2011 Victoria School Sec 4 Prelim 2 Answer Scheme 2011 Paper 1

Paper 2 Section A A1a) Yes.

Carbon, C is oxidised to carbon dioxide, CO2. The oxidation state of carbon increases from 0 in C to +4 in CO2. Sulfur, S is reduced to potassium sulfide, K2S. The oxidation state of sulfur decreases from 0 in S to -2 in K2S. OR Potassium nitrate, KNO3 is reduced to nitrogen, N2. The oxidation state of nitrogen decreases from +5 in KNO3 to 0 in N2.

1b) The reaction produces a large volume of gas. The exothermic reaction

produces large amount of heat energy resulting in the sudden expansion of the hot gases causing the explosion.

1c)

1d)

+ 2- Key: electron of K electron of S 2

ΔH = -

2KNO3(s)+ S(s)

+3C(s)

K2S(s)+ N2(g) +3CO2(g)

Ea

K

S

Replace qn with energy

profile diagram

A2

(a) (i) CFCs production decreased from year 1980 to 1982 and increased from year 1982 to 1988.

CFCs production then decreased sharply from year 1988 to 1996 but the decreased was more gradual from year 1996 to 2006.

(ii) There was no definite correlation between the CFC production and the amount of high-level ozone in atmosphere because:

when CFCs production decreased sharply from year 1990 to 1996, the amount of high level ozone remained relatively constant / decreased very slightly over the same period of time.

when CFCs production decreased from year 1998 to 2002 but the amount of high level ozone increases slightly over the same period of time

(b) (i) 2O3 3O2

(ii) A higher pressure means that there will be an increase the number of ozone molecules per unit volume.

Hence frequency of effective collisions between the ozone molecules increases, leading to increased rate of reaction.

(c) The graph shows the mass and amount of carbon, fluorine and chlorine atoms in one mole of a CFC compound.

carbon fluorine chlorine

Ar 12.0 19.0 35.5

No. of moles 2.00 4.00 2.00

Mass of element in one mole of CFC / g

2.00 x 12.0 = 24.0

4.00 x 19.0 =76.0

2.00 x 35.5 = 71.0

Molecular formula is C2F4Cl2

A3 (a) & (b)

CH4(g) + H2O(g) CO(g) + 3H2(g) ΔH = +206 kJ/mol

(a) Change in concentration of hydrogen gas (b) Formation of carbon monoxide in the presence of catalyst

(c) 206 kJ of energy is absorbed for one mole of CH4 No. of moles of methane required for 51.5kJ of energy to be absorbed = 51.5/206 = 0.250 mol Mass of CH4 required = 0.250 X (12+4) = 0.250 x 16 = 4.00 g

A4

(a) (i) Positive electrode: 4OH-(aq) - 4e 2H2O(l) + O2(g)

(ii) Negative electrode: 2H+(aq) + 2e H2(g)

(b) Negative electrode: Pb2+(aq) + 2e Pb(s)

(c) At the copper anode, the copper atoms lose electrons to form copper (II) ions which dissolve into the electrolyte, resulting in the formation of a blue solution.

At the copper cathode, the copper(II) ions gain electrons and are discharged as copper atoms which coats onto the cathode. The copper(II) ions in the electrolyte decreases and the blue solution fades.

Ions present:

Pb2+ OH-

Ca2+ ?

H+

(d) (i)

The ionic equations for the process occurring at each electrode are: H2 (g) 2H

+ (aq) + 2e

O2 (g) + 4H+

(aq) + 4e 2H2O (l)

(ii) Electricity is generated in the fuel cell in hydrogen powered cars through the movement of electrons from the electrode(anode) where hydrogen gas is oxidised, through the external circuit to the electrode(cathode) where oxygen gas is reduced.

clockwise arrow (from hydrogen to oxygen)

A5

(a) B Forms a basic oxide

(b) A

It has the lowest density.

It is a gas at room temperature.

It has a low boiling point.

(c) C has a giant ionic structure consisting of positive and negative ions. In the molten and aqueous states, the ions are free to move / mobile to conduct electricity. E has a simple molecular structure consisting of simple neutral molecules. E does not contain mobile electrons and hence is not able to conduct electricity.

(d) D & F; D and F have the same boiling points.

(e) Test the electrical conductivity of the substance when it is in solid; and then in molten/aqueous; - Giant substance consisting of ions will conduct in molten/aqueous

only, but not in solid state. - Giant structure of consisting of atoms would conduct in solid state

(giant metallic structure or giant molecular structures (eg. graphite)).

A6

(a) (i) Ra + 2H2O Ra(OH)2 + H2

(ii)) Vigorous effervescence of colourless and odourless formed

Metal becomes smaller in size.

(iii) pH 10-14

(iv) Radium will react more vigorously than barium Radium is below barium in Group II of the Periodic Table. This means that an atom of radium has an additional electron shell than an atom of barium. Hence, the 2 valence electrons from a radium atom is lost more easily due to increase in shielding effect caused by the decrease in electrostatic forces of attraction between the positive nucleus and the valence electron.

(b) (i) Gas turns orange acidified aqueous potassium dichromate(VI) to green. Gas turns purple acidified aqueous potassium manganate(VII) to colourless. / Gas decolourises acidified aqueous potassium manganate(VII).

(ii) Some bromine molecules on surface of liquid will have sufficient energy to overcome the weak forces of attraction between molecules to exist as a gas. These gaseous bromine molecules with enough kinetic energy can diffuse and move quickly to fill up any available spaces between the air molecules in the room allowing one to smell the vapour.

