Pahang STPM Trial 2011 Chemistry Paper 2 (w Ans)

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962/2 TRIAL STPM 2011

(PEPERIKSAAN PERCUBAAN STPM 2011)

CHEMISTRY (KIMIA) PAPER 2 (KERTAS 2) Two and a half hours (Dua jam setengah)

Instructions to candidates: DO NOT OPEN THIS QUESTION PAPER UNTIL YOU ARE TOLD TO DO SO.

For examiner’s use 1

Answer all questions in section A. Write your answers in the spaces provided. All working should be shown. For numerical answers, units should be quoted wherever they are appropriate.

2 3

Answer any four questions in section B. Write your answers on the answer sheets provided. Begin each answer on a fresh sheet of paper and arrange your answers in numerical order. Tie your answer sheets to this question paper.

4 5 6 7 8 9 10 Total

This question paper consists of 12 printed pages.

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2 Section A [ 40 marks ] Answer all questions in this section.

1.

(a) Draw and name the shape of these following molecules : (i) BeCl2

(ii) CO32-

Shape :

Shape :

(iii) NH4+

(iv) SF6

Shape :

Shape :

[ 4 marks ]

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3 (b)

An element A reacts with another element B to form a compound of formula AB2. The element B exist as molecules of formula B2. Some properties of A, B2 and AB2 are tabulated below: Substance

A High (in the range of 700°C - 1200°C)

Melting point

B2 Very low (less than -50°C)

Electrical conductivity of the salt

High

Very low

Electrical conductivity of molten material

High

Very low

-

-

Electrical conductivity of aqueous solution of the material

(i)

AB2 Moderate (in the range of 400°C -700°C) Very low High

High

Which particles will move when a potential difference is applied across a sample of (I) solid, [ 1 marks ] (II)

………………………………………………… molten AB2 ?

[ 1 marks ]

………………………………………………… (ii)

Explain why the electrical conductivity of molten AB2 is high, whereas that of the solid is very low. [ 2 marks ] …………………………………………………………………………………. ………………………………………………………………………………….

(iii)

Electrolysis of an aqueous solution of AB2 with Pt electrodes give A at the cathode and B2 at the anode. Suggest possible name for the elements A and B2 consistent with all the above information. [ 2 marks ] A : ………………………………. B2 : ……………………………..

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4 2.

As a first step in a manufacture of nitric acid it has been suggested that nitrogen monoxide NO, can be formed from nitrogen and oxygen in a reversible reaction.

(a)

Write an equation for this reaction and deduce an expression for the [ 2 marks ]

equilibrium constant Kc. Equation

…………………………………………………………………………. Kc

(b)

=

The sketch graph below shows how the value of Kc for this reaction changes with temperature. Kc

Temperature (ºC) Use this graph to deduce whether the reaction is exothermic or endothermic. Explain your answer.

[ 2 marks ]

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5 (c) When cooled, nitrogen monoxide reacts with oxygen to form gaseous nitrogen dioxide NO2, in a reversible reaction. (i)

Write an equation for this reaction.

[ 1 mark ]

……………………………………………………………………………………….. (ii)

State how an increase in pressure would change the position of the equilibrium and the value of the rate constant for this reaction.

[ 2 marks ]

Change in equilibrium position : ……………………………………………………………………………………….. Change in rate constant : ……………………………………………………………………………………….. (d)

Titration curves labelled A, B, C and D for combinations of different acids and bases are shown below. All solutions have a concentration of 0.1 mol dm–3.

14 12 10 8 pH 6 4 2 0

0

10

20 30 40 Volume/cm 3

50

14 12 10 8 pH 6 4 2 0

0

10

20 30 40 Volume/cm 3

A 14 12 10 8 pH 6 4 2 0

0

10

50

B

20 30 40 Volume/cm 3

50

14 12 10 8 pH 6 4 2 0

0

10

20 30 40 Volume/cm 3

C

50

D

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6 (i)

Select from A, B, C and D the curve produced by the addition of ammonia to 25 cm3 of hydrochloric acid

[ 1 mark ]

...................................................................................................................... ethanoic acid to 25 cm3 of sodium hydroxide

[ 1 mark ]

...................................................................................................................... (iii)

A table of acid–base indicators and the pH ranges over which they change colour is shown below.

