GCE O Level Chemistry SPA

1

2

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A

B

C

B

B

B

A

D

D

D

11

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C

A

C

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A

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A

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D

21

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C

C

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B

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B

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C

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A

Question

Explanations

4

Question involving ink does not necessary have to be related to chromatography. The question wants the separation of pure water from ink and thus, the answer would be distillation.

6

The atom and ion of sodium do not contain the same number of electrons. Thus, we would need to look at the number of neutrons which is not affected by the gain or loss of electrons.

9

Read the question carefully. This question is asking for the total number of electrons in the covalent bonds surrounding the S atom. There are six covalent bonds (double bond = two covalent bonds) surrounding the S atom and so there are (6x2=12) electrons.

12

The electrodes have been changed to copper and so, instead of discharging OH^- ions at the anode, the copper metal of the anode is oxidized.

15

The question concerned the rates of reaction only and not how pressure affects rates and position of equilibrium. Therefore, increasing the pressure of the system would increase the speed of both forward and backward reaction.

33

Calcium sulphate was the only compound which was not a base and therefore did not change the pH of an acidic soil.

38

Addition reactions usually produce one product and option A was the only reaction that has more than one product.

Qn

Answers

A1

(a)  potassium

(b)  lead (II) bromide/ zinc oxide

(c)  zinc oxide

(d)  lead (II) bromide

(e)  carbon

(f)  oxygen

(g)  hydrogen

A2

(a)  80^oC

(b)  It remains constant until all the substance has melted.

(c)  It is because the melting point of the substance is 80^oC and boiling point is 215^oC. [1] The melting point of water is 0^oC and the boiling point of water is 100^oC. [1]

       {Common mistake: never compare the melting or boiling point of water and the substance. Need to discuss both water and substance to earn the full marks}     

A3

(ai)  It refers to a reversible reaction.

(aii) {not in syllabus} Removal of carbon dioxide causes the equilibrium to shift to the right thereby producing more calcium oxide.

(bi)  CaO (s) +  H₂O (l) ® Ca(OH)₂ {slaked lime is calcium hydroxide}

(bii) It is used to neutralize acidity in soil.

       {Rejected answers: testing for carbon dioxide, making chalk, making paint, as a fertilizer and in the blast furnace}

A4

(ai)  Brown gas (bromine gas) moves up the other gas jar. After some time, both layers have a uniform brown colour.

       {Note: bromine gas is brown and not red. Must mention the colour of bromine to earn the mark}

(aii) Diffusion has taken place. The bromine molecules move from a region of higher concentration to a region of lower concentration.

(bi)  The rate of change would be faster.

(bii) This is because the molecular mass of nitrogen dioxide is smaller than bromine. Therefore, nitrogen dioxide diffuses at a faster rate.

A5

(a)  It is a process in which oxygen is added to the substance. OR A reaction in which there is an increase in oxidation state.

(bi)  Name: Propanol    Structure:    H  H H

                                                           |    |   |

                                                      H-C-C-C-O-H

                                                           |    |   |

                                                          H  H H

(bii) It is higher than the boiling point of ethanol. This is because as the molecules become bigger, the intermolecular forces are stronger. More energy is required to overcome the forces. Therefore, propanol would have a higher boiling point.

(ci)   ethyl ethanoate

(cii)  CH₃COOH + C₂H₅OH D CH₃COOC₂H₅ + H₂O

         {Common mistake: missed out water from the equation}

A6

(a)  gaseous state

(b) 

      {Common mistake: inner electrons drawn instead of showing only outer electrons}

(ci) H⁺

(cii) The solution turns reddish brown.

        {Common mistake: the brown solution was decolourised}

(ciii) Cl₂ + 2Br^- ® 2Cl^- + Br₂

A7

(a)  nitrogen is needed for the formation of protein for healthy plant growth.

(b)  The ammonium and nitrate ions will break free from each other in the lattice arrangement and move about freely and occupy spaces between the water molecules.

       {Common mistake: never explain in terms of particles}

(c)  molar mass of ammonium nitrate = 14 + (1x4) + 14 + (16x3) = 80g

      % of nitrogen in NH₄NO₃ = (28/80) x 100% = 35%

      % of nitrogen in urea = 47% (given)

      Therefore, 1 kg of urea contains more nitrogen than 1 kg of ammonium nitrate.

