Stpm Che2 Ans (Sbh)

8/14/2019 Stpm Che2 Ans (Sbh)

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Suggested Answers For Chemistry STPM

Section A

1 (a) (i) nucleus has no (zero) influence over the electron. 1m

(ii) Energy required = 1310 / 6.0 x 1023

= 2.18 x 10-21

kJ

or 2.18 x 10-18 J

1m

(iii) Difference in energy (J per electron) between n = 2 and n = 4

= (1230 981) x 103

/ 6.0 x 1023

J per electron transition.

= 4.15 x 10-19

J per electron transition

E = hf

f= 4.15 x 10-19

/ 6.3 x 10-34

= 6.26 x 1014

s-1

1m

1m

(b) (i) P1:35

Cl+

ion and P3:37

Cl+

1m

(ii) Peak height P2 : Peak height P4

100 : 33 = 3 : 1

3 : 1 which conforms to the relative abundance of35Cl and 37Cl

1m

1m

(c) (i) At very high pressures, the volume occupied by the gas

molecules cannot be ignoredbecause the volume of the gas is small

and the molecules are closer together.

1m

(ii) At medium pressure :

-negative deviation caused by stronger intermolecular forces of

attraction since HCl is a bigger molecule than hydrogen.

- HCl is a polar molecule; hence stronger intermolecular forces

exist .

1m

1m

10m

2 (a) (i) 0 = 2(-127) + 2Hf(ClO2)- 2 (-25)

Hf(ClO2) = + 102 kJ mol-1

1m

1m

(ii) The enthalpy of formation of ClO2 is endothermic, hence ClO2 is

unstable compared to chlorine and oxygen.

1m

(b) (i) Fe3+

(aq) + e Fe3+

(aq); E

= +0.77 V

ClO2 (aq) + 4H+ (aq) + 5 Fe3+ Cl- (aq) + 5Fe2+ + 2H2O ;

E.m.f of the cell = 1.50 (+0.77 ) = + 0.73 V

Since e.m.f of the cell is positive, ClO2 oxidises Fe2+

to Fe3+

spontaneously.

1m

1m

(ii) S + 2H+

+ 2e H2S (aq); E

= +0.14 V

ClO2 + 4H2S 2Cl-(aq) + 5S + 4H2O + 2H

+

E.m.f of the cell = 1.50 (+0.14 ) = + 1.36V

Since e.m.f of the cell is positive, ClO2 oxidises H2S to sulphur

spontaneously.

1m

1m

(iii)


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* Correct arrangement of apparatus as well as functional; 1 mark

* correct materials and prperly labelled : 1 mark

2m

(c) 2ClO2 + 2OH-

ClO2-

+ ClO3-

+ H2O 1m

10m

3 (a) The minimum energy required to displace/remove one mole ofelectron from one mole of atom in the gaseous state 1m

(b) 1. atomic size increases

2. screening effect (shielding effect) increases

1m

1m

(c) -Valence electron configuration:

Mg: 3s2

; Al: 3s23p

1

- more energy is required to remove an electron from a fully occupied

s-orbital than from a singly occupiedp orbital.

1m

1m

(d) -the small boron ion will distort the electron cloud of its neighbouring

anions, giving a covalent characteristic to its compounds.

- the big Ba2+

ion will not show much polarization; thus barium

compounds are more ionic.

1m

1m

(e) (i)

1m

(ii) Mg3N2 + 6 H2O 2NHs(g) + 3Mg(OH)2 2m

10m

4 (a)

C H Br

No. of moles 35.2/12 = 2.9 6.5/1 = 6.5 58.3/80 = 0.73

Mole ratio 4 9 1

Molar mass : 136.9

Molecular formula : C4H9Br

1m

1m

(b) (i)

Mg2+ N

3-x

x

x

x


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CH3

CH3 - CH - CH2Br

1m

(ii) Nucleophilic substitution SN2 (primary halogen alkanes) 1m

(c) (i)

H H

CH3 - C = C- CH3 or CH3 CH = CHCH3

1m

(ii)

CH3

CH3 - C - CH3

Br

1m

(iii) Isomer: CH3 CH2 CH = CH2 or CH3- C = CH2

CH3

Equation:CH3 CH2 CH = CH2 + 5[O] CH3 CH2 COOH +CO2+ H2O

Or C(CH3)2=CH2 + 4[O] (CH3)2C=O + CO2 + H2O

1m

1m

(d) -Warm with aqueous acidified KMnO4.