Indicator

pH range

Thymol blue

1.2 – 2.8

Bromophenol blue

3.0 – 4.6

Methyl red

4.2 – 6.3

Cresolphthalein

8.2 – 9.8

Thymolphthalein

9.3 – 10.5

Select from the table an indicator which could be used in the titration which produces curve A but not in the titration which produces curve B. [ 1 mark ] ..............................................................................................................................

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7 3+

3. (a) (i) Write the electron configuration of the Scandium atom and Scandium (III) ion, Sc . [ 2 marks ] Sc :…………………………………………………………………….. Sc

3+ :

……………………………………………………………………..

(ii) State the colour of Scandium (III) ion, Sc

3+

and explain why the colour is such.

[ 2 marks ]

…………………………………………………………………………………………………... …………………………………………………………………………………………………... (b) Chlorine is a greenish yellow poisonous gas which is slightly soluble in water. (i)

Write a balanced equation for the reaction that occur when chlorine gas is passed into water. [ 1 mark ] ……………………………………………………………………………………………………

(ii) What would you observe when an excess of potassium iodide is added to the solution obtained from the reaction and write the equation that occurs. [ 2 marks ] …………………………………………………………………………………………………… ……………………………………………………………………………………………………

(c) A sodium halide reacts with cold concentrated sulphuric acid to give colourless fumes.When heated, the mixture produces brown fumes. (i)

Name the halide ion in the sodium salt.

[ 1 mark ]

………………………………………………………………………………………………….. (ii) Write two balanced equations to explain the observations above.

[ 2 marks ]

………………………………………………………………………………………………….. …………………………………………………………………………………………………..

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8 4. (a) A compound M has the composition by mass : C, 80.0% ; H, 6.7% ; O, 13.3%. (i) Calculate the empirical formula of M.

[ 1 mark ]

3

(ii) 0.305g of liquid M is vaporized at 480K. Its vapour occupies a volume of 100cm at 101kPa. Calculate the relative molecular mass of M and deduce the molecular formula. [ 2 marks ]

(b) When warmed with iodine and aqueous sodium hydroxide, M gives a yellow precipitate. M can reduced to compound N. (i) What is the yellow precipitate? [ 1 mark ] ………………………………………………………………………………………………… (ii)

State the functional groups possesses by M and N.

[ 1 mark ]

………………………………………………………………………………………………… (iii)

Draw the structural formulae of M and N.

M:

[ 2 marks ]

N:

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9 (c) When N is heated with concentrated sulphuric acid, a colourless liquid P is obtained. P decolourises bromine water. [ 3 marks ] (i)

Draw the structural formula of P.

(ii) What is the type of reaction that occurred in the conversion of N to P? ………………………………………………………………………………………………….. (iii) When P is heated with a catalyst, a viscous liquid with high relative molecular is produced by addition polymerization. Draw the repeating unit of the substance in the viscous liquid.

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10 Section B [ 60 marks ] Answer any four questions in this section.

5.

