯⎯⎯⎯→ Y ⎯⎯⎯⎯→ Z ∆ ∆ Z forms an orange precipitate with 2,4-dinitrophenylhydrazine and a yellow precipitate with an alkaline solution of iodine. What are X, Y and Z?

A B C D

X

Y

Z

CH3CH2CH2CH2CH2Cl CH3CH2CH(Cl)CH2CH3 CH3CH(Cl)CH2CH2CH3 (CH3)2C(Cl)CH2CH3

CH3CH2CH2CH2CH2OH CH3CH2CH(OH)CH2CH3 CH3CH(OH)CH2CH2CH3 (CH3)2C(OH)CH2CH3

CH3CH2CH2CH2CHO CH3CH2COCH2CH3 CH3COCH2CH2CH3 CH3COCH2CH2CH3

35 An organic acid decolourises bromine water. 1.16 g of the acid in aqueous solution requires 20.0 cm3 of a 1.0 mol dm–3 sodium hydroxide solution for complete neutralisation. What is the possible structural formula of the acid? [Relative atomic mass of H is 1.0, C is 12.0 and O is 16.0] A HCOOH B HOOC—COOH C CH2=CHCOOH D HOOCCH=CHCOOH 36 When a white solid P, with molecular formula C13H10O2, is refluxed with an aqueous solution of sodium hydroxide and then acidified with hydrochloric acid, compounds Q and R are formed. What are Q and R? Q

R

A CH2OH

B

O C

OH OH

6

OH

Actual 2008 STPM Chemistry Examination Paper

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 6

C O ONa

C ONa

D O CH3CH2OH

C OH

37 A 2.0 cm3 volume of sodium hydroxide solution is added to 0.1 g of ethanamide in a test tube and then heated. Which observation about the reaction is true? A The gas evolved forms white fumes with hydrogen chloride. B The gas evolved turns blue litmus paper red. C A liquid with a smell of vinegar is formed. D A layer of colourless oil is formed. © Majlis Peperiksaan Malaysia 2008

21/01/2009 2:10:17 PM

38 What is the compound formed when (CH3)2C=CHCN reacts with hydrogen gas in the presence of platinum? A (CH3)2CHCH2CN B (CH3)2C=CHCH2NH2 C (CH3)2CHCH2CH2NH2 D (CH3)2CHCH2NHCH3 39 GABA is a neuro-transmitter released by red algae which encourages shellfish larvae to settle on the ocean bed. Its structural formula is H2NCH2CH2CH2COOH. Which reagent reacts only with the acidic group of GABA?

A B C D

HCl HNO2 CH3CH2Cl CH3CH(OH)CH3

40 Ziegler-Natta catalyst is used in addition polymerisation of ethene. Which statement is not true of the process? A It requires a low pressure. B It requires a low temperature. C It produces high density poly(ethene). D It is an example of free radical polymerisation.

Section B For each question in this section, one or more of the three numbered statements 1 to 3 may be correct. The responses A to D should be selected as follows: A

B

1 only is 1 and 2 correct. only are correct.

C

D

2 and 3 only are correct.

1, 2 and 3 are correct.

P

Q

1 Trichloromethane Propanone 2 Benzene Methylbenzene 3 Ethanol Water 42 The paper chromatogram of samples W, X, Y and Z which is obtained using ethanol as an eluent is shown below.

41 The phase diagram of a mixture of two liquids P and Q is as follows:

Solvent front

Vapour pressure/ atm W

1.0 Q

Mole fraction

What could P and Q be?

© Majlis Peperiksaan Malaysia 2008

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 7

1.0 P

X

Y

Z

Which statement(s) is/are true of the samples W, X, Y and Z? 1 W has a larger Rf value than Y. 2 X is a mixture of W and Y. 3 Z has the lowest solubility in ethanol.

Actual 2008 STPM Chemistry Examination Paper

7

21/01/2009 2:10:18 PM

43 Which property(ies) of the elements in Period 2 and Period 3 of the Periodic Table change(s) periodically? 1 Electronegativity 2 Electrical conductivity 3 Enthalpy of vapourisation 44 The bond angles in methane, ammonia and water molecules are shown below. H

N

C

107°

109.5°

H

H

H

H H

H

O 104.5°

H

46 [Al(H2O)6]3+ ion is hydrolysed in aqueous solution according to the equation [Al(H 2 O) 6 ] 3+ (aq) + H 2 O(l) [Al(H2O)5(OH)]2+(aq) + H3O+(aq) Which statement(s) about the reaction is/are true? 1 The [Al(H2O)5(OH)]2+ ion is not stable with respect to hydrolysis. 2 The reaction is a redox reaction. 3 The rate of reaction increases when an acid is added. 47 A solution is prepared by dissolving a small amount of sodium chloride and sodium iodide in water. The solution formed is added with an aqueous solution of silver nitrate followed by an aqueous solution of ammonia. The reaction(s) involved is/are 1 Ag+(aq) + I–(aq) → AgI(s) 2 Ag+(aq) + Cl–(aq) → AgCl(s) 3 AgCl(s) + 2NH3(aq) → Ag(NH3)2+(aq) + Cl–(aq) 48 Which compound(s) and reagent(s) with reaction condition(s) react to form a compound with halogen atoms attached directly to its benzene ring? Compound

