CSEC Chemistry — Paper 2 Solutions

Electrolysis is the breakdown (decomposition) of an ionic compound (electrolyte) — when molten or in aqueous solution — by passing a direct electric current through it.

1 mark for "decomposition / breakdown of an ionic compound"; 1 mark for "by passing a direct current / using electrical energy".

Graphite (carbon) — cheap, conducts electricity, doesn't react with the electrolyte or the products. (Platinum is also accepted.)

The data: (10, 2.40), (20, 3.60), (30, 4.80), (40, 6.00), (50, 7.20), (60, 8.40).

Plot on Figure 2 (page 7): time / minutes on the x-axis; volume of O₂ / cm³ on the y-axis. The points lie on a perfect straight line that, when extended back, passes through the origin.

Marks: 1 for axes labelled with units; 1 for sensible scale; 2 for plotting all six points correctly; 1 for a single ruled line of best fit.

From the table, every 10 minutes liberates an extra 1.20 cm³ of O₂. So 1.20 cm³ takes 10 min ⇒ 5.50 cm³ takes 5.50 ÷ 1.20 × 10 ≈ 45.8 min. Check the graph confirms ≈ 46 min.

Time ≈ 46 minutes (accept 45–47 min from the line of best fit).

CuSO₄(aq) is dissolved in water, so we must include the ions from water too:

Cu²⁺(aq), SO₄²⁻(aq), H⁺(aq) and OH⁻(aq).

1 mark for the two CuSO₄ ions; 1 mark for remembering the H⁺ and OH⁻ from water.

From Figure 1, the + terminal of the battery is connected to electrode P and the − terminal to Q.

P = anode (+); Q = cathode (−).

The anode is positively charged and attracts negative ions (anions).

SO₄²⁻ (or OH⁻) drifts towards the anode.

At the cathode (Q), Cu²⁺ is preferentially discharged over H⁺ (lower in the electrochemical series):

Cu²⁺(aq) + 2e⁻ → Cu(s)

1 mark for correct species; 1 mark for correctly balanced (the 2e⁻).

I = 3.5 A. Time = 1 hour = 3 600 s.

Q = I × t = 3.5 × 3 600 (1 mark)

Q = 12 600 C (1 mark)

From the cathode half-equation: Cu²⁺ + 2e⁻ → Cu. So 1 mole of Cu requires 2 moles of electrons (= 2 × 96 500 = 193 000 C).

Moles of electrons = Q ÷ F = 12 600 ÷ 96 500 = 0.1306 mol e⁻. (1 mark)

Moles of Cu = ½ × moles of e⁻ = 0.1306 ÷ 2 (1 mark)

Moles of Cu deposited = 0.0653 mol (≈ 0.065 mol) (1 mark)

Mass = moles × RAM = 0.0653 × 64

Mass of Cu deposited = 4.18 g (≈ 4.2 g).

Some H⁺ ions were also discharged at the cathode (releasing H₂) but most current went to depositing Cu — yet the student's measurement may also include impurities or undeposited solution still clinging to the electrode.

Other accepted answers: the cathode was not fully dried before weighing (water added to the mass); current fluctuated above 3.5 A during the experiment, so more charge actually flowed than calculated; loss of solution by splashing was actually balanced by deposition continuing past the timer.

With inert electrodes, Cu²⁺ is steadily removed at the cathode (deposited as Cu metal) but no copper is replenished from the anode (only O₂(g) is liberated).

The blue colour of the CuSO₄(aq) gradually fades — the solution becomes paler and eventually colourless as the Cu²⁺ ions are used up.

• Anode: use the impure copper as the anode (it dissolves: Cu → Cu²⁺ + 2e⁻; impurities like Ag, Au fall off as anode sludge). (1 mark)
• Cathode: use a thin strip of pure copper as the cathode (Cu²⁺ + 2e⁻ → Cu deposits as 99.99% pure copper). (1 mark)

The CuSO₄(aq) electrolyte concentration stays roughly constant because Cu²⁺ is replaced as fast as it is removed.

An acid is a substance that produces hydrogen ions (H⁺) as the only positive ions when dissolved in water. (Brønsted-Lowry alternative: a proton donor.)

A salt is an ionic compound formed when the hydrogen ion (H⁺) of an acid is replaced by a metal ion (or by NH₄⁺).

1 mark for "ionic compound from an acid"; 1 mark for "H⁺ replaced by metal/ammonium ion".

Sodium bicarbonate solution is alkaline (basic). Bases turn red litmus blue but have no effect on blue litmus (which is already blue).