(iii) Colourless aqueous potassium bromide turns brown / orange brown / reddish brown.

A7 (a) C6H12O6 2CO2 + 2C2H5OH

(b) No. of moles of C6H12O6 = 25 / [ (6(12) + 12 + 6(16)] = 25 /180 = 0.1389 mol No. of moles of C2H5OH = 2/1 x 0.1389 = 0.2778 mol Mass of C2H5OH = 0.2778 x [ 2(12) + 5(1) + 16+ 1] = 0.2778 x 46 = 12.8 g

(c)

(d) Ethanol produced will react with oxygen in the air and undergo bacterial oxidation to form ethanoic acid which cause the mixture to turn sour.

(e) Ester : ethyl butanoate

Section B

B8

(a) Limestone and Brine

(b) Sodium carbonate (Soda) and Calcium chloride

NaCl + NH3 + H2O + CO2 NaHCO3 + NH4Cl Which product in this reaction is the least soluble in water?

(c) NaHCO3 or sodium hydrogencarbonate

(d) 2NaHCO3 Na2CO3 + H2O + CO2

(e) Ammonium chloride is heated with calcium hydroxide to form calcium chloride and ammonia gas

(f) Add aqueous sodium carbonate to the sodium chloride solution contaminated with magnesium ions. The magnesium ions will react with the carbonate ions to form a precipitate, magnesium carbonate which can be removed by filtration.

(g) carbon dioxide/ammonia/ammonium chloride can be recycled/reused.

(h) SO2 + Na2CO3 Na2SO3 + CO2 Sulfur dioxide reacts with moist sodium carbonate to form sodium sulfite and carbon dioxide.

2Na2SO3 + O2 2Na2SO4 The sodium sulfite is further oxidised to form sodium sulfate before being removed.

2SO2 + O2 2SO3 If sulfur dioxide is released into the environment, it will react with oxygen in the air to form sulfur trioxide.

SO3 + H2O H2SO4 Sulfur trioxide dissolves in rain water to form acid rain. Acid rain which causes corrosion of buildings made of limestone or metallic structures or kill aquatic life.

B9

(a) For the first 12 minutes, the pH drops sharply from 7 to 5.1/5.2/5.3. - The pH increases initially as the sweet reacts with the water to

release hydrogen ions causing the concentration of hydrogen ions in the mouth to increase.

After 12 minutes, pH then rises gradually to 7.2/7.3; - Upon chewing, saliva is gradually produced to neutralise the acid /

remove the hydrogen ions from the sweet causing the pH to increase.

(b) (i) Calcium carbonate reacts with citric acid in lemon juice to give

carbon dioxide gas which causes the fizzing. (ii) This is to remove the excess lemon juice which will contaminate

the calcium citrate.

(iii) Heat the solution of citric acid until saturation point.

Allow the hot citric acid to cool for crystallisation of citric acid crystals to take place.

Filter mixture to collect the citric acid crystals as residue.

Rinse crystals with cold deionised water.

Dry the crystals between sheets of filter paper.

(c)

(c) (i) Malic acid is less soluble in the solvent than lactic acid.

(ii) Sample 2 is older. Sample 2 contains only lactic acid but sample 1 contains both malic and lactic acid. This means that all the malic acid in sample 2 has been converted to lactic acid but not in sample 1.

B10

(a) (i) Although aluminium is a reactive metal, however it is protected by a layer of non-porous aluminium oxide which will prevent it from undergoing any displacement reaction with any of the solutions.

(ii) The hydrochloric acid will react first the surface layer aluminium oxide to form soluble aluminium chloride salt and water. This will eventually expose the inner aluminium metal which will react quickly with the hydrochloric acid to give hydrogen gas seen as the vigorous bubbling.

(b) Copper, Iron, Nickel, Manganese, Aluminium

(c) X : Pinkish-brown solid formed. Bluish-green solution turns yellow. Y : Grey / Silver Grey solid formed. Green solution turns yellow. Z : Grey solid / silver grey solid formed. Green solution turns pink.

(d) Mn(s) + Cu2+(aq) Cu(s) + Mn2+(aq)

(e) Nickel can be extracted from its metal oxide ore by reduction by carbon/ coke / carbon monoxide OR Electrolysis of molten nickel(III) oxide.

(f) Yes Manganese being more reactive than iron, will lose electrons more readily than iron. This means that manganese will corrode in place of iron and protect iron from rusting.

B10 Fumaric acid

(a) Fumaric acid will decolourise red-brown / brown aqueous bromine rapidly in absence of UV light.

O O II II H – O – C C – O - H I I H – C C – H I I Br Br

(b) (i) HO2CCH=CHCO2H + 2NaOH NaO2CCH=CHCO2Na + 2H2O

(ii) No. of moles of NaOH = 18 x 10-3 x 0.200 = 0.00360 mol No. of moles of fumaric acid = 0.00360/2 = 0.00180 mol concentration of fumaric acid = 0.00180 / (60 x 10-3) = 0.0300 mol/dm3 (3. s.f.)

(c) Structural formula of the polymer made when fumaric acid reacts with ethane-1,2-diol, HO(CH2)2OH.

Ester linkage

(d) Structural formula of the repeating unit of fumaric acid

(e) (i) Polymers are not biodegradable /do not decompose naturally. This will lead of long term litter problem as more space needed for landfill to dispose the polymers. OR Burning polymers produces harmful gases such as carbon monoxide.

(ii) Difficulty in sorting the different types of polymer and hence not economical. It may be cheaper to produce new polymers than to recycle.