H

The difference in the bond angles in the three molecules is due to 1 the number of lone pair electrons in the central atom. 2 the hybridisation of the central atom. 3 the electronegativity of the central atom. 45 When a current of 0.8 A is passed through a molten ionic compound for 1 hour, 1.00 × 10–2 mol of a metal is deposited at the cathode. What could the metal be? [Faraday constant is 9.65 × 104 C mol–1.] 1 Fe 2 Cr 3 Pb 8

Actual 2008 STPM Chemistry Examination Paper

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 8

Reagent/Reaction condition

1

CH2CH3

Br2/Fe

2

CH2CH3

Cl2/Light

3

CH(OH)CH3

I2/NaOH, ∆

© Majlis Peperiksaan Malaysia 2008

21/01/2009 2:10:18 PM

49 A breath analyser is used to determine the alcohol content in a driver’s body. Which statement(s) is/are true? 1 The alcohol to be determined is ethanol. 2 The chemical substance used could be acidified solution of potassium dichromate(VI). 3 The chemical reaction involved is a redox reaction.

© Majlis Peperiksaan Malaysia 2008

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 9

50 Butyl lithium, with molecular formula C4H9Li, is a very useful organometallic compound in organic synthesis. Which statement(s) is/are true of butyl lithium? 1 It is more reactive than Grignard reagents. 2 It is a starting substance in the synthesis of silicone. 3 It can be used to prepare alcohols.

Actual 2008 STPM Chemistry Examination Paper

9

21/01/2009 2:10:18 PM

PAPER 2 Section A [40 marks] Answer all questions in this section. 1 (a) “If two or more orbitals with equal energy are available, the orbitals are each filled with one electron, with the electrons having parallel spins, before a second electron of the opposite spin is added.” (i) Name the above rule. [1 mark] (ii) Draw a diagram showing valence shells filled with valence electrons for an atom X with proton number 28. [1 mark] (iii) State the number of unpaired electrons in atom X. [1 mark] (iv) Why does the total energy of atom X decrease when there is a transfer of an electron from one subshell to another? [1 mark] (b) The decomposition of aqueous hydrogen peroxide can be accelerated by the addition of a small amount of an acidic solution of sodium bromide. The catalysis steps are as follows: 2Br– + H2O2 + 2H+ → Br2 + 2H2O Br2 + H2O2 → 2Br– + 2H+ + O2 (i) Write an overall equation for the above exothermic reaction. [1 mark] (ii) Sketch and label the energy profiles for the catalysed and uncatalysed reactions on the axes below. Energy

Reaction coordinate

(iii) State two observations for the catalysed reaction.

[3 marks] [2 marks]

2 (a) Dmitri Mendeleyev is acknowledged by scientists as the founder of the modern Periodic Table. (i) What is the basis of Mendeleyev’s Periodic Table? [1 mark] (ii) Based on his Periodic Table, Mendeleyev successfully predicted the properties of some elements that had not been discovered. Name one of the elements. [1 mark]

(b) A graph of first ionisation energy versus proton number for several elements in Period 3 of the Periodic Table is given below. First ionisation energy /kJ mol–1

Y

Proton number

10

Actual 2008 STPM Chemistry Examination Paper

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 10

© Majlis Peperiksaan Malaysia 2008

21/01/2009 2:10:18 PM

(i) Identify the element labelled Y. [1 mark] (ii) Why is the first ionisation energy of Y higher than that of the next element? [1 mark]

(c) Cyclopropane decomposes into propene through the ring opening of cyclopropane according to the equation C3H6(g) → CH2=CHCH3(g) rate constant for the decomposition is 15.12 s–1 at 400 °C. Write the rate equation for the decomposition of cyclopropane. [1 mark] Calculate the half-life of cyclopropane. [2 marks] If 1.0 mol of cyclopropane is introduced into a 1 dm3 container, what is the concentration of cyclopropane at 400 °C after two half-lives? [1 mark] (iv) What is the relationship between cyclopropane and propene? [1 mark] (v) Name a chemical compound, other than cyclopropane, that may be used to produce propene. [1 mark]

The (i) (ii) (iii)

3 (a) Silicon compounds are mainly used in making glass. The most common and cheapest glass is soda glass which is manufactured by fusing sand, sodium carbonate and limestone. (i) State the components of soda glass, and write the equation for the reaction involved. [2 marks] (ii) State the main component of borosilicate glass and its property. [2 marks] (b) Silicones are oils or rubber-like materials used commercially in a wide variety of products. Commercial silicones have infinite O-Si-O chains with alkyl or aryl groups attached to the silicon atoms. (i) Draw the structure of chain silicone showing three repeating units with alkyl groups. [2 marks] (ii) State one property of silicone. [1 mark] (iii) Explain why silicone is used in the manufacture of waterproof fabrics. [3 marks]