Sodium bicarbonate solution is a weak base, typical pH about 8–9.

pH > 7 (alkaline / basic).

Experiment 1: iron + dilute H₂SO₄.

1 mark per correct inference.

1. Silvery-grey metallic colour with a shiny lustre when freshly cut.
2. Magnetic at room temperature (one of the few elements that is — Fe, Co, Ni).
3. Other accepted: hard solid; high melting point (~1538 °C); high density; good conductor of heat and electricity; malleable and ductile.

1 mark each.

Fe(s) + H₂SO₄(aq) → FeSO₄(aq) + H₂(g)

Test: Bubble the gas through limewater (calcium hydroxide solution, Ca(OH)₂(aq)). (1 mark)

Observation: The colourless limewater turns milky/cloudy (a white precipitate of CaCO₃ forms) — this confirms CO₂. (1 mark)

2 NaHCO₃ reacts with 1 H₂SO₄ (because H₂SO₄ is dibasic):

2NaHCO₃(s) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l) + 2CO₂(g)

1 mark for correct products; 1 mark for balancing; 1 mark for state symbols.

C₂H₄ is ethene — an alkene.

General formula of alkenes: C_nH_2n

1. Same functional group ⇒ similar chemical reactions.
2. Successive members differ by one −CH₂− unit (relative molecular mass differs by 14).
3. Gradual change in physical properties as molar mass increases (boiling/melting points rise; density rises; volatility decreases).

1 mark each.

C₄H₁₀ fits the formula C_nH_2n+2 ⇒ alkane.

Alkanes (Compound B is butane, C₄H₁₀).

Compound A — ethene (C2H4)         (2 marks)

   H        H
    \      /
     C  =  C
    /      \
   H        H

Compound B — butane (C4H10)        (2 marks)

   H   H   H   H
   |   |   |   |
H− C − C − C − C −H
   |   |   |   |
   H   H   H   H
      

2 marks each — must show every C, H and bond. The C=C must be drawn as a clear double line for ethene.

Test tube 1 (ethene + KMnO₄): The purple KMnO₄ solution is decolorised (turns colourless). The C=C of ethene is oxidised by the manganate(VII) — manganate is reduced from purple Mn(VII) to colourless Mn(II). (1 mark)

Test tube 2 (butane + KMnO₄): No reaction / no colour change — the solution remains purple, because alkanes are saturated and do not react with KMnO₄ under these conditions. (2 marks: 1 for "no change", 1 for "stays purple".)

Saturated — butane has only single C−C bonds and the maximum number of hydrogens for 4 carbons.

• Compound A (ethene): raw material for making poly(ethene) plastic; also used commercially to ripen fruit (e.g. bananas, mangoes). (1 mark)
• Compound B (butane): fuel — bottled gas (LPG) for camping stoves and cigarette lighters. (1 mark)

Silicon, atomic number 14: 14 electrons fill the shells as 2, 8, 4.

Electronic configuration of Si: 2,8,4. (1 mark)

Justification: Si has electrons in three shells, so it appears in Period 3 of the periodic table — the period number equals the number of occupied electron shells. (1 mark)

Each fluorine atom (Group VII, 7 outer electrons) needs one more electron to complete its octet. The two F atoms share one pair of electrons (a single covalent bond) and each F retains 3 lone pairs.

        ••              ××
       •  •            ×  ×
      • F •  •×   ו  × F ×
       •  •            ×  ×
        ••              ××

      one shared pair shown in the middle:
      F (• •)(• ×) F  — each F has 6 outer
      electrons of its own + 2 shared (octet OK).
      

1 mark for showing the single shared pair (one dot + one cross between the two F nuclei); 1 mark for showing the three lone pairs on each F atom.

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons (so they have the same atomic number but different mass numbers).

Isotopes have the same proton number (Z = 6 for C) but different mass numbers.

¹²C, ¹³C and ¹⁴C are all isotopes of carbon. ¹⁴N is nitrogen, not carbon.

2 marks per radioisotope + use pair (1 for the isotope, 1 for the correct use).

Calcium (a Group 2 metal) reacts with cold water to form calcium hydroxide (slightly soluble) and hydrogen gas:

Ca(s) + 2H₂O(l) → Ca(OH)₂(aq) + H₂(g)

1 mark for products; 1 mark for balancing (the 2 in front of H₂O); 1 mark for state symbols.

Reactivity of Group 2 metals increases down the group: as you go down, the outer 2 electrons are further from the nucleus and easier to lose.

X is below Ca (which is below Mg) ⇒ X is the most reactive of the three.