(a) A The reaction : 2NO(g) + Cl2(g) following data were obtained : Eksperiment Number 1 2 3

Initial concentration [NO], mol dm-3 0.10 0.10 0.20

2NOCl was studied at -10°C, and the Initial concentration [Cl2], mol dm-3 0.10 0.20 0.20

Initial rate of formation of NOCl, mol dm-3 min-1 0.18 0.35 1.45

What is the order of reaction with respect to NO and with respect to Cl2 and what is value of the rate constant ? State clearly the units of rate constant. [ 4 marks ] (b) Sucrose decomposes in acid solution into glucose and fructose according to a first order rate law, with a half-life of 3.33 h at 25°C. What fraction of a sample of sucrose remains after 9.00 h ? [ 3 marks ] (c) A current of 2.00 A is passed through a potassium sulfate solution for 30.0 minutes at 25.0°C using graphite electrodes. Calculate the volume, in cm3, of the gas evolved at the anode at 101 kPa. [ 3 marks ] (d) The values of Ksp for the slightly soluble salts MX and QX2 are each equal to 4.0 x 10-18. which salt is more soluble ? Explain your answer fully. [ 5 marks ] 6.

(a) Elements P and Q form an ionic compound PQ. The enthalpy changes involved in the formation of this compound under standard conditions are as below : P(s) P(g) : ½ Q2(g) Q(g) : P(g) P+(g) + e- : Q(g) + eQ-(g) : P(s) + ½Q2(s) PQ(g) :

∆H° = +77 kJ ∆H° = +121 kJ ∆H° = +419 kJ ∆H° = - 364kJ ∆H° = -436 kJ

Write equation which represent the lattice energy of compound PQ and calculate the value of the lattice energy with the aid of a Born-Haber cycle. [ 6 marks ] (b) Nitrogen dioxide, NO2, is brown gas that is a constituent of smogs caused by cars in cities. Suggest how NO2 is formed in a car engine and how it may be removed from exhaust gases. [ 3 marks ] 962/2 * This question paper is CONFIDENTIAL until the examination is over.

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11 (c) In the gaseous state, NO2 can dimerise as follows. (i) At a temperature of 320 K and a pressure of 1.0 x 105 Pa(1.0 atm), 0.50 g of the gaseous NO2/N2O4 mixture takes up a volume of 190 cm3. Calculate the average mass, Mr of the mixture. [ 3 marks ] (ii) at another temperature, it is found that the partial pressures of NO2 and N2O4 at equilibrium are 0.4 atm and 0.6 atm respectively. Write an expression for Kp and calculate its value, giving units. [ 3 marks ] 7.

Benzene and methylbenzene form a solution that obeys Raoult’s law. The vapour pressure of pure benzene and pure methylbenzene are 12.7 and 3.9 kPa respectively at 298 K. (a) State Raoult’s law [ 2 marks ] (b) Draw and label, on a graph paper the vapour pressure-composition graph for a mixture of benzene and methylbenzene. [ 3 marks ] (c) Determine the vapour pressure of a solution containing 2.0 moles of benzene and 3.0 moles of methylbenzene. [ 3 marks ] (d) By using a sketch of boiling point-composition curve, describe how a mixture of benzene and methylbenzene can be separated. [ 7 marks ]

8.

(a) State the changes in acid-base properties of oxides of the third period elements, that is from sodium to chlorine in the Periodic Table. Write an equation to represent each typical property of the oxides. [ 6 marks ] (b) Between hydrogen chloride and hydrogen sulphide, which is the stronger acid in aqueous solution. Explain your answer. [4 marks ] (c) Draw the possible structures of isomers of the following complexes. State the name of each isomer. + (i) [Cr(NH3)4Cl2] (ii) [Cr(C2O4)3]

3-

[ 5 marks ]

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12 9. Two organic compounds X and Y have the following formulae. CH3

O2N X

H2N

COOH Y

a) State which of these two compounds has the higher melting point. Explain your answer. [ 3 marks ] b) State the reagents used and the conditions, the observations obtained and write equations for the reactions that occur for the following. (i) Formation of X from benzene. [ 4 marks ] (ii) Formation of Y from X. [ 4 marks ] (iii) A simple test to differentiate between X and Y. [ 2 marks] c) Which of the compounds X and Y exists as a zwiterrion? Write the formula of the zwiterrion. [ 2 marks ] 10. a) Explain the following observations and write equations for the reactions involved. (i) 1-Butanol is insoluble in aqueous sodium hydroxide but phenol is soluble. [ 3 marks ] (ii) Methanoic acid decolourises the purple colour of aqueous potassium manganate( VII). [ 3 marks ] (iii) Methylamine is soluble in hydrochloric acid.

