CSEC Chemistry — Paper 2 Solutions

Rate of reaction is the change in concentration of a reactant (or product) per unit time — i.e. how quickly reactants are used up, or products formed, in a given time interval.

A catalyst is a substance that increases the rate of a chemical reaction by providing an alternative pathway with a lower activation energy, and is itself chemically unchanged at the end of the reaction.

1 mark for "speeds up the reaction"; 1 mark for "not consumed / unchanged at the end".

Read each stop-clock face. The hand position gives the time, in seconds, for the H₂O₂ to fully decompose.

1 mark per correct time = 5 marks.

Rate = 1 ÷ Time. Use the times from (b):

• Exp 1: 1 ÷ 60 = 0.017 s⁻¹
• Exp 2: 1 ÷ 45 = 0.022 s⁻¹
• Exp 3: 1 ÷ 30 = 0.033 s⁻¹
• Exp 4: 1 ÷ 25 = 0.040 s⁻¹
• Exp 5: 1 ÷ 20 = 0.050 s⁻¹

1 mark per correct calculation = 5 marks.

On the printed grid (Figure 2): rate (s⁻¹) on the y-axis (0–0.050), concentration (mol/dm³) on the x-axis (0–0.45).

Plot the five points: (0.10, 0.017), (0.15, 0.022), (0.20, 0.033), (0.25, 0.040), (0.30, 0.050).

The points form a near-straight line passing close to the origin. Draw a single straight line of best fit through them.

Marks: 2 for axes labelled with units + sensible scale; 2 for plotting all five points correctly; 1 for a clean, ruled line of best fit.

The rate of reaction is directly proportional to the concentration of hydrogen peroxide. As the H₂O₂ concentration increases, the rate of reaction increases linearly. (Graphically: a straight line through, or close to, the origin.)

1 mark for the direction (rate increases as concentration increases); 1 mark for "directly proportional / linear".

Read off the graph: at rate = 0.045 s⁻¹, draw a horizontal line until it meets the line of best fit, then drop a vertical line to the x-axis.

[H₂O₂] ≈ 0.275 mol dm⁻³ (accept 0.27–0.28 mol/dm³).

The standard volume of solution used in this kind of experiment is 1 dm³ (1 L). So the moles of H₂O₂ in 1 dm³ at 0.275 mol/dm³ = 0.275 mol.

Mass = moles × molar mass = 0.275 × 34.01

Mass of H₂O₂ = 9.35 g (in 1 dm³ of solution).

Hydrogen peroxide decomposes into water and oxygen gas. KI is the catalyst — it is written above the arrow but does NOT appear in the balanced equation.

2H₂O₂(aq) → 2H₂O(l) + O₂(g)

1 mark for correct reactants and products; 1 mark for correct balancing (the 2 in front of H₂O₂ and H₂O); 1 mark for all three state symbols correct.

Solid (ice), Liquid (water), Gas (water vapour / steam).

• Solid: Particles have the least kinetic energy. They vibrate about fixed positions in a regular lattice but cannot move freely from place to place.
• Liquid: Particles have more kinetic energy than in a solid. They can slide past one another but are still held close together by attractive forces.
• Gas: Particles have the most kinetic energy. They move rapidly in all directions, are far apart, and the forces of attraction between them are negligible.

1 mark per correctly described state.

Carbon dioxide skips the liquid phase at atmospheric pressure: solid CO₂ (dry ice) turns directly into gaseous CO₂.

Sublimation (the reverse — gas directly to solid — is called deposition).

In solid CO₂, the molecules are closely packed in a regular lattice and only vibrate about fixed positions.

During sublimation, the molecules gain enough kinetic energy to overcome the weak intermolecular forces. They break free of the lattice and become widely spaced and randomly arranged, moving rapidly in all directions in the gaseous state.

1 mark for describing the closely-packed regular solid arrangement; 1 mark for describing the widely-spaced random gas arrangement.

Sodium chloride lattice (3 marks): Draw a 3-D cubic arrangement of alternating Na⁺ ions (smaller circles, labelled +) and Cl⁻ ions (larger circles, labelled −). Each Na⁺ is surrounded by 6 Cl⁻ ions and vice-versa (octahedral coordination). Label the ions with their charges and indicate that this is held together by strong electrostatic ionic bonds.

• 1 mark — alternating arrangement of two types of ion.
• 1 mark — correctly labelled Na⁺ and Cl⁻.
• 1 mark — 3-D cubic / giant lattice shown.

Diamond (3 marks): Draw each carbon atom covalently bonded to four other carbon atoms in a tetrahedral arrangement, forming a giant 3-D network. Label "C" on the atoms and use solid lines for the four covalent bonds at each carbon.

