CONFIDENTIAL* 962/2 1

CHEMISTRY MARKING SCHEME

STPM TRIAL 2012

1 (a) (i) N : 1s2 2s

2 2p

3 [1]

(ii)

NCl3 PCl5

NCl3 : Polar molecule PCl5: non-polar molecule

[1]+[1]

[1]+[1]

(iii) Overlapping orbital of N2H4

Label the hybrid orbital

Show overlapping

Label sigma(σ) bond

[1]

[1]

[1]

(b) (i)

[1]+[1]

TOTAL MARKS

10

Increasing frequency

N N

H

H

H

H

σ

σ

σ

σ

P

• x

• x

• x

• x

• x

Cl

Cl

Cl

Cl

Cl

• •

• • • •

• •

• •

• •

• • •

•

• •

• •

• •

• •

• •

• •

• •

P

• x

•x

• x

x x

• •

•• ••

••

••

••

• •

• •

• •

N Cl

Cl

Cl

CONFIDENTIAL* 962/2 2

2 (a) (i)

[1]

[1]

(ii) Direction of electron flow from Cu(s) to Ag(s) (must be in/close to wire)

[1]

(iii) Cu(s)Cu2+

(aq)// Ag+(aq)Ag(s)

[1]

(iv) Cu(s) + 2Ag+(aq) Cu

2+(aq) + 2Ag(s)

[1]

(v) Eocell = (– 0.34) + ( +0.80) or

= + 0.46 V

[1]

(vi) Nernst Equation

2

2

][

][log

2

059.0

Ag

CuEE o

cellcell

or

V40.0

10.0

00.1log

2

059.0)46.0(

2

[1]

[1]

(c) Standard electrode potential for chlorine is more positive than Fe3+

therefore it is a better oxidising agent than Fe3+

(do not accept Eo is

larger or smaller)

Standard electrode potential for iodine is less positive than Fe3+

therefore it is a poorer oxidising agent than Fe3+

.

(Accept release of electrons/equilibrium arguments)

Or calculation

[1]

[1]

TOTAL MARKS 10

CONFIDENTIAL* 962/2 3

3 (a) (i) An ion which formed when a metallic ion is bonded to a group of ligand

by dative bonds

[1]

[1]

(ii) Ligand that can donate more than one lone pair electrons

[1]

(b) (i)

[Cu(H2O)6]2+

; [Cu(H2O)4(OH)2] ; [Cu(NH3)4]2+

3 corrects

2 corrects

[2]

[1]

(ii) H2O < OH- < NH3

A stronger ligand is able to displace a weaker ligand from the complex

ion

[1]

[1]

(iii) Transition metal ions have incomplete 3d-subshell / d1 to d

9

d-d electronic transition is possible

non transition metal / other metal have either completely filled 3d orbitals or

no 3d electron or no d-d electronic transition

[1]

[1]

[1]

TOTAL MARKS 10

CONFIDENTIAL* 962/2 4

4 (a) (i) Carboxyl, amino, ester and amide [4]

(ii) Amine and carboxylic acid.

This amino acid can exist as zwitterions

Attraction between this ion with polar water molecules is responsible for

it solubility

[1]

[1]

(b) (i) Heat with dilute sulphuric acid / H2SO4 / H+ [1]

(ii)

[1]

+ _

H3N – CH –COO

|

CH2

|

[1]

[1]

TOTAL MARKS 10

OH

|

H - C - H

|

H

+ _

H3N – CH – COO | CH2 | COOH

CONFIDENTIAL* 962/2 5

P/atm

T/°C

Liquid

54 Critical point

0.175 Triple point

-169 -63

5 (a) (i)

Correct x-axis , y-axis and label Correct curve All point label All phases label

[1]

[1]

[1]

[1]

(ii) Conditions of temperature and pressure under which a substance can exist in the solid, liquid and gas in equilibrium

[1]

(iii) Liquid

Gas

[1]

[1]

(iii) Cannot Because room temperature is above critical temperature

[1]

[1]

(b) (i) Positive deviation

Because hydrogen bonds /attractive forces between ethanol and water

molecules are weaker than those between the pure molecules

Hence the ethanol and water molecules easily escape from this mixture.

or

total vapour pressure is higher than predicted

[1]

[1]

[1]

(ii) Fractional distillation.