[ 2 marks ]

b) Using 1, 4-dibromobutane, Br(CH2)4Br, as the starting material, write the equations for all the reactions involved in the synthesis of nylon-6,6. [ 7 marks ]

END OF QUESTION PAPER

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MARKING SCHEME OF PAPER 2 CHEMISTRY TRIAL STPM 2011 1.

(a) (ii)CO32-

(i)BeCl2

O || Cl

Be

Cl

C -

Shape : linear

O

O

-

Shape : trigonal planar

(iii) NH4+

(iv) SF6 H+

F

N

F

F S

H

H

F

H

F F

Shape : tetrahedral

Shape : octahedral [4M]

(b)

(i) electrons Ions / A2+ and B-

[2M]

(ii) Ions in molten AB2 can move freely, in solid ions are held closely in a lattice position. (iii) A: Mg B2:Cl2

1

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[1M] [1M] [2M] ------------------Total : 10M

2.

(a)

Equation Kc

(b) (c)

(d)

=

N2 + O2 2NO (1) [NO]2 [N2] [O2] (1)

[2M]

As temperature increases Kc increases (or yield increases) (1) Hence reaction endothermic (1) (i)

2NO + O2

(ii)

Change in equilibrium position: Change in rate constant:

(i)

B;

(ii)

cresolphthalein OR thymolphthalein;

[2M]

2NO2 (1)

(1)

Displaced to the right (1) No change (1)

C;

(1)

[3M]

[2M] [1M] -------------Total : 10 M

3. (a) (i) Sc : 1s2 2s2 2p6 3s2 3p6 3d1 4s2 Sc3+ : 1s2 2s2 2p6 3s2 3p6 (ii) Sc3+ is a colourless ion because there is no electron in 3d subshell (b) (i) Cl2 + H20 HCl + HOCl (ii) Brown precipitate of iodine is formed, iodine is oxidised Cl2 + 2I2Cl- + I2 (c) (i) Sodium bromide (ii) NaBr + H2SO4 NaHSO4 + HBr (iii) 2HBr + H2SO4 SO2 + Br2 + 2H2O

4. (a) (i) C H O 80.0 6.7 13.3 12 1 16 = 6.67 = 6.67 = 0.83 8 8 1 Empirical formula : C8H8O

[1M] [1M] [1M] [1M] [1M] [1M] [1M] [1M] [1M] [1M] ----------------Total : 10 M

[1M]

(ii)

pV = nRT 101 x 0.1 = n (8.314) x 480 n = 0.025 RMM of M is 0.305 = 122 0.025 Molecular formula of M : C8H8O (b) (i) triiodomethane (ii) M has carbonyl group N is an alcohol (iii) M : CO CH3

[1M] [1M] [1M] [1M] [1M] [1M] 2

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(c) (i)

N:

CH(OH)CH3

[1M]

P:

CH=CH2

[1M]

(ii) Dehydration

[1M]

(iii) –CH-CH2-

[1M] -----------------Total : 10M

5. (a) From expt 1 and 2 to determine the order with respect to Cl2, ( 1.0 )m = 0.18 ; (½ )m = ½ ; 0.2 0.35 m ≈ 1 , the reaction is first order with respect to Cl2 From expt 2 and 3 to determine the order with respect to NO, ( 1.0 )m = 0.35 : (½ )m = ¼ ; 0.2 1.45 m ≈ 2 the reaction is second with respect to NO Rate = k[NO][Cl2]2 From expt 1,

[1M]

[1M]

[1M]

0.18 = k(0.10)(0.10)2 k = 180 mol-2dm6

[1M]

(b) ln[sucrose] = -kt + ln[sucrose]0 (1) ln[0.5] = - k (3.33) + ln[1] (2) ln[sucrose] = - k (9) + ln[1]