• 1 mark — each C has 4 bonds.
• 1 mark — tetrahedral / 3-D network.
• 1 mark — correctly labelled covalent bonds.

For A to displace Fe from a solution of Fe²⁺, A must be more reactive than Fe. A loses one electron (to form A⁺); Fe²⁺ gains 2 electrons. We need 2 × A for every 1 × Fe²⁺ to balance the electrons.

2A(s) + Fe²⁺(aq) → 2A⁺(aq) + Fe(s)

1 mark for correct species; 1 mark for correct balancing (the 2A and 2A⁺) and state symbols.

Compound A — five C atoms, all single bonds (C₅H₁₂) — that's pentane.

Compound B — has the −COOH group at the end (C₂H₅COOH) — that's propanoic acid.

• Compound A: Alkanes (1 mark)
• Compound B: Carboxylic acids (alkanoic acids) (1 mark)

Compound C is ethanol, CH₃CH₂OH (or C₂H₅OH). Complete combustion gives carbon dioxide and water.

C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)

1 mark for correct formulae of reactants/products; 1 mark for correct balancing.

Compound C (ethanol) is more soluble in water (1 mark).

Reason: Ethanol contains the polar −OH group, which can form hydrogen bonds with water molecules. Pentane (Compound A) is a non-polar hydrocarbon — only weak van der Waals forces — and cannot form hydrogen bonds with water, so it is essentially insoluble. (1 mark)

Compound B (propanoic acid) reacts more vigorously with sodium (1 mark).

Reason: Carboxylic acids are stronger acids than alcohols — the O−H bond in −COOH ionises far more readily than the O−H in alcohols, so Compound B releases H⁺ more easily and reacts faster with the very reactive sodium metal, producing hydrogen gas more vigorously. (1 mark)

Ethanol reacts with sodium to form sodium ethoxide and hydrogen gas. Two ethanol molecules per H₂ to balance.

2C₂H₅OH(l) + 2Na(s) → 2C₂H₅ONa(s) + H₂(g)

1 mark for products; 1 mark for balanced equation with state symbols.

The gas produced is hydrogen (H₂).

Test: Collect the gas in a test-tube and bring a lighted splint to the mouth of the tube. (1 mark)

Observation: The gas burns with a "squeaky pop" sound — confirms hydrogen. (1 mark)

This is an esterification reaction (carboxylic acid + alcohol → ester + water).

Catalyst: concentrated sulfuric acid (H₂SO₄).

Propanoic acid (CH₃CH₂COOH) + ethanol (C₂H₅OH) → ethyl propanoate + water.

The ester linkage is −C(=O)−O−. Draw every C, H and O atom with all bonds shown:

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

   propyl group   |   ethyl group
   from acid B    |   from alcohol C
      

That is CH₃−CH₂−C(=O)−O−CH₂−CH₃ drawn with every bond shown. 1 mark for the correct ester linkage (−COO−); 1 mark for all hydrogens shown on every carbon.

Sulfur is a non-metal and its oxide (SO₂) is a simple molecular covalent compound. SO₂ consists of small, discrete molecules held together by strong covalent bonds within the molecule, but only weak intermolecular forces (van der Waals / dipole–dipole) between separate molecules. Only these weak intermolecular forces need to be overcome on melting, so very little energy is needed — the melting point is very low (−72°C). (3 marks)

Magnesium is a metal and oxygen is a non-metal, so MgO is an ionic compound consisting of Mg²⁺ and O²⁻ ions. The ions are held in a giant ionic lattice by very strong electrostatic forces of attraction between oppositely charged ions. A large amount of energy is required to break this lattice and melt the solid, so the melting point is very high (2852°C). (3 marks)

Marks: 1 for "covalent / molecular" for SO₂; 1 for "weak intermolecular forces"; 1 for "low MP / little energy needed"; 1 for "ionic" for MgO; 1 for "strong electrostatic forces in giant lattice"; 1 for "high MP / large energy needed".

SO₂ (covalent molecular): Does not conduct electricity in any state — it has no charged particles (no free ions, no delocalised electrons). However, when SO₂ dissolves in water it forms sulfurous acid (H₂SO₃), which ionises to give H⁺ and HSO₃⁻; the resulting solution conducts electricity. (2 marks)

MgO (ionic giant lattice): Does not conduct electricity when solid (the ions are locked in fixed positions). It does conduct electricity when molten or dissolved in water, because the Mg²⁺ and O²⁻ ions are then free to move and carry charge. (2 marks)

The partial diagram already shows a battery (+ and − terminals) connected to two electrodes A and B. To turn it into a working test rig you must add:

1. A beaker containing the test liquid, with electrodes A and B dipped into it. (1 mark)
2. An ammeter (or a bulb) in the circuit, in series with the battery, to show whether current is flowing. (1 mark)

Wires must complete a closed loop: + terminal → ammeter → electrode A → liquid → electrode B → − terminal.