Distillate is an azeotropic mixture / 96% ethanol.

Pure ethanol can be obtained by adding anhydrous calcium oxide to

remove water.(any other drying agent)

[1]

[1]

[1]

15

Solid

Gas

CONFIDENTIAL* 962/2 6

6 (a) (i) The standard enthalpy of neutralization, ΔHneut is the enthalpy

change when one mole of hydrogen ions, H+(aq), reacts with one

mole of hydroxide ions, OH-(aq),

to form one mole of water under standard condition.

[1]

[1]

(ii) KOH(aq) + HCl(aq) KCl(s) + H2O(l) [1]

(iii) Molar mass of KOH = 39.1 + 16.0 + 1.0 = 56.1

Amount of KOH used =

= 0.0179 mol

Or = 1.79 x 10-2

mol

Heat evolved = mcѲ

Or = 50.0 x 4.0 x (40.0 -30.0)

= 2000 J or

= 2.00 kJ

ΔH = -

Or = -112 kJ mol-1

[1]

[1]

[1]

(iv) Polystyrene cup is a good heat insulator and has a low heat capacity.

[1]

(v) Stirring speeds up dissolving and so minimize heat loss during the

dissolving process by reducing the duration of the experiment.

Or

It also provides even distribution of temperature.

[1]

(b) (i) The standard enthalpy of combustion is the heat released when one

mole of octane is completely burned in excess oxygen under

standard condition.

[1]

(ii) 2C8H18(l) + 25O2(g) 16CO2(g) + 18H2O [1]

(iii) 2C8H18(l) + 25O2(g) 16CO2(g) + 18H2O

ΔHf : 2(-250) 0 16(-394) 18(-286)

or ΔH = [18ΔHf(H2O) + 16ΔHf(CO2)] – [2ΔHf(C8H18) +

25ΔHf(O2)]

= [18(-286) + 16(-394)] – [2(-250) + 25(0)]

= -10952 kJ mol-1

= -1.09 x 104 kJ mol

-1

[1]

[1]

[1]

(iv) When storing chemical substances with high enthalpy of combustion,

they must be protected from sunlight or sources of heat.

Or to present explosion

[1]

[1]

TOTAL MARKS 15

CONFIDENTIAL* 962/2 7

7

(a) (i) Aluminium is a good conductor.

Silicon and phosphorus are non-conductors.

Aluminium has metallic bond with delocalized electrons.

Silicon & phosphorus have localized electrons /do not have

delocalized electrons.

[1]

[1]

[1]

[1]

(ii) Dative bond

Electron pair/lone pair donated from Cl- to Al/AlCl3

Diagram

[1]

[1]

[1]

(b) (i) Beryllium chloride is covalent.

Magnesium chloride is ionic.

The beryllium atom is much smaller than the magnesium atom,

more difficult for it to transfer electrons to the chlorine atoms,

instead it shares the electrons

[1]

[1]

[1]

(ii) Solubility of the group 2 sulphates decreases down the group from

beryllium to barium.

Going down the group,

lattice enthalpy decreases slowly (since the sulphate ion is large )

but hydration enthalpy shows a larger decrease

hence the enthalpy of solution becomes more positive.

[1]

[1]

[1]

(iii) BaSO4(s) Ba2+

(aq) + SO42-

(aq) (1)

Each mole of barium sulphate dissolves to give 1 mole of barium

ions and 1 mole of sulphate ions in solution.