4 (a) Phenylamine, C6H5NH2, reacts with nitric acid in the presence of concentrated hydrochloric acid at a temperature of 0 °C to 5 °C to produce a salt used in dyeing industry. (i) Name this salt. [1 mark] (ii) Write an equation to show the formation of this salt. [1 mark] (iii) When an aqueous solution of this salt is heated to 70 °C, a vigorous reaction occurs. Name the organic product formed. [1 mark] (iv) When an aqueous solution of this salt is added to an alkaline solution of phenol, a yellow precipitate is formed immediately. Name the chemical reaction which occurs, and draw the structure of the yellow precipitate [2 marks] (b) Insulin is a hormone which controls the concentration of glucose in the blood. It is a globular protein which has two interlinked polypeptide chains. (i) Draw the structural formula of the basic unit in a polypeptide. [1 mark] (ii) What is meant by a polypeptide? [1 mark] (iii) Explain why insulin is easily denatured under a strong alkaline or acidic condition. [2 marks] (iv) Name one type of bonding which can contribute to the rigidity of the structure of the insulin molecules. [1 mark] © Majlis Peperiksaan Malaysia 2008

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 11

Actual 2008 STPM Chemistry Examination Paper

11

21/01/2009 2:10:19 PM

Section B [60 marks] Answer any four questions in this section. 5 A methane molecule dissociates homolytically in the presence of a chlorine free radical to form a methyl free radical. (a) Using the valence-shell electron pair repulsion theory, compare the shape of a methyl free radical to that of a methane molecule. [6 marks] (b) The standard enthalpies of atomisation of several species are given in the following table. Species

Standard enthalpy of atomisation/kJ mo1–1

CH4

+ 1740

•CH3

+ 1305

HC1

+ 431

(i) Calculate the standard enthalpy of reaction for the homolytic dissociation of [6 marks] methane by a chlorine free radical. (ii) Sketch the enthalpy diagram for the reaction. [3 marks] 6 (a) Ammonia is a nitrogen fertiliser which is very soluble in water. It is prepared commercially by the Haber process. 1.0 mol of nitrogen gas and 1.0 mol of hydrogen gas are introduced into a 2.0 dm3 volumetric flask at 450 °C. The amount of hydrogen gas is 0.40 mol at equilibrium. Calculate the equilibrium constant for the reaction involved at this temperature. [7 marks]

(b) A colourless gas X is an oxide of nitrogen. It changes to a brown gas Y when heated and forms X again when cooled. Identify gases X and Y, and explain the observations. [4 marks] (c) Gases X and Y are allowed to react in a closed container until equilibrium is reached. (i) State and explain what would be observed when the pressure of the container is increased at constant temperature. [2 marks] (ii) Explain the equilibrium position when a small amount of argon gas is added to the equilibrium mixture at constant volume. [2 marks] 7 (a) The table below shows the formulae of ions with proton numbers 7 to 13 except proton number 10. Proton number

7

8

9

11

12

13

Formula of ion

N3–

02–

F–

Na+

Mg2+

A13+

(i) Sketch a graph of the ionic radius against proton number. [2 marks] (ii) Explain your graph. [3 marks] (b) In industry, one of the stages in nitrogen extraction is the liquefaction of air by applying high pressure and low temperature. The liquefied air contains mainly nitrogen, oxygen and argon. 12

Actual 2008 STPM Chemistry Examination Paper

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 12

© Majlis Peperiksaan Malaysia 2008

21/01/2009 2:10:19 PM

(i) Why is carbon dioxide not found in the liquefied air? [1 mark] (ii) Explain how pure nitrogen is obtained from the liquefied air, and state one of the uses of liquid nitrogen. [3 marks] (iii) Give a reason for the inertness of nitrogen, and explain a natural phenomenon [3 marks] which produces nitrogen oxides. (c) The relative molecular mass of aluminium chloride calculated from its vapour density at different temperatures are shown in the table below. Temperature /°C

