OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2017: Question 17

✅ Correct Answer

Ba(OH)₂ + 2HCl → BaCl₂ + 2H₂O

💡 Key Knowledge

• Barium hydroxide is a strong base containing the divalent OH⁻ ion.
• Hydrochloric acid provides H⁺ ions to form water alongside barium chloride salt.

❌ Common Errors

• Writing an incorrect formula for barium hydroxide (e.g., BaOH ) due to forgetting Group 2 valency.
• Balancing the equation incorrectly by missing the coefficient 2 in front of HCl and H₂O .

✅ Correct Answer

1. Atomic size: Atomic radius increases down the group / more electron shells.
2. Attraction: Outer electrons experience more shielding and weaker nuclear attraction.
3. Ionisation Energy: