2025 Cambridge IGCSE Chemistry (0620) 模擬試題連答案詳解

In the Contact process, sulfur dioxide is reacted with oxygen to produce sulfur trioxide. The compromise temperature used to achieve an optimal balance of rate and equilibrium yield is 450 °C.

Award [1 mark] for 450 (allow 400 to 450).

Insoluble salts are prepared by mixing two soluble salts together to form an insoluble solid precipitate. This method is called precipitation.

Award [1 mark] for precipitation. Reject crystallization / filtration.

The halogens (Group VII) show a trend of becoming darker down the group: fluorine is pale yellow, chlorine is yellow-green, bromine is red-brown, iodine is grey-black, and therefore astatine is expected to be black.

Award [1 mark] for black / dark grey.

The empirical formula mass of \(\text{CH}_2\) is \(12 + (2 \times 1) = 14\). Dividing the relative molecular mass by the empirical mass: \(56 / 14 = 4\). Therefore, the molecular formula is \(4 \times \text{CH}_2 = \text{C}_4\text{H}_8\).

Award [1 mark] for \(\text{C}_4\text{H}_8\) or C4H8.

A homologous series is a family of organic compounds that share the same general formula, functional group, similar chemical properties, and show a trend in physical properties.

Award [1 mark] for homologous series. Reject functional group.

A sulfur atom has an atomic number of 16 and a configuration of 2,8,6. To form a sulfide ion with a \(2-\). charge, it gains 2 electrons, resulting in an electronic configuration of 2,8,8.

Award [1 mark] for 2,8,8 (accept 2.8.8 or 2 8 8).

Delocalised electrons are valence electrons that are not bound to any specific atom in a metallic structure and are free to move throughout the giant metallic lattice.

Award [1 mark] for electrons that are free to move / mobile valence electrons. Reject just 'free electrons' without any mention of mobility/movement.

The reaction of ethene with steam to form ethanol is catalysed by phosphoric acid (specifically phosphoric(V) acid, \(\text{H}_3\text{PO}_4\)).

Award [1 mark] for phosphoric acid / phosphoric(V) acid / \(\text{H}_3\text{PO}_4\). Reject sulfuric acid / iron / nickel / yeast.

In the industrial manufacture of sulfuric acid (the Contact Process), vanadium(V) oxide (\(\text{V}_2\text{O}_5\)) is used as a catalyst to speed up the reversible oxidation of sulfur dioxide to sulfur trioxide.

1 mark: vanadium(V) oxide. Accept: vanadium pentoxide, vanadium(5) oxide or \(\text{V}_2\text{O}_5\). Reject: vanadium.

The halogens show a clear trend in physical state down the group, changing from gas (fluorine and chlorine) to liquid (bromine) to solid (iodine). Since astatine lies below iodine, it is expected to be a solid at room temperature and pressure.

1 mark: solid. Reject: gas, liquid, semi-solid.

(a) The balanced equation with state symbols is:
2SO2(g) + O2(g) <=> 2SO3(g)

(b) The standard industrial conditions are:
- Temperature: 450 degrees Celsius
- Pressure: 1 - 2 atm (or 100 - 200 kPa)
- Catalyst: Vanadium(V) oxide (V2O5)

(c) According to collision theory, increasing the temperature increases the kinetic energy of the particles. Consequently:
1. Particles move faster and collide more frequently per unit time.
2. A larger proportion of colliding particles have energy greater than or equal to the activation energy (E_a), resulting in more successful collisions.

(d) The forward reaction is exothermic (Delta H = -197 kJ/mol).
1. An increase in temperature shifts the position of equilibrium to the left (endothermic direction) to oppose the change, reducing the equilibrium yield of SO3.
2. A very low temperature would give a high equilibrium yield, but the rate of reaction would be extremely slow.
3. Therefore, 450 degrees Celsius is a compromise temperature that provides a sufficient rate of reaction with an acceptable yield of SO3.

(e) (i) A catalyst has no effect on the position of equilibrium because it increases the rate of both forward and reverse reactions by the same factor.
(ii) It increases the rate of the forward reaction by providing an alternative pathway with a lower activation energy.

(a) [3 marks total]:
- Correct formulae of reactants and products with correct balancing: 2SO2 + O2 <=> 2SO3 [1 mark]
- Reversible reaction symbol used [1 mark]
- Correct state symbols (g) for all reactants and products [1 mark]

(b) [3 marks total]:
- Temperature: 450 degrees Celsius (accept 400 - 500 degrees Celsius) [1 mark]
- Pressure: 1 - 2 atm (accept 1 - 5 atm or 100 - 500 kPa) [1 mark]
- Catalyst: Vanadium(V) oxide / vanadium pentoxide / V2O5 [1 mark]

(c) [3 marks total]:
- Particles have more kinetic energy / move faster [1 mark]
- More frequent collisions / more collisions per unit time [1 mark]
- A higher proportion of particles have energy greater than (or equal to) the activation energy [1 mark]

(d) [3 marks total]:
- Forward reaction is exothermic, so higher temperature decreases yield of SO3 (or shifts equilibrium left) [1 mark]
- Lower temperature gives a higher yield, but the reaction rate is too slow to be practical [1 mark]
- 450 degrees Celsius is a compromise temperature between yield and rate [1 mark]

(e) [2 marks total]:
- (i) No effect (or remains unchanged) [1 mark]
- (ii) Increases the rate [1 mark]

(a) CuCO3(s) + H2SO4(aq) -> CuSO4(aq) + CO2(g) + H2O(l)

(b) Practical preparation steps:
1. Add excess copper(II) carbonate powder to a measured volume of dilute sulfuric acid in a beaker and stir until no more effervescence is seen and solid remains undissolved.
2. Filter the mixture to remove the excess unreacted copper(II) carbonate solid as residue.
3. Transfer the filtrate to an evaporating basin and heat to evaporate water until the crystallization point is reached.
4. Leave the concentrated solution to cool slowly so that crystals of hydrated copper(II) sulfate form.
5. Filter the crystals from the remaining liquid, rinse with a small amount of cold distilled water, and dry the crystals with filter paper.

(c) (i) Moles of CuCO3 = mass / M_r = 6.20 / 124 = 0.0500 mol.
From the balanced equation, 1 mole of CuCO3 produces 1 mole of CuSO4.
Therefore, the maximum theoretical yield of copper(II) sulfate is 0.0500 mol.
(ii) Theoretical mass of CuSO4 . 5H2O = moles * M_r = 0.0500 * 250 = 12.5 g.
Actual mass obtained = 10.0 g.
Percentage yield = (Actual mass / Theoretical mass) * 100% = (10.0 / 12.5) * 100% = 80.0%.

(a) [3 marks total]:
- Correct reactants and products: CuCO3 + H2SO4 -> CuSO4 + CO2 + H2O [1 mark]
- State symbols correct: CuCO3(s), H2SO4(aq), CuSO4(aq), CO2(g), H2O(l) [1 mark]
- Correctly balanced (1:1 ratio) [1 mark]

(b) [5 marks total]:
- Add excess CuCO3 (to ensure all acid is reacted) [1 mark]
- Filter to remove unreacted solid/residue [1 mark]
- Heat/evaporate filtrate to crystallization point [1 mark]
- Leave to cool to crystallize [1 mark]
- Filter crystals, wash with cold distilled water, and dry with filter paper [1 mark]

(c) (i) [2 marks total]:
- Moles of CuCO3 = 6.20 / 124 = 0.05 mol [1 mark]
- Theoretical moles of CuSO4 = 0.05 mol [1 mark]

(ii) [4 marks total]:
- Calculation of theoretical mass of crystals: 0.05 * 250 = 12.5 g [1 mark]
- Correct formula for percentage yield [1 mark]
- Correct substitution: (10.0 / 12.5) * 100 [1 mark]
- Evaluation to 80.0% [1 mark]

(a) Reactivity of the halogens decreases down Group VII. As you go down, the number of electron shells increases, making the atomic radius larger. This increases shielding, meaning there is a weaker attraction between the positive nucleus and an incoming electron. Thus, it is harder for the atom to gain an electron to form a 1- halide ion.

(b) (i) The colorless solution turns orange/yellow/brown due to the formation of aqueous bromine.
(ii) Cl2(g) + 2Br-(aq) -> 2Cl-(aq) + Br2(aq)
(iii) Bromide ions (Br-) are oxidized because they lose electrons (2Br- -> Br2 + 2e-). Chlorine molecules (Cl2) are reduced because they gain electrons (Cl2 + 2e- -> 2Cl-).

(c) (i) Physical state: solid (as physical state goes gas -> liquid -> solid down the group).
Color: black / dark grey (as colors get darker down the group).

(a) [4 marks total]:
- Reactivity decreases down the group [1 mark]
- Number of electron shells increases / atomic radius increases [1 mark]
- Outer shell is shielded more from positive nucleus [1 mark]
- Attraction for incoming electron is weaker / harder to gain an electron [1 mark]

(b) [8 marks total]:
- (i) Solution turns orange / yellow / brown [1 mark]
- (ii) Reactants and products correct: Cl2 + 2Br- -> 2Cl- + Br2 [1 mark]
- Balancing correct [1 mark]
- State symbols correct: Cl2(g), Br-(aq), Cl-(aq), Br2(aq) [1 mark]
- (iii) Bromide / Br- is oxidized AND loses electrons [2 marks]
- Chlorine / Cl2 is reduced AND gains electrons [2 marks]

(c) [2 marks total]:
- State: solid [1 mark]
- Color: black / dark grey [1 mark]

(a) A homologous series is a family of similar organic compounds which have the same functional group, similar chemical properties, a trend in physical properties, and can be represented by a general formula.

(b) (i) Conditions for fermentation:
- Yeast catalyst
- Temperature between 25 and 35 degrees Celsius
- Absence of oxygen (anaerobic conditions)
(ii) C2H4 + H2O -> C2H5OH

(c) (i) Displayed structure must show every single bond explicitly: H3C-CH2-CH2-OH, with a circle drawn around the -O-H (or -OH) group.
(ii) 2C3H7OH + 9O2 -> 6CO2 + 8H2O (or C3H7OH + 4.5O2 -> 3CO2 + 4H2O)
(iii) Exothermic. More energy is released when making bonds in the products (CO2 and H2O) than is absorbed to break bonds in the reactants (C3H7OH and O2).

(a) [3 marks total]:
- Same functional group / similar chemical properties [1 mark]
- Same general formula / consecutive members differ by -CH2- [1 mark]
- Trend in physical properties [1 mark]

(b) [5 marks total]:
- (i) Yeast [1 mark]
- Temperature in range 20 - 40 degrees Celsius [1 mark]
- Anaerobic / absence of oxygen [1 mark]
- (ii) Correct reactants: C2H4 + H2O [1 mark]
- Correct product: C2H5OH [1 mark]

(c) [6 marks total]:
- (i) Fully displayed structure of propan-1-ol showing all bonds [1 mark]
- -OH group correctly circled [1 mark]
- (ii) Correct formulae of reactants and products: C3H7OH + O2 -> CO2 + H2O [1 mark]
- Correctly balanced coefficients (2, 9 -> 6, 8) [1 mark]
- (iii) Exothermic [1 mark]
- More energy is released when making bonds than is absorbed when breaking bonds [1 mark]

(a) Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons (or different mass numbers).

(b) For 69Ga:
- Protons = 31
- Neutrons = 69 - 31 = 38
- Electrons = 31
For 71Ga3+:
- Protons = 31
- Neutrons = 71 - 31 = 40
- Electrons = 31 - 3 = 28

(c) Metallic bonding is the electrostatic attraction between positive metal ions (cations) arranged in a giant lattice and a sea of mobile, delocalized electrons.
- Malleability: Metals are malleable because positive ions are arranged in regular layers which can slide over each other when a force is applied, without breaking the metallic bonding.
- Electrical conductivity: Metals conduct electricity because they contain free/delocalized electrons that are mobile and can move to carry electrical charge.

(d) (i) Nitrogen and oxygen gases from the air react together under very high temperatures.
(ii) Oxides of nitrogen cause acid rain / photochemical smog / respiratory irritation.

(a) [2 marks total]:
- Same number of protons / same element [1 mark]
- Different number of neutrons / different mass number [1 mark]

(b) [4 marks total]:
- 69Ga: Protons = 31 AND Electrons = 31 [1 mark]
- 69Ga: Neutrons = 38 [1 mark]
- 71Ga3+: Protons = 31 [1 mark]
- 71Ga3+: Neutrons = 40 AND Electrons = 28 [1 mark]

(c) [6 marks total]:
- Giant lattice of positive ions (cations) [1 mark]
- Surrounded by a sea of delocalized electrons [1 mark]
- Electrostatic attraction between positive ions and delocalized electrons [1 mark]
- Malleability: layers of ions can slide over each other [2 marks]
- Conductivity: delocalized electrons are free to move / mobile [1 mark]

(d) [2 marks total]:
- (i) Nitrogen and oxygen (from the air) react together at high temperatures [1 mark]
- (ii) Acid rain / photochemical smog / respiratory problems [1 mark]