-Alcohol P does not decolourise the purple solution.

-2-methyl-1- propanol decolourises the purple solution. (oxidation of

primary alcohols)

1m

1m

10m


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SECTION B

5 (a) (i) Relative atomic mass

= Mass of one atom of element X

1/12 x Mass of one atom of12

C

1mark(ii) Isotopes means atoms of an element having the same number of protons but

different number of neutrons.

2 marks

(b) Ethanol molecule: CH3CH2OH

During bombardment by high speed electrons, various bonds can be broken,

positive ions which are fragments are formed.

m/e Ion

15 CH3+

28 CH3CH+

31 CH2OH+

45 CH3CH2O+

46 CH3CH2OH+

5marks

(c) (i) Water molecule undergoes autoionisation.

H2O H+

+ OH-H = Endothermic

Ionic product of water, Kw = [H3O+

] [OH-]

= 1 x 10-14

mol2dm

-6at 25

0C

When temperature increases, the position of equilibrium shifts to the right

forming more H+

and OH-. Hence the value of Kw increases.

4marks

(ii) Ca2+

(aq) + 2e Ca(s) E

= -2.87V

The standard electrode potential of calcium is the potential difference obtained

when the half cell of Ca/Ca2+

(aq) is connected to a standard hydrogen electrode

under standard conditions.

3marks

6 (a) (i)

KNO3(s) + aq K+

(aq) + NO3-(aq)

The forward reaction of dissolution of KNO3 is an endothermic

process as the temperature drops when it dissolves.

The backward reaction of crystallization is an exothermic process.

On cooling a saturated solution, heat is given off/ removed from the

system. By Le Chateliers Principle, processes occur in the system tocounteract the lost of heat by favouring the reaction that gives out heat.

Since the crystallization is an exothermic process, it is favoured and

crystals formed.

1m

1m

(2)

(ii) Ca2+

(aq) + SO42-

(aq) CaSO4(s)

When concentrated sodium sulphate solution is added to a saturated

solution of calcium sulphate, concentration of sulphate ions increases.

By Le Chateliers Principle, processes occur in the system to

1m


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counteract this increase of sulphate ions by favouring the process that

decreases the concentration of sulphate ions. The equilibrium shifts to

the right to favour the precipitation reaction, a white precipitate of

calcium sulphate forms.

1m

1m

(3)

(iii) 2NO2 (g) N2O4(g) H ve

(brown) (colourless)Since cooling favours the exothermic reaction which gives out heat,

the forward reaction is exothermic.

On compressing the gaseous mixture, total pressure in the system

increases; By Le Chateliers Principle, the forward reaction that

produces less molecules of gas is favoured, the colour fades as the

brown NO2 gas forms the colourless dimer.

On warming, heat is supplied to the system. According to Le

Chatelier, the process that absorbs heat is favoured. Since backward

process is endothermic, it is favourd. The brown colour is restored as

colourless N2O4 dissociates to form brown NO2.

1m

1m

1m

(3)

(b) C(s) + H2O(g) H2(g) + CO (g)

Initial partial press./atm 1.0 0 0

Equilm

partial press./atm 0.1 0.9 0.9

Change in partial pressure of steam = 1.0- 0.1 = 0.9 atm.