200

600

800

Relative molecular mass

269

163

133

Explain why the relative molecular mass of aluminium chloride shows different values at the different temperatures. [3 marks] 8 (a) What is meant by a transition element? [1 mark] (i) State and explain what would be observed when an aqueous solution of potassium thiocyanate is added to an aqueous solution of iron(III) ions. [4 marks] 2+ – (ii) A Fe ion reacts with CN ions to form the complex ion hexacyanoferrate(II). Draw the shape of this complex ion. State the number of 3d suborbitals which are not filled with electrons, and give reasons for your answer. [4 marks] (b) The stability constants, expressed as their logarithms, of the complex ions [CuC14]2– and [Cu(NH3)4]2+ are 5.6 and 13.2 respectively. (i) Explain what happens when an aqueous solution of ammonia is added to an aqueous solution of [CuCl4]2–. [4 marks] (ii) Predict the stability constants, expressed as their logarithms, of the complex ion [Cu(EDTA)]2–, and give a reason for your answer. [2 marks] 9 Compound A, with the molecular formula C5H12O, which has a straight chain structure is an optically active alcohol. When A is heated with alumina, compounds B and C are formed. B shows geometrical isomerism but C does not. When C is heated with an acidified potassium manganate(VII) solution, compound D is formed and a gas which turns lime water chalky is evolved. When D is heated with an aqueous solution of ammonia, compound E is formed which reacts with lithium tetrahydridoaluminate(III) to form compound F. (i) Write the structural formulae of compounds A, B, C, D, E and F. Justify your answers. [10 marks] (ii) Write the structural formulae of the cis and trans isomers of compound B. [1 mark] (iii) Write the equations for all the reactions above. [4 marks] 10 (a) Propene polymerises in the presence of benzoyl peroxide, C6H5COO-OOCC6H5, to form polypropene. Explain, with chemical equations, the reactions which occur in the polymerisation of propene. [7 marks] (b) An optically active compound Z, with molecular formula C4H9C1, reacts with ethanolic potassium hydroxide solution to form three isomers of unsaturated organic compounds. (i) Write the structural formula of Z, and explain its reaction with ethanolic potassium hydroxide solution. [5 marks] (ii) Determine the isomer of Z which reacts with ethanolic potassium hydroxide solution to form only one organic compound. Give a reason for your answer. [3 marks] © Majlis Peperiksaan Malaysia 2008

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 13

Actual 2008 STPM Chemistry Examination Paper

13

21/01/2009 2:10:19 PM

SUGGESTED ANSWERS PAPER 1 Section A

Energy

1. D Let relative molecular mass of haemoglobin molecule, HbFe4 = Mr. 4 × 55.8 % Fe = ———— × 100 = 0.335% Mr Mr = 6.66 × 104 2. A 2B + 3H2 → B2H6 1.00 g boron forms 1.278 g B2H6 ∴ Mass of H = 0.278 g

NO2 + O2 Reaction coordinate

10. B A Lewis base is an electron-pair donor. NH3 has a lone pair that is donated to the empty orbital in BF3. 92.0 11. B [C2H5OH] = ——– = 2 mol dm–3 46

1.00 —— 1 Ar ⇒ ——— = — ⇒ Ar = 10.8 0.278 3 ——– 1

120.0 [CH3COOH] = ——— = 2 mol dm–3 60 C2H5OH + CH3COOH CH3COOC2H5 + H2O Let x = number of moles of ethanol that has reacted

Let % abundance of 10B : 11B = x% : (100 – x)%

4. A

5. C

6. A

10x + 11(100 – x) ∴ ————–——— = 10.8 100 ⇒ x = 20% ∴ 10B : 11B = 1 : 4 First ionisation energy is the amount of energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of unipositive ions. Valence electronic configuration of 3d3 4s2 indicates that vanadium can have oxidation states of +1 to +5. The oxidation states of V in VO42– is +6, in VO3– is +5, in VO2+ is +5 and in VO2+ is +4. Fajan’s rule for a compound to exhibit high covalent character: (i) Small cation (ii) Large anion (iii) High ionic charge PCl3, ClO3– and SO32– are trigonal pyramidal in shape.

7. B 3H

C O

CN

+ NC

H—P—H H

CN Fe

NC

CN CN

8. C The first graph shows the relationship: Rate = k[X] ⇒ 1st order The second graph shows the relationship: Rate = k[Y]2 ⇒ 2nd order 9. D

14

Actual 2008 STPM Chemistry Examination Paper

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 14

+(10.5 + 200) kJ

–200 kJ

mol B 2 ——— = — mol H 6

3. A

+10.5 kJ +10.5kJ

NO + O3

12. D 13. C

14. A 15. C

x2 —————— = 4.0 (2 – x)(2 – x) ⇒ x = 1.333 ∴ Mass of C2H5OH left = (2 – 1.333) × 46 = 30.7 g For Kc = Kp, number of moles of gases on right-hand side = left-hand side. pH = 3.5 ⇒ [H+] = 3.16 × 10–4 Let concentration of monobasic acid = c mol dm–3 ∴ 25.00 × c = 27.50 × 0.10 ⇒ c = 0.11 mol dm–3 + [H ] = Ka × c Ka = 9.08 × 10–7 mol dm–3 A weak base and its salt forms an alkaline buffer solution. Let x = mass of X extracted in the first extraction x —— 100 ————— = 3.00 ⇒ x = 3.60 g 6.00 – x ———— 200 ∴ Mass of X remaining in aqueous solution = 6.00 – 3.60 = 2.40 g Let y = mass of X extracted in the second extraction y —— 100 ————— = 3.00 ⇒ y = 1.44 g 2.40 – y ———— 200 Total mass of X extracted = 3.60 + 1.44 = 5.04 g © Oxford Fajar Sdn. Bhd. (008974-T) 2008