[1M]

(2) ÷ (1) ln[sucrose] -k(3.33) -------------- = ------------ln[0.5] -k(9) [sucrose] = 0.153

[1M]

Fraction remaining = 0.153/1 =0.153

[1M]

(c) Q = It = 2.00 x 30 x 60 =3 600 C Anode – oxygen gas 4OH- ----- > 2H2O + O2 1 mol O2 requires 4F

[1M]

3 600 C =

3600 mol of O2 = 0.00933 mol 4 x 96500 Volume of O2 produced = 0.00933 x 24 = 0.224 dm3 3

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[1M] [1M]

M+ + X -

(d) MX

Ksp = [M+][X- ] = 4.0 x 10-18 The solubility of MX = [M+] = 2.0 x 10-9

[1M] [1M]

Q2+ + 2X-

QX2

Ksp = [Q2+][X- ]2 = 4.0 x 10-18 The solubility of MX = [M+] = 1.0 x 10-6

[1M] [1M]

QX2 is more soluble

6. (a)

P+(g)

+

[1M] --------------Total : 15M

Q-(g)

∆H° = +419 kJ P(g)

+

∆H° = +77 kJ P(s)

+

∆H° = -

kJ

Q(g) :

∆H° = +121 kJ ½Q2(s)

∆H° = -436 kJ

PQ(g) P (g) + Q (g) +

-

PQ(g) :

∆H° = lattice energy

Lattice energy = -436 – 77 – 121 – 419 - (-364) = - 689 kJ/mol

(b) The temperature in the car engine is very high. The high temperature cause the N2 and O2 in the air to react to form NO2. NO2 can be removed form exhoust gaswes by using catalytic converter which reduce NO2 to N2.

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[4M] [1M] [1M]

[1M] [1M] [1M]

(c) (i) Let n be the number of mol of gasseous molecules present. PV = nRT

(ii)

1.0 x 105 x 190 x 10-6 = n x 8.31 x 320

[1M]

n = 0.00714

[1M]

average mass = mass ÷ mol = 0.5 ÷ 0.00714 = 70.0

[1M]

PN2O4 Kp= ---------------PNO2

[1M]

0.6 Kp = --------------- = 3.75 atm-1 (0.4)2

[2M]

7. (a) the vapour pressure PA of a liquid in a mixture of a miscible liquids is given by the products of the vapour pressure of that pure component P°A and its mole fraction PA = XAP°A [2M] (b)

pressure/kPa

12.7

P(t) 3.9

P(benzene) P(methylbenzene)

0

1 Mol fraction of benzene

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[3M]

Partial pressure of benzene in the mixture = 2/5 x 12.7 = 5.08 kPa

[1M]

Partial pressure of benzene in the mixture = 3/5 x 3.9 = 2.34 k

[1M]

Total pressure = 7.42 kPa

[1M]

(c) A mixture of benzene and methylbenzene which forms an ideal solution, can be separated by fractional distillation. Benzene, which is more volatile, will be distilled over first, followed by methylbenzene. Temperature/°C

[1M]

Boiling point vapour

of benzene

Boiling point of methyl benzene liquid

0

1.0

[2M]

mol fraction of benzene

[1M] A mixture of benzene and methylbenzene with C1 composition is being heated to boil at T1 temperature. This will produce a vapour with C2 composition which has more benzene. The vapour will be cooled in the column and become liquid. [1M] A mixture of benzene and methylbenzene with C1 composition Reheating will vapourise the C2 liquid to form vapour at T2 temperature with C3 composition [1M] which even rich in benzene. Repeating processes of boiling, cooling and condensation will separate benzene [1M] as distillate and methylbenzene as residue ---------------Total: 10M 8. (a) Na2O and MgO are basic oxides. MgO is insoluble in water but it is soluble in dilute acid Na2O + H2O 2NaOH MgO + 2HCl MgCl2 + H2O Al2O3 is amphoteric. It dissolves in both dilute acids and alkalis 6

