Chemistry p2 Mark Scheme

8/3/2019 Chemistry p2 Mark Scheme

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PAHANG TRIAL 2009 EXAMINATION

CHEMISTRY PAPER 2 MARKING SCHEMES

SECTION A - Structural Questions:

Question 1.

(a) (i) The presence of isotopes 1M

(ii) Let the abundance of 63X be a %.The % abundance of 65X. = ( 100 a ) 1M

Relative atomic mass = ( 62.93 x a) + ( 64.93 x ( 100 -a) ) 1M

100

63.55 = 62.93a + 6493 -64.93a100

6355 = -2a + 6500a = 69.0% 1M

The % abundance of 65X = 100- 69.0

= 31.0 %

Relative abundance 63X : 65X1 : 2 1M

(iii)

RelativeAbundance

63 64 65 Relative mass /m/e2M


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Species protons neutrons Electrons

20 Ne

10

10 10

10

16 O2-

8

8 8 10

2 M

The species have same number of electrons or isoelectronic. 1M----------------

10M

2. (a) (i) H2O2 + 2H+ + 2 I- 2H2O + I2 1M

(ii) Rate = k [H2O2] [I-] 1M(iii) 0.2 1M

0.1 1M

(iv) second order 1M

(b) (i) 12 1M

(ii) 1s2 2s2 2p6 3s2 . 1M

(iii) +2 , X has two valence electrons 2M(iv) X is a better electricity conductor. 1M

----------------

10M

3.(a) Atomic size increases, screening effect increases with more inner shells of

electrons 1M

effective nuclear charge decreases, ionisation energy lowered, valence

electrons are more easily removed. 1M

(b) i. Be2+ (aq) + 4H2O (l) [ Be (H2O)4 ]2+ (aq) 1M

ii. It is acidic, acting as a Bronsted-Lowry acid 1M

The Be2+ ion has a high charge density 1M

and can strongly polarise large anions due to its smaller size. 1M

The ions of other Group 2 elements have larger sizes and charge densities andweaker polarising power


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(d) i. platinum and rhodium 1M

ii. 4NH3(g) + 5O2(g) 4NO(g) + 6H2O (g) 1M

iii. low temperature 1M

low pressure 1M(Note : The reaction is exothermic reaction. According to le Chatelier

principle, a low temperature will favour the formation of NO. For gaseous

equilibrium, a decrease in pressure will favour the reaction which producesmore gaseous molecules. Thus in the above equilibrium a low pressure will

favour the formation of NO.)

________

10M

4.(a) i. A is CH3CH2CH2COOH 1MB is CH3CH2CH2COCl 1M

C is CH3CH2CH2COOCH2CH 1M

ii. butanoyl chloride 1M

iii. Formation of ester:

CH3CH2CH2COCl + CH3CH2OH CH3CH2CH2COOCH2CH3 + HCl 1M

(b) i. H3N

+CH2COOH + H2NCH(CH3)COOH

H2NCH2CONHCH(CH3)COOH + H2O 1M

Glycylalanine 1M

( Note: Alanylglycine can also be formed )

ii. The amino group NH2 which is basic group reacts with hydrochloricacid to form the ammonium chloride salt of alanine 1M

HOOCCH(CH3)NH2 + HCl HOOCCH(CH3)NH3+Cl- 1M

___________

10 M


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SECTION B - ESSAY

5.(a) (i) Orbitals with the same energy 1M

Example : 2p or 3d s orbitals 1M

(ii) Nitrogen atom has 7 electrons 1M

Fill 1s orbital with 2 electrons 1MFill 2s orbital with 2 electrons 1M

Fill 2px,2py and 2pz orbitals with 3 electrons 1M / 6

1M

(b) Fe 2+ 1s2 2s2 2p6 3s2 3p6 3d4 1M

Fe 3+ 1s2 2s2 2p6 3s2 3p6 3d5 1M

In terms of electronic configuration, Fe 3+ is more stable than Fe 2+ 1M

Because it has half-filled 3d orbital which is more stable 1M./ 4

(c ) The valence electronic configuration of the electrons for nitrogen atom

is 2s2 2px1 2py1 2pz1 1MNitrogen atom uses sp3 hybrid orbitals for forming covalent bonds

between N and H atoms.