21/01/2009 2:10:19 PM

16. D E o cell = +0.80 – (–0.76) = +1.56 V At equilibrium 0.059 ⇒ 0 = E o cell – ——— log Kc 2 ⇒ Kc = 7.61 × 1052 dm3 mol–1 17. C Plastic is a non-conductor while graphite is a conductor. 18. B C2H5OH + O2 → CH3COOH + H2O ∆H = Σ ∆Hf (products) – Σ ∆Hf (reactants) = [–487 + (–286)] – [(–278) + 0] = –495 kJ mol–1 19. D The small F – ion has a very exothermic enthalpy of hydration. 20. B Descending Group 2: • Ionic size increases, thus charge density decreases • Solubility of sulphates decreases • Thermal stability of carbonates and nitrates increases 21. D Descending Group 14: • Conductivity increases as metallic character increases • Electronegativity decreases • Stability of +4 oxidation state decreases • Carbon can undergo catenation but not other elements 22. A Si is in Period 3. It has empty 3d orbitals to accept lone pairs of electrons from water molecules. 23. C Reaction between N2 and H2 is exothermic. Thus when temperature increases, the value of Kc decreases. 24. C When an NH4+ compound is heated with a base, NH3 gas will be liberated. 25. C Ionisation energy involves the formation of positive ions. 26. B & C X has one oxidation state only (Sc). Hence, it will form colourless hydrated ions. Y has an oxidation state of +6 in K2YO4. But the oxidation states of Y are +2, +3 and +4 only. Z2O means Z has oxidation state of +1 but Z does not have oxidation state of +1. 27. D The presence of NH4+ ions cause a common ion effect which decreases the concentration of OH– ions. This causes the ionic product of [Zn2+][OH–]2 < Ksp of Zn(OH)2. 28. D There are three chiral centres. There is only one functional group (–OH). The –OH group does not react with NaOH. It is a secondary alcohol that can be oxidised. CH3

* *

* OH

CH(CH3)2 © Oxford Fajar Sdn. Bhd. (008974-T) 2008

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 15

29. D The aromatic nucleus undergoes electrophilic substitution while the methyl side chain can undergo free radical substitution. 30. B CH3CH2CH=CH2 +

31. A

CH2CH3

CH3CH2CHCH3

COOH

+ 6[O]

+ CO2 + 2H2O

white precipitate

32. C Ease of hydrolysis : RCOCl > RCl > C6H5Cl 33. B C6H5OH (phenol) forms a white precipitate with bromine water but ethanol does not. 34. C Z forms an orange precipitate with 2,4-dinitrophenylhydrazine and a yellow precipitate with I2/OH– indicates that Z is a ketone with CH3C=O group. Thus, Y is a secondary alcohol with CH3CH(OH)group, and X is a haloalkane with CH3CHCl- group. 35. D Decolourises bromine water ⇒ alkene Number of moles of NaOH = 0.02 1.16 Number of moles of acid = ——– Mr 0.02 If acid is monobasic ⇒ ——– = 1 1.16 —— Mr ⇒ Mr = 58 ≠ Mr of (C) 0.02 If acid is dibasic = ——– = 2 1.16 —— Mr ⇒ Mr = 116 = Mr of (D) 36. B C O O (P)

NaOH(aq)

+

+

C O– Na + Na –O O

HCl(aq)

C OH + HO O (Q)

(R)

37. A When ethanamide is heated with NaOH, NH3 gas is liberated and sodium ethanoate is formed. NH3 gas forms white fumes with HCl gas. 38. C ( C H 3 ) 2 C = C H C ≡ N + 3H2 → (CH3)2CHCH2CH2NH2 39. D –COOH is the acidic group. It can react with an alcohol. 40. D Ziegler-Natta catalyst produces HDPE at low temperature and pressure via coordination polymerisation. Actual 2008 STPM Chemistry Examination Paper

15

21/01/2009 2:10:20 PM

Section B 41. A The mixture shows negative deviation. Thus, intermolecular forces between molecules in the mixture are stronger than those between molecules in the pure liquids. CHCl3 and propanone molecules are held together by hydrogen bonds which are stronger than the van der Waals forces between CHCl3 molecules or propanone molecules. 42. No answer W has a smaller Rf value than Y. X is not a mixture of W and Y. Z and W seem to have the same (lowest) solubility in ethanol. 43. D Electronegativity, electrical conductivity and enthalpy of vapourisation are periodic functions of the elements across a period. 44. A The difference in the bond angles is due to the number of lone pairs in the molecules. According to VSEPR theory, lone pairlone pair repulsion > lone pair-bond pair repulsion > bond pair-bond pair repulsion 45. B Quantity of electricity =I×t = (0.8 × 1 × 60 × 60)/ 9.65 × 104 = 2.98 × 10–2 F To deposit 1 mol of the metal, quantity of 2.98 × 10–2 electricity required = ————— =3F 1.00 × 10–2 The metal ion has a charge of +3. 46. A [Al(H2O)5(OH)]2+ can be hydrolysed to Al(OH)3 under basic conditions. 47. D Ag+ ions form precipitates with Cl– and I– ions. AgCl dissolves in ammonia to form [Ag(NH3)2]+ complex ions. 48. A The reaction is an electrophilic substitution reaction of the aromatic nucleus. 49. D Ethanol can be consumed. K2Cr2O7 oxidises ethanol to ethanoic acid and is itself reduced to Cr3+ ions. 50. D Organolithium compounds are more reactive than Grignard reagents and they can be used to prepare alcohols. Silicones are also prepared from chlorosilanes. PAPER 2 Section A 1. (a)