Order of reactivity, most → least: X > Ca > Mg.

1 mark for placing X first; 1 mark for putting Ca above Mg.

1. Crude oil (petroleum) — Trinidad & Tobago is a major Caribbean producer.
2. Natural gas — primarily methane, often found alongside crude oil.
3. Other accepted: coal; tar sands; oil shale.

1 mark each.

1 mark per correctly named fraction. (Any TWO of the three is fine.)

One long C₇H₁₆-style alkane breaks into a shorter alkane (C₅H₁₂) plus a smaller alkene (C₂H₄).

Cracking (specifically, thermal cracking when no catalyst is used).

1. High temperature (~700–900 °C). (1 mark)
2. High pressure (~70 atm). (1 mark)

(Catalytic cracking uses lower temperature and pressure with a zeolite catalyst — but the question says "absence of a catalyst", so thermal cracking conditions apply.)

1. Component of petrol (gasoline) for motor vehicles.
2. Used as a solvent in laboratories and industry (e.g. extracting oils from plant material).
3. Other accepted: blowing agent for polystyrene foam; in geothermal power stations as the working fluid; as a starting material for further industrial reactions.

1 mark each.

Free-radical substitution: one H atom on pentane is replaced by Br. The product (drawing the substitution at C-1, giving 1-bromopentane) is:

   H   H   H   H   H
   |   |   |   |   |
Br−C − C − C − C − C −H
   |   |   |   |   |
   H   H   H   H   H

           1-bromopentane (C5H11Br)
      

1 mark for one H replaced by Br; 1 mark for fully displayed structure showing all C, H and bonds.

Free-radical substitution (one of the H atoms is substituted/replaced by Br; sunlight/UV initiates the radicals).

Propane burns in excess O₂ to form CO₂ and H₂O:

C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l)

1 mark for correct reactants/products; 1 mark for the balancing (5 O₂, 3 CO₂, 4 H₂O); 1 mark for state symbols.

Aluminium is high in the electrochemical/reactivity series — it holds onto its electrons very tightly. Carbon is not reactive enough to displace Al from Al₂O₃. So Al must be extracted by electrolysis of molten aluminium oxide (in cryolite, Na₃AlF₆) — electricity provides the energy needed to reduce the very stable Al³⁺ ions to Al metal at the cathode. (2 marks)

Iron is lower in the reactivity series, below carbon. The Fe³⁺ ions in iron(III) oxide (Fe₂O₃) can be reduced by carbon (in the form of coke / carbon monoxide) — so iron is extracted by reduction in the blast furnace: Fe₂O₃ + 3CO → 2Fe + 3CO₂. This is much cheaper than electrolysis. (2 marks)

Lead is below carbon in the reactivity series — about as reactive as copper.

Carbon reduction (chemical reduction) is more suitable for extracting Pb from PbO₂. (1 mark)

Reasons: Pb is below C in the electrochemical series, so carbon (as coke / CO) is reactive enough to reduce Pb⁴⁺ in PbO₂ to Pb metal. (1 mark) Carbon reduction is much cheaper and uses much less energy than electrolysis — there is no need to use the expensive electrolytic process when a simple smelting reaction will do. (1 mark)

Step 1 — coke burns in the furnace to make carbon monoxide (the active reducing agent):

2C(s) + O₂(g) → 2CO(g) (2 marks)

Step 2 — CO reduces lead(IV) oxide to lead metal:

PbO₂(s) + 2CO(g) → Pb(s) + 2CO₂(g) (2 marks)

(An alternative for step 2 is direct reduction with carbon: PbO₂(s) + C(s) → Pb(s) + CO₂(g).)

Usefulness — IRON: Iron is essential to life. It is the central atom in the haemoglobin molecule of red blood cells, where it binds O₂ reversibly and transports it from the lungs to every tissue. Iron is also part of cytochrome enzymes that carry out cellular respiration. A diet low in iron causes anaemia (low haemoglobin, fatigue, breathlessness). (2 marks)

Harmfulness — LEAD: Lead and its compounds are toxic to humans and animals. Lead enters the body through contaminated water (old lead pipes, lead-based paint dust, leaded petrol exhaust) and accumulates in bones and soft tissue. It interferes with the enzymes that synthesise haemoglobin, damages the nervous system (especially in children — causing reduced IQ, learning difficulties, behavioural problems) and damages the kidneys. (2 marks)

(Aluminium is generally inert to most living systems but high concentrations have been linked to neurological effects; either Al or Pb is acceptable as the harmful example.)