• Conductors: aqueous ammonia (weak electrolyte — partly ionises to NH₄⁺ + OH⁻) and aqueous lead(II) nitrate (strong electrolyte — fully ionised to Pb²⁺ and NO₃⁻). (2 marks)
• Non-conductor: ethanol — it is a covalent molecular liquid with no free ions or delocalised electrons. (1 mark)

Anaerobic fermentation is the breakdown of sugars (such as glucose) by yeast (or other microorganisms) in the absence of oxygen, to produce ethanol and carbon dioxide.

1 mark for "breakdown of sugars by yeast/microbes"; 1 mark for "in the absence of oxygen / without air".

High temperatures denature the enzymes (zymase) in the yeast. Once denatured, the enzymes can no longer catalyse the conversion of glucose to ethanol, and fermentation stops. (Optimum temperature is around 25–37 °C.)

One mole of glucose produces two moles of ethanol and two moles of CO₂:

C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g)

(yeast / zymase enzyme above the arrow.) 1 mark for products; 1 mark for correct balancing and state symbols.

Alkaline hydrolysis of an ester gives the sodium salt of the carboxylic acid + the alcohol. Ethyl ethanoate hydrolyses to give sodium ethanoate and ethanol:

Product 1 — sodium ethanoate (2 marks)

   H   O
   |   ||
H− C − C − O⁻ Na⁺
   |
   H

Product 2 — ethanol (2 marks)

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

2 marks each for fully displayed structure (every C, H, O bond shown).

When a fat (triglyceride) is hydrolysed by NaOH, three fatty-acid sodium salts (the soap) are produced together with one molecule of glycerol (propane-1,2,3-triol).

By-product: glycerol.

Soaps form an insoluble scum with the Ca²⁺ and Mg²⁺ ions in hard water (so they don't lather easily and waste cleaner), whereas soapless detergents do not form a scum — they remain soluble and lather readily even in hard water.

Amino acids contain both −NH₂ and −COOH groups; they join by losing a water molecule between two monomers each time.

Condensation polymerisation.

Protein (or polypeptide).

Propene is an alkene (C=C) — it is unsaturated and reactive. Polypropene is fully saturated (no C=C) and very unreactive.

1. Test 1 — Bromine water: Add a few drops of orange/brown bromine water to each sample. Propene decolorises the bromine water (turns it colourless) because the C=C adds across Br₂; polypropene gives no colour change (stays orange/brown). (1 mark)
2. Test 2 — Acidified potassium permanganate (KMnO₄): The purple solution is decolorised by propene (oxidation of C=C), but polypropene gives no colour change — it stays purple. (1 mark)

Magnesium is the central metal ion in the chlorophyll molecule, the green pigment in plant leaves. Without Mg²⁺, the plant cannot make chlorophyll. (1 mark)

Chlorophyll is essential for photosynthesis — it absorbs sunlight and converts CO₂ + water into glucose + oxygen, providing the food the plant uses to grow. (1 mark)

1. Without enough Mg²⁺, the plant produces less chlorophyll — the leaves turn yellow (a condition known as chlorosis), starting between the veins of older leaves. (1 mark)
2. With less chlorophyll, the plant cannot absorb sunlight efficiently, so the rate of photosynthesis falls. Less glucose is produced. (1 mark)
3. Less glucose means less energy and fewer building blocks for plant growth — the plant grows slowly, produces fewer/smaller fruits or vegetables, and the overall yield drops. (1 mark)

2 marks per metal-ion + deficiency pair = 4 marks.

1. Single-use plastic shopping bags (LDPE / HDPE).
2. Plastic drink bottles and bottle caps (PET).
3. Other accepted answers: drinking straws, plastic cutlery, polystyrene takeout containers, fishing nets and ropes, microbeads from cosmetics, cigarette filters.

1 mark per example.

1. Ingestion of plastic by marine animals. Sea turtles mistake floating plastic bags for jellyfish; fish and seabirds swallow micro-plastics. The plastic blocks the digestive tract, gives a false feeling of fullness, and the animals starve to death. Toxic chemicals leached from the plastic also accumulate up the food chain — eventually reaching humans who eat seafood. (2 marks)
2. Entanglement in plastic debris. Discarded fishing nets ("ghost nets"), six-pack rings and ropes wrap around the necks, fins or flippers of seals, dolphins, whales and turtles. Animals are unable to swim, feed or breathe properly and often drown or die from infected wounds. (2 marks)
3. Other accepted answers: Plastics smother coral reefs and block sunlight from sea-grass; micro-plastics accumulate persistent organic pollutants (POPs) which disrupt hormones in marine organisms.