(i) Hund’s rule (ii) 3d 8

4s 2

atom X increases (not decrease as in the question). (b) (i) 2H2O2 → 2H2O + O2 (ii) energy E2

E1 2H2O2 ∆H 2H2O + O2 reaction coordinate E2 = activation energy without catalyst E1 = activation energy with catalyst ∆H = enthalpy change

(iii) The solution turns reddish-brown at the beginning (due to formation of Br2) then becomes colourless (Br2 is reduced to Br–). 2. (a)

(i) Properties of elements are periodic functions of atomic mass. (ii) Germanium (b) (i) Phosphorus (ii) P has a stable half-filled p subshell. (c) (i) Rate = k [C3H6] (This is a first order reaction because the unit for the rate constant k is s–1.) ln 2 (ii) t 1 = —— — k 2 0.693 = ——— 15.12 = 0.0458 s (iii) 0.25 mol dm–3 (1.0 → 0.5 → 0.25) (iv) Both have the same molecular formula. They are structural isomers. (v) Propanol (dehydration of propanol forms propene)

3. (a)

(i) Soda glass consists of SiO2, Na2O, CaO. SiO2 + Na2CO3 → Na2SiO3 + CO2 (ii) Main component is B2O3 and SiO2. Has low expansion coefficient, can withstand temperature/heat change without cracking. (b) (i) R R R O

(iii) 2 (iv) When an electron is transferred from one subshell to another, the electron gains energy and is promoted to a higher subshell. The total energy of 16

Actual 2008 STPM Chemistry Examination Paper

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 16

Si

O Si O

Si

R

R

R

where R = alkyl group (ii) High viscosity liquid / stable towards heat / chemically inert © Oxford Fajar Sdn. Bhd. (008974-T) 2008

21/01/2009 2:10:20 PM

pyramidal shape with smaller bond angle of < 109.5°.

(iii) When applied to fabric, the O atoms of silicone form hydrogen bonds with molecules on the surface of fabrics. The hydrophobic organic groups act as barrier on the surface of fabrics and repel water. Silicone fluid shows spreading properties due to low surface tension, forming a film on the fibre that water cannot wet. The hydrophilic –OH group is replaced by the hydrophobic organic group (–CH3) that repels water. Thus, water falling on the fabric roll off instead of penetrating through and wetting it. 4. (a)

c H

(b)

-N=N-

(b) (i)

-O

(i) Enthalpy of atomisation is the heat energy required to convert one mole of a covalent compound (or species) into its free gaseous atoms. ∆H CH4(g) + Cl•(g) ⎯→ •CH3(g) + HCl(g)

+1740 kJ

+1305 kJ

+431kJ

C(g) + 4H(g) + Cl(g)

(i) Benzene diazonium chloride (ii) C6H5NH2 + HNO2 + HCl → C6H5N2+ Cl– + 2H2O (iii) Phenol (iv) Coupling reaction –

H H

Enthalpy of reaction, ∆H = +1740 – (+1305) – (+431) = + 4 kJ mol–1 (ii) energy

H H2N C COOH

R The basic unit in polypeptide is amino acid. (ii) Long chain of amino acids bonded by peptide linkages (iii) Hydrogen bonds are disrupted and hydrolysis occurs on the –CONHgroup. (iv) Hydrogen bond between C=O group of an insulin molecule with N–H group of another insulin molecule. Section B 5. (a) VSEPR (valence-shell electron-pair repulsion) theory states that the geometry of a molecule is determined by the repulsion among the electron-pairs associated with a central atom. The electron-pairs arrange themselves as far apart as possible to minimise the repulsion. Repulsion between electron-pairs increases in the order: bond pair – bond pair < bond pair – lone pair < lone pair – lone pair CH4: Has four bonding electron-pairs and no lone pair. Hence, tetrahedral shape with bond angle of 109.5°. H

C

H

H

H

•CH3: Has three bonding electron-pairs and one lone electron. Hence, trigonal © Oxford Fajar Sdn. Bhd. (008974-T) 2008

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 17

•CH3(g) + HCl(g) ∆H = +4 kJ mol–1

CH4(g) + Cl•(g)

reaction coordinate

6. (a) N2(g) + Initial: 1 mol At equilibrium: 0.8 mol 0.8 ≡ [ ] in 2 dm3 —— mol 2