      {Common mistake: never compare the % of nitrogen of both compounds}

A8

(ai) Limestone {Common mistake: haematite (iron ore is given in the question!)}

(aii) To produce calcium oxide which is then used to remove acidic impurities

       {Note: need to mention the name of the impurities (sand or silicon dioxide) removed or state acidic impurities to earn the mark. Common mistake: vague answers such as ‘to remove impurities’ or ‘to produce slag’}

(bi) carbon monoxide

(bii) CO₂ + C ® 2CO

(c)  An alloy is a mixture of two or more metals.

      {Common mistake: never mention ‘mixture’ in the answer}

(d)  mild steel: it will rust; aluminium: it is very light

      {Common mistake: cars blowing away because aluminium was so light!}

Section B

B9

(ai)  catalytic cracking

(aii) It acts as catalyst.

       {common mistakes: for decomposition/ to distribute heat/ to make a bigger surface are}

(bi) C₃H₈ ® C₂H₄ + CH₄; Bubble the gas into aqueous bromine. The aqueous bromine will decolourise.

       {Common mistake: just mention ‘bromine’ and not ‘aqueous bromine/ bromine water’}

(bii) 1 volume of C₃H₈ produces 1 volume of C₂H₄.

        100 cm³ of C₃H₈ produce 100cm³ of C₂H₄.

        {Common mistake: using M_r values of propane and ethene to give 100 x 28/44= 66cm³}

(ci)  Polypropene

(cii) Addition polymerization

                 H  H              H  H                    H H      H H

                  |    |                |    |                       |   |       |   |

     … +     C=C       +    C=C + … ®  …-C-C ¾ C-C-…

                  |    |                |    |                       |   |       |   |

                 H  CH₃              H  CH₃                 H CH₃ H CH₃

         {Common mistake: drawing poly(ethene) instead of poly(propene)}

(ciii) They cannot be decomposed by bacteria in the soil.

         {Common mistake: Never mention ‘bacteria’ in the answer which is the ‘bio’ part of the term ‘biodegradable’}

B10

(ai)  C₄H₁₀ + 13/2 O₂ ® 4CO₂ + 5H₂O

(aii)  no. of moles of butane = volume/molar volume = 16/24 = 2/3

        1 mole of butane gives off 2880kJ of heat.

        2/3 mole of butane gives (2/3 x 2880) = 1920 kJ of heat

(aiii) The value of ∆H is negative showing that the reactants have more energy than the products.

(bi) {not in syllabus} Silver metal is formed.

(bii) The silver ion is reduced to silver. Ag⁺ + e ® Ag

(biii) used to coat photographic film

         {note: must mention the term ‘film’ to earn the mark}

B11

(a)  Cryolite acts as a solvent to dissolve the aluminium oxide. It lowers the melting point of aluminium oxide so that less energy is needed. It melts the aluminium oxide and maintains the electrolyte in a molten state.

(b)  Cathode: Al³⁺(l) + 3e ® Al (l)

       Anode: 2O^2-(l) – 4e ® 2O₂ (g)

(c)  The oxygen gas given off at the anodes reacts with carbon anode to form carbon dioxide.

(di) 

       The impure copper is used as the anode and pure copper, the cathode.

      {Note: must mention the electrolyte as aqueous copper (II) sulphate and not just copper (II) sulphate.}

(dii) At the anode: The anode dissolves in solution. Cu ® Cu²⁺ + 2e The copper (II) ions move to the cathode.

        At the cathode. The Cu²⁺ ions are discharged and pure copper is formed on the cathode. Cu²⁺ + 2e ® Cu

B12

(a)   fractional distillation; difference in boiling point

       {Common mistake: never mention ‘difference’ in the boiling points of the gases}

(bi)  vanadium (V) oxide

        {Common mistake: omission of oxidation state of vanadium}

(bii)  2 SO₂ + O₂ ® 2 SO₃

         no. of moles of SO₂ = 3200/64 = 50

         no. of moles of SO₃ = 50

         mass of SO₃ = 50 x (32+16x3)g

                              = 4000g = 4kg

(biii) Sulphur trioxide is dissolved in concentrated sulphuric acid to form oleum.

       H₂SO₄ + H₂O ® H₂S₂O₇

       Oleum is then converted to sulphuric acid by adding water to it.

       H₂S₂O₇ + H₂O ® 2H₂SO₄

{Common mistake: omission of ‘concentrated’ for the sulphuric acid in the first step}

(ci)  Sulphur dioxide dissolves in water to form acid rain. Acid rain will corrode limestone buildings and kill fishes in lakes. Sulphur dioxide will also cause breathing difficulties.

(cii)   Neutralisation reaction {Note: SO₂ is an acidic oxide}