Grade 12 PHYSICAL SCIENCES

Exam Paper 2 - Suggested Solution

SECTION A

QUESTION 1

1.1 B PP (2)

1.2 B PP (2)

1.3 A PP (2)

1.4 B PP (2)

1.5 A PP (2)

1.6 C PP (2)

1.7 B PP (2)

1.8 A PP (2)

1.9 B PP (2)

1.10 C PP (2) [20]

1

QUESTION 2

2.1 Six electrons P (1)

2.2 Cu(NO3)2 P (1)

2.3 Has no effect on P (1)

2.4 0,15 V P (1)

2.5 Sulphuric acid P (1) [5]

TOTAL FOR SECTION A - 25 marks

SECTION B

QUESTION 3

3.1 From magnesium to iron PP (2)

3.2 Mg P (1)

3.3 Mg (s) Mg2+ (aq) + 2e– PP (2)

3.4 It decreases. PMagnesium is oxidised to soluble magnesium salts in the wet soil. PP (3)

3.5 Fe2+ (aq) + 2e– Fe (s) PP (2) [10]

QUESTION 4

4.1 2H2O + 2e– H2 + 2OH – PP (2)

4.2 2Cl– Cl2 + 2e– PP (2)

4.3 E cell = E oxidising agent – E reducing agent P

= –0,83 – (1,36) PP

= –2,19 V P (4)

4.4 E cell = 2,19 V PP (2)

4.5 The reaction is not spontaneous. PP (2) [12]

2

QUESTION 5

5.1 Cu PP (2)

5.2 (a) Cu (s) Cu2+ (aq) + 2e–

Cu2+ ions move away from the electrode into the solution. PP (2)

(b) Ag+ (aq) + e– Ag (s)

Ag+ ions move towards the Ag electrode. PP (2)

5.3 E cell = E

oxidising agent – E reducing agent P

= 0,80 – (0,34) PP

= 0,46 V P (4)

5.4 Cu (s) Cu2+ (aq) + 2e–

2 moles e– obtained from 1 mole Cu

0,25 moles e– obtained from x moles Cu P

x = 0,25

= 0,125 mol P

n = mM P

= 0,125 63,5

= 7,94 g P (4) [14]

3

QUESTION 6

6.1 Aluminium at cathode; O2 at anode PP (2)

6.2 Cathode: 2Al3+ + 6e– 2Al PP

Anode: 3O2– O2 + 6e– PP (4)

6.3 In the solid phase, ions are held in a fixed position in the crystal lattice. PP (2)

6.4 Q = It P

= (5)(32 60) P

= 9 600 C P

= 0,1 F P

1 mol Al3+ requires 3 F

Mass of aluminium deposited = P

= 0,9 g P (6)

Alternative solution:

Q = It P

= (5)(32 60) P

= 9 600 C P

Number of e– = = 5,99 1022 e– P

6e– give 2 Al atoms.

5,99 1022 e– give 2 1022 Al atoms = 0,033 mol Al P

m = nM

= 0,033 27

= 0,89 g P (6) [14]

4

QUESTION 7

7.1 Any four of the following :

Job creation (P)

Local production of materials that can supply many secondary industries. (P)

Less duties on imported raw materials. (P)

Investment in the youth, not only via job creation, but stimulating local awareness in terms of skilled career opportunities. (P)

Foreign investment opportunities. (P) (4)

7.2 Benzene is an unsaturated compound due to the presence of double bonds. An ordinary alkene readily adds Br2, but benzene does not do so under similar conditions. Benzene has double bonds that resonate, giving extra stability. Benzene is thus not as reactive as anticipated. Benzene gas as a pollutant affects the respiratory system adversely by not readily reacting. PP (2)

7.3 The formation of a polymer requires a high temperature and high pressure in the presence of a catalyst. P These are not conditions easily achieved in the atmosphere with escaping gases. P (2)

7.4 Coal is a natural resource in South Africa and refineries like SASOL use the resource not only to make a wide range of fuels but also compounds used in other industries. P

However, the resource is not readily renewable. P

The fuels contribute significantly to greenhouse gas emissions, which are not good for the planet. P

Alternative, renewable energy resources like wind energy, solar energy and nuclear energy must be explored. P (4) [12]

5

QUESTION 8

8.1 Contact process PP (2)

8.2 2SO2 (g) + O2 (g) 2SO3 (g) PPP (3)

8.3 SO2 (g) + H2O (l) H2SO3 (aq) P

SO3 (g) + H2O (l) H2SO4 (aq) P

Both gases form acid when they react with water, i.e. acid rain forms. P

Damages and may kill crops, damages buildings. P (4)

8.4 Acid is corrosive. It eats into skin and other materials. PP (2)

8.5 Reaction between H2SO4 and water is exothermic. P

Heat given off can crack bowl and acid can fall on student, or acid can boil and spill out of bowl. P (2) [13]

6

QUESTION 9

9.1 P – Air P

Q – Methane gas P (2)

9.2 Haber process P (1)

9.3 N2 (g) + 3H2 (g) 2NH3 (g) PPP (3)

9.4 Exothermic PP (2)

9.5 Nitric acid PP (2)

9.6 As fertilisers. Good source of nitrogen, which stimulates growth. PP (2)

9.7 Water pollution caused by: P (any one of the following)

Excessive algae growth in water; (P)

Algae die and bacteria decompose them; (P)

Fish die due to lack of oxygen (P) (2)

9.8 Releases oxygen, a supporter of combustion, when heated PP (2)

9.9 NH4NO3 NH3NO2 + O2 PPP (3) [19]

7

QUESTION 10

10.1 High energy combustion/combustion releases huge amounts of energy/highly exothermic PP (2)

10.2 Graph of boiling points versus number of carbon atoms:

10.3 Boiling point increases with number of carbon atoms. PP (2)

10.4 Hydrogen bonds between alcohol molecules increase with increase in molecular size. PP (2)

10.5 Hydrogen bonds between alcohol molecules stronger than van der Waals forces between alkane molecules. PP (2)

10.6 Petrol has a low boiling point, vapourises easily / is volatile / flammable / easily combustible / vapours have a higher density than oxygen and when swallowed vapours can cause suffocation. PP (2)

8

10.7 Ethanol can be produced by fermentation of plant material e.g. maize and sugar cane – both renewable.

Alkanes are fossil fuels which are non-renewable PP (2)

QUESTION 11

11.1 When one metal is coated with another by electrolysis PP (2)

11.2 Silver electrode P (1)

11.3 AgNO3 or any solution of silver compound P (1)

11.4 Ag+ PP (2)

11.5 Ag+ + e– Ag PP (2)

11.6 Ag Ag+ + e– (2)

11.7 Ag + Ag+ Ag+ + Ag (2)

11.8 Electrolytic cell (2)

[14]

Total marks: 150

9