3H2(g) ↔ 1 mol 0.4 mol 0.4 —— mol 2

2NH3(g) 0 mol 0.4 mol 0.4 —— mol 2

[NH3]2 Kc = ————– [N2] [H2]3 0.4

2

冢—— 2 冣

= —————— 0.8 0.4 3 —— —— 2 2 = 12.5 mol–2 dm6 (b) Gas X is N2O4. Gas Y is NO2. Decomposition of N2O4 to NO2 is an endothermic process. N2O4(g) 2NO2(g) ; ∆H = +ve When heated, the equilibrium position shifts to the right, colourless N2O4 decomposes to the brown NO2. When cooled, the equilibrium position shifts to the left, colourless N2O4 is formed. (c) N2O4(g) 2NO2(g) (i) When pressure is increased, the mixture turns light brown / pale yellow as more N2O4 is formed. The equilibrium position shifts to the left

冢

冣冢

冣

Actual 2008 STPM Chemistry Examination Paper

17

21/01/2009 2:10:20 PM

as the reaction to the left is accompanied by a reduction in the number of moles of the gases. (ii) When argon gas is added to the equilibrium mixture at constant volume, the equilibrium position is unaffected because the partial pressure of each component gas remains unchanged.

Dissociation of dimer Al2Cl6 occurs between 400 – 800 °C. 8. (a) Transition element is an element that forms at least a stable simple ion with partially filled d orbitals. (i) [Fe(H2O)6]3+(aq) + SCN–(aq) → [Fe(H2O)5SCN]2+(aq) + H2O(l) Yellow solution becomes blood-red. An aqueous solution of iron(III) contains [Fe(H2O)6]3+. SCN– is a stronger ligand than H2O. Hence, SCN– displaces a H2O ligand, forming the complex ion [Fe(H2O)5SCN]2+ which is red in colour. (ii) [Fe(CN)6]4–

ionic radius

7. (a) (i) N3– O2–

F– Na+ Mg2+ 3+ Al

7

8

9

10

11

12

13

proton number

(ii) All the ions are isoelectronic (having the same number of electrons, 10 e–), hence almost similar shielding effect but the nuclear charge increases with increasing proton number. Hence, the effective nuclear charge increases. Therefore, the ionic size decreases in the order: N3–, O2–, F–, Na+, Mg2+, Al3+. (b) (i) Under very low temperatures, CO2 is in solid form and is filtered off. (ii) Pure nitrogen is obtained by fractional distillation of liquefied air. N2 is more volatile and is distilled out while O2 is left in the residual liquid air. Liquid nitrogen is used as refrigerant to freeze food rapidly / as biological preservation. (iii) Nitrogen is inert due to the strong triple covalent bond in N2 molecule that has a high bond energy and requires a lot of energy to break. Nitrogen oxides are produced during thunderstorms. Lightning provides very high energy to break the strong covalent bonds in N2 and O2, thus enabling nitrogen and oxygen gases to react to form nitrogen oxides. N2(g) + O2(g) → 2NO(g) Further oxidation: 2NO(g) + O2(g) → 2NO2(g) (c) RMM of AlCl3 = 27.0 + (3 × 35.5) = 133.5 At 200 °C, aluminium chloride exists as a dimer Al2Cl6 as its rmm is twice that of AlCl3. At 800 °C, aluminium chloride exists as monomer AlCl3 as its rmm is 133. At 600 °C, both monomer AlCl3 and dimer Al2Cl6 exist in equilibrium as its rmm is >133 but < 269. 18

Actual 2008 STPM Chemistry Examination Paper

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 18

4-

CN NC

CN Octahedral shape

Fe NC

CN CN

Two of the 3d orbitals are not filled with electrons. There are six electrons in Fe2+. The electrons in the 3d subshells rearrange to form vacant orbitals in order to accept lone pair electrons from the CN– ions. orbitals hybridised to accept electrons from CN– ions

Fe2+ 3d

(b)

4s

4p

(i) Yellow solution turns a deep blue colour. [CuCl4]2–(aq) + 4NH3(aq) → yellow solution

[Cu(NH3)4]2+(aq) + 4Cl–(aq) deep blue solution

[Cu(NH3)4]2+ has a higher stability constant than [CuCl4]2– which implies that the complex ion [Cu(NH3)4]2+ is more stable. Cl– is a weaker ligand and is displaced by the stronger NH3 ligand. (ii) The stability constant of [Cu(EDTA)]2– complex, expressed in logarithm, is higher than 13.2. [EDTA]4– is a stronger ligand than Cl– and NH3 because it is a polydentate / chelating ligand that is attached to the metal ion by several bonds. © Oxford Fajar Sdn. Bhd. (008974-T) 2008

21/01/2009 2:10:21 PM

9. (i) H

H

H

H

OH H

C

C

C

C

C

H A

H H H 2-pentanol

H

H H

H

H C

C

C

H B

H H H 2-pentene

H

H

H

H

C

C

C

C

C

H C

H H H 1-pentene

H

H

H

H

O

C

C

C

C

H D

H H butanoic acid

H

H

H

O

H

C

C

C

C

N

H E

H H butanamide

H

H

H

H

H

C

C

C

C

N

H F

H H H butanamine

H

H

H

C3H7

H

C

H

A is an optically active alcohol implies that A has a chiral carbon atom that is attached to four different groups.