[Ba2+

] = 1.05 x 10-5

mol dm-3

[SO42-

] = 1.05 x 10-5

mol dm-3

Ksp = [Ba2+

][SO42-

]

[1]

[1]

CONFIDENTIAL* 962/2 8

= (1.05 x 10

-5)(1.05 x 10

-5) or

= 1.10 x 10-10

mol2 dm

-6

Use:

contrast medium used for X-ray imaging of the intestines/

gastrointestinal tract OR

white pigment for paints OR

component of oil well drilling fluid

[1]

[1]

TOTAL MARKS 15

8 (a) (i) High pressure or 600 atm

Low temperature or 200oC

[1]

[1]

(ii) Iron filling [1]

(b) (i) Yield or NO decrease

No of gas molecules increase (9 to 10)

The equilibrium shift to the left / backward

[1]

[1]

[1]

(ii) yield is reduced/ decrease

Forward reaction is exorthermic so

the equilibrium to the left

[1]

[1]

[1]

(iii) If temperature is reduced the yield is increased.

At lower temperature the rate of reaction will be very slow.

[1]

[1]

(c) X – Green, Ni(OH)2

Y – Blue, Ni(NH3)62+

[1+1]

[1+1]

TOTAL MARKS 15

CONFIDENTIAL* 962/2 9

9 (a) (i) CxHy + [ x + y/4]O2 → x CO2 + y/2 H2O

anhydrous CaCl2 absorbs water and KOH absorbs carbon dioxide-the

mass of water is 0.72 g and the mass of carbon dioxide is 1.76 g

Mass of C =

OR

= 0.48 g

The mass of C is 0.48 g.

Mass of H =

OR

= 0.08 g

The mass of H is 0.08 g.

Element C H

Mass 0.48 0.08

Number of moles

0.48

12.0

=0.04

0.08

1.0

=0.08

Mole ratio 1 2

Empirical formula is CH2

[1]

[1]

[1]

[1]

[1]

(ii)

The mass of hydrocarbon = mass of C + mass of H

= 0.48 + 0.08

= 0.56 g

Relative molecular mass = mass

mol

= 0.56 OR

0.01

= 56

(CH2)n = 56

or 14n = 56

or n = 4

Molecular formula is C4H8

[1]

[1]

[1]

[1]

CONFIDENTIAL* 962/2 10

(b)

(i)

(ii)

(iii)

(CH3)2C(OH)2 (CH3)2C=O

[1]

[1+1]

[1]

(c)

(i)

(ii)

(iii)

OH

|

CH3CCH2-

|

H

OH

|

CH3CCH2- --OH

|

H

O

║

+NaO

- CCH2- =O and CHI3

[1]

[1]

[1+1]

TOTAL MARKS

MAX

17

15

NH2

CH(OH)CH2CH3

OH

CONFIDENTIAL* 962/2 11

10 (a)

(i)

[1]

(ii)

(iii)

(iv)

(v)

(vi)

C= C or double bond

harder

more elastic

less sticky

[ any two ]

hard and brittle (both)

sulphur

Elastomer is polymer that stretches when pulled and return to their

length or shape when the force is released. /elastic

[1]

[1]+[1]

[1]

[1]

[1]

[1]

(b)

(i)

(ii)

(iii)

Rate = k [ C3H7-C(Br)(CH3)-C2H5 ]

constant or no change

because OH- ion does not involve in the rate of determining step

OR only reactant (3-bromo-3-methylhexane) affected by the rate of

hydrolysis

Nucleophilic subtituition

[1]

[1]

[1]

[1]

CH3 H

C = C

--- CH2 CH2----

CH CH2

S C = C CH3 H

CH CH2

C = C S CH3 H CH CH2

C = C CH3 H

CH2 CH

C = C CH3 H

CONFIDENTIAL* 962/2 12

(iv) Optical isomer

C3H7 C3H7

| |

C C

Br C2H5

CH3 C2H5 CH3 Br

[1]

[1]+[1]

TOTAL MARKS 15