Since 1 mol of steam produces 1 mol of H2 and 1 mol of CO, partial pressure

of H2 = partial pressure of CO = partial pressure of steam used = 0.9 atm

Kp = 0.9 x 0.9 / 0.1 = 8.1

1m

1m

1m

(3)

(c) Ksp = [Mg2+

] [ OH-]2

At pH 9,

pOH = 14-9 = 5 and [OH-] = 10

-5= 1 x 10

-5mol dm

-3

Max. value of [ Mg2+

] = Ksp / [OH-]

2= 1.8 x 10

-12/ (1 x 10

-5)2

= 1.8 x 10-2 mol dm-3

At pH 6.5,

pOH = 14-6.5 = 7.5; [OH-] = 10-7.5 = 3.16 x 10-8 mol dm-3

Max value of [ Mg2+

] = Ksp / [OH-]2

= 1.8 x 10-12

/ ( 3.16 x 10-8

)2

= 1803 mol dm-3

Peat based soil is preferred.

1m

1m

1m

1m(4)

(15m)


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7. (a) Silver bromide is used in black and white photography. The surface of

the paper is spread with a layer of silver bromide. When exposed to

sunlight, the bromide ion will be oxidized to bromine gas, leaving a layer

of silver on the surface of the paper.

5m

(b) (i) [Co(NH3)3(Cl)3]

(ii)

NH3

Cl NH3

Cl NH3

Cl

fac-triamminetrichlorocobalt(III)

NH3

Cl NH3

Cl Cl

NH3

mer-triamminetrichlorocobalt(III)

1m

2m

2m

(c) (i) Observation: no visible change

Explanation: There is no reaction between the solution and silver

nitrate because there is no chloride ion present.

(ii) Observation: White precipitate forms when silver nitrate is added

Explanation: Ag+

will react with the chloride ion from the complex

to form AgCl solid which appears as a white precipitate.

2m

3m

(15m)

9 (a) HCl or H2SO4or H+

oracid

conc(if HCl only)/dilute/aqueous + heat

1m

1m

(b) two rings only (1 ring around the -C of tyrosine & 1 around the -C of lysine) 1m

(c)+

NH3CH2CO2- (or displayed formula) 1m(d) (i) NH2CH2CO

2-(Na

+) (either -CO

2-Na

+or-CO2Na but NOT CO-O-Na)

(ii) (Na+) O-C6H4-CH2CH(NH2)CO

2-(Na

+)

(iii) (Cl-)

+NH3(CH2)4CH(NH3

+)CO2H (Cl

-)

(iv) HO-C6H2Br2-CH2CH(NH2)CO2H (if shown, Br at 2,6 to OH group)

1m

1m+1m

1m+1m

1m

(6m )

Co

Co


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(e)

2m

(f)

3m

15m

10 (a) (i) AlCl3/FeCl3/Al/Fe/I2 (+ heat) (N.B. NOT AlBr3 etc.)

(or names)

1m

(ii) (sun)light/hf/UV 1m

(b) SOCl2/PCl3/PCl5

(or names)

1m

(c) (i) C > B > A 1m

(ii) (acyl chloride fastest) highly

+ carbon atom joined to 2electronegative atoms

or

addition-elimination mechanism is possible

(aryl chloride slowest) delocalisation of lone pair over ring stronger C-

Cl bondOr impossibility of backside attack on the C-Cl bond

1m

1m

(i) C6H5-CO2C6H5 1m


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(d) (ii) C6H5-CONHCH3(iii) C6H5-CONHCH3

Or structural formula

1m

1m

(3m)

(e) (i) E 1m

- CH3CH2CH2CO2-

(Na+) [NOT C3H7COO

-Na or C3H7COOH]

but allow CH3CH2CH2CO2Na]- CHl3 or name

1m

1m

(ii) the alcohol from E has four different groups around a carbon atom

it is chiral/asymmetric or it is produced as a 50:50 mixture of mirror

images

or its mirror images are non-superimposable.

1m

1m

1m

(15m )

END OF MARKING SCHEME

Stpm Che2 Ans (Sbh)

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