C

C3H7

C*

H

C* H

HO

H

OH

CH3

CH3

When heated with alumina, A undergoes elimination to alkenes B and C. H

C is an alkene with C=C double bond at one end. Under strong oxidation (when heated with KMnO4), the C=C double bond breaks and carboxylic acid D is formed and CO2 gas that turns lime water chalky is evolved.

OH

When carboxylic acid D is heated with ammonia, an amide E is formed. Amide E with LiAlH4 is reduced to an amine F. H

(ii)

CH3 C

C2H5

CH3

C

H

C

H H cis-2-pentene

C

H C2H5 trans-2-pentene

H

(iii)

2H

H

H

H

H

H

OH H

C

C

C

C

C

H

H

H H A

H

H

H

H

C

C

C

C

C

H

H

H H C

H

H

H

H

O

C

C

C

C

H

H D

H

H

Al2O3 䉭

H + 5[O]

OH + NH3

© Oxford Fajar Sdn. Bhd. (008974-T) 2008

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 19

H

H

H

C

C

C

C

C

H

H

H H B

H

H

H

H

O

C

C

C

C

H

H D

H

KMnO4 䉭

䉭

H

H

H H + H

H

H

H

C

C

C

C

C

H

H

H H C

H

H + 2H2O

OH + CO2 + H2O

H

H

H

O

H

C

C

C

C

N

H

H

H E

H + H2O

Actual 2008 STPM Chemistry Examination Paper

19

21/01/2009 2:10:21 PM

H

H

H

H

O

H

C

C

C

C

N

H

H

H E

H + 4[H]

LiAlH4

H

H

H

H

H

H

C

C

C

C

N

H

H

H F

H

H + H2O

10. (a) Free radical addition polymerisation. (i) Initiation step: Benzoyl peroxide, C6H5COO–OOCC6H5 acts as initiator, undergoes photochemical/thermal decomposition to form free radicals C6H5COO OOCC6H5 → 2C6H5COO• → 2C6H5• + 2CO2 C6H5• + CH3CH = CH2 → C6H5CHCH2• CH3 (ii) Propagation steps: The free radical reacts with propene monomer to form another free radical. The chain propagates as successive monomers add on until a long polymeric chain is formed. C6H5CHCH2• + CH CH2 → C6H5 CHCH2CHCH2• CH3

CH3

CH3

CH3 CH3

冢

C6H5CHCH2CHCH2• + CH CH2 → C6H5CHCH2 CH CH2 CH CH2• → → C6H5CHCH2 CH CH2 CH3

CH3

CH3

CH3

CH3

CH3

CH3

冣nCH

CH2•

CH3

(iii) Termination step: Two free radicals combine to form a molecule. C6H5CHCH2• + C6H5 CHCH2• → C6H5 CHCH2CH2CHC6H5 CH3 (b)

CH3

CH3

CH3

(i) Z is

H

H

H

H

H

C

C

C

C

H

H

Cl H

H

Z is optically active implies that Z has a chiral carbon that is attached to four different groups. C2H5

C2H5

C* Cl

C* H CH3

H

Cl CH3

Z undergoes elimination reaction with ethanolic potassium hydroxide solution to form two different alkenes (unsaturated organic compounds): 1-butene, CH3CH2CH=CH2 and 2-butene, CH3CH=CHCH3. CH3CH2CH(Cl)CH3 + KOH → CH3CH2CH=CH2 + KCl + H2O CH3CH2CH(Cl)CH3 + KOH → CH3CH=CHCH3 + KCl + H2O 2-butene forms two geometrical (cis-trans) isomers because it has different groups on the unsaturated carbon atom. 20

Actual 2008 STPM Chemistry Examination Paper

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 20

© Oxford Fajar Sdn. Bhd. (008974-T) 2008

21/01/2009 2:10:22 PM

H Hence the three isomers are

CH3

H

H

C

C

C

C

H

H

H

H

C

H

CH3

CH3

C

, H

C H and H

H C CH3

(ii) There are three possible isomers of Z that give only one organic compound with ethanolic KOH. CH3

CH3

CH3CH2CH2CH2Cl / CH3 C CH3 / CH3 CH CH2Cl. Cl Reaction with ethanolic KOH: CH3CH2CH2CH2Cl

CH3 CH3 CH CH2Cl

ethanolic CH3CH2CH=CH2 + KCl + H2O KOH 1-butene CH3 CH3 C CH3

CH3 ethanolic KOH

CH3 C=CH2 + KCl + H2O methylpropene

Cl The products formed do not have geometrical isomers because they have the same group on the unsaturated carbon atoms.

© Oxford Fajar Sdn. Bhd. (008974-T) 2008

Ace Ahead STPM Chem Vol 2 08 Q&A 4th.indd 21

Actual 2008 STPM Chemistry Examination Paper

21

21/01/2009 2:10:22 PM