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[1M]

[1M] [1M]

Al2O3 + 6H+

+ 2OH + 3H2O

Al2O3

(b)

2Al3+

+ 3H2O

[1M]

-

2Al(OH)4

The other oxides like SiO2, P4O10, SO3 and Cl2O7 are all acidic They dissolve in water to produce acidic solutions. SO3 + H2O H2SO4

[1M]

HCl is stronger acid in aqueous solution The H-Cl bond is more polar than the H-S bond. Cl is more electronegative than S H-Cl ionises completely whereas H2S ionises only partially in aqueous solution. HCl + H2O H3O+ + Cl H2S + H2O H3O+ + HS

[1M] [1M] [1M]

(c)

+

Cl

H3N

[1M]

+

Cl

H3N

NH3

[1M]

NH3 [1M + 1M]

Cr

Cr

Cl NH3

NH3

H3N

Cis

C

O

C

O

O

O

3-

O

3-

O

C

C

C

O

O

O

O

O

O O C C O

Cr

Cr

C

[1M]

Trans O

O

NH3

Cl

O O C C O

O

C O

O O

O

[1M + 1M] --------------Total : 15M

C

Mirror 9. (a) Y Because molecules of Y form intermolecular hydrogen bonds whereas the Intermolecular forces between molecules of X are the weak van der Waals forces. (b) (i) Step 1 Reagent: Chloromethane in the presence of anhydrous aluminium chloride Equation :

H + CH3Cl

CH3 + HCl

Step 2 Reagent: Concentrated sulphuric acid and concentrated nitric acid Condition : 550C 7

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[1M] [1M] [1M]

[1M] [1M]

[1M]

Equation: CH3 + HNO3 H3C NO2 + H2O (ii) Step 1 Reagent: Add tin, concentrated HCl followed by NaOH (aq) Equation : O2N CH3 + 6[H] H2N CH3 + 2H2O Step 2 Reagent: Acidified potassium manganate (VII) solution and reflux Equation : H2N CH3 + 3[O] H2N COOH + H2O

(iii) Y will dissolve in hydrochloric acid whereas X will not. HOOC NH2 + HCl HOOC NH3Cl (c) Y +

H3 N

n

ONa + H2O

(ii) Methanoic acid contains aldehyde group in its structure. Hence just like aldehydes it can be oxidised by acidified potassium manganate (VII) solution to carbonic 1M acid which decomposes to carbon dioxide and water. H2CO3

CO2 + H2O

(iii) Methylamine is a base solution and will react with dilute HCl to produce A water-soluble ionic salt. CH3NH2 + HCl

[1M] [1M]

[1M] [1M]

[1M] --------------Total : 15 M

OH + NaOH

+ [O]

[1M] [1M]

[1M]

COO

10. (a) (i) The benzene ring has electron withdrawing effect, this makes the O-H group bond quite polar to donate proton. With NaOH (aq) phenol forms a stable salt of sodium phenoxide. On the other hand, 1-butanol is neutral. The butyl group is rather non-polar And this prevent its reaction with NaOH (aq).

H –C=O OH

[1M]

CH3NH3Cl

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[1M] [1M]

[1M]

[1M] [1M]

[1M]

[1M]

[1M]

(b)

Br(CH2)4Br

NaCN (alcohol), reflux

NC(CH2)4CN Dilute H2SO4

reflux

HOOC(CH2)4COOH

Br(CH2)4Br

NaCN (alcohol), reflux

NC(CH2)4CN

[1M] [1M]

[1M]

[1M]

LiAlH4, H2O/H+

[1M]

H2NCH2(CH2)4CH2NH2

[1M]

ether

n [H2NCH2(CH2)4CH2NH2] + n [HOOC(CH2)4COOH]

[1M] --------------Total : 15 M

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