Energy

2p

sp3 hybrid porbitals

N(ground state) 1M

In sp3 hybrid orbitals of nitrogen atom,one of the orbitalsIs occupied by a lone pair of electrons and three sp3 orbitals

are half filled 1M

Each N-H atom is formed by the overlapping of the s orbital

of hydrogen atom with one of the half filled sp3 orbitals

to give the ammonia molecule 1M

Diagram of the bond formation in NH3 molecule. 1M /.5

-------------------Total : 15 M


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6. (a) Dynamic equilibrium ....

a reversible reaction , in a closed system

forward and backward reactions have the same rate of reaction. 2M(b) (i) N2O4 2NO2

Kc = [NO2]2

= [0.12]2

[N2O4] [0.04]

= 0.36 mol dm-3 5M

(ii) Using PV =nRT where n = 0.12 +0.04 = 0.16 mol

P = 0.16 (8.31) (383)

10 -3

= 509.24 kPa .3M

(c) N2(g) + 3H2(g) 2NH3(g)

- at low temperatures, % NH3 is higher

- forward reaction is exothermic

- equilibrium position shifts to the right at higher temperature

- forward reaction is accompanied by a reduction in volume of gas

- at higher pressures, equilibrium position shifts to the right

- at high pressures, % NH3 is higher 5M---------------

Total : 15MNo. 7

(a) (i) Aluminium metal is extracted by electrolysis

The electrolyte is molten bauxite in sodium hexafluoroaluminate.The electroyte has aluminium ion and oxide ions.

Anode : 2O2- --- > O2 + 4e

Cathode : Al3+ + 3e --- > Al 5M

(ii) (Any 2 points)light

Resistant to corrosion

Strong alloy 2M

`(b) aluminium :

A giant metallic structure, strong metallic bonf.

Silicon : giant 3 D covalent structure.

Strong covalent bond between silicon atomes.

higher melting point

Phosphorus and sulphur - Both are simple molecules.

Weak van der waals between molecules

Sulphur has a stronger intermolecular forces

S8 larger than P4 8M


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No 8. (a) chlorine strong oxidation agent

Bromide is oxidized to bromine

E of chlorine is more positive than that of bromine.Cl2 + 2Br

- ---- > 2Cl- + Br2 4M

(b) iodine forms triodes complex in KI.

I2 + I- ---- > I3

-

Iodine does not form any complex ions in water.

I2 + 2H2O --- > I- + HIO + H3O

+ 4M

(c) HCl is released in cold acid

NaCl + H2SO4 NaHSO4 + HCl

If heated more HCl released.

NaHSO4 + NaCl - Na2SO4 + HCl 4M

(d) Iodide is oxidized to iodine

Purple Iodine is releasedPungent smell of H2S is detected 3M

--------------

Total : 15M

9.( a ) ( i ) order : W, Y, X

W, Y, X act as Lewis bases.

X is the strongest base because ethyl group is an electron donor byinductive effect.

Y is more basic than W because the lone pair electron on the N atom

is not delocalised.

W is less basic than Y because the lone pair electron on the N atomis delocalised into the benzene ring. 5M

( ii ) pKb value > 9.39

Z is a weaker base than W.

Presence of Cl - an electron withdrawing group reduces the donating

potential of lone pair electron on the N atom through inductive effect. 4M

( b ) Concentated H2SO4 and HNO3., 55C

Mechanism: HNO3 + H2SO4 NO2+ + HSO4

+ H2O

NO2+ is an electrophile.

H+ NO2+ NO2

H

NO2 + HSO4 NO2 + H2SO4

+ HNO3 NO2 + H2O 6M


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Total : 15 marks

10.( a ) ( i ) Terylene/Dacron

~~~~O - CH2 - CH2 - O - C C - O - CH2 - CH2 - O - C C~~~~ 3M|| || || ||

O O O O

( ii ) Condensation polymerisation

To make cloth/sleeping bags, etc 2M

( b ) ( i ) K: functional group : -OH

isomers : CH3CH2CH2OH and CH3CHCH3OH

warm isomers separately with alkaline iodine,

CH3CHCH3OH gives a yellow precipitate but CH3CH2CH2OH does not.

CH3CH2CH2OH + 4I2 + 6OH

CHI3 + 5I + 5H2O + CH3COO

5M

(ii ) L : functional group : C = O

|

Isomers : CH3CH2CHO and CH3COCH3

warm isomers separately with Tollens reagent.

CH3CH2CHO gives a silver mirror but CH3COCH3 does not.

CH3CH2CHO + 2[Ag(NH3)2]2+ + OH CH3CH2COO

+ 2Ag + 2NH4+ + 2NH3

5M

Note: Can also accept other suitable chemical test.

Total : 15 marks

Chemistry p2 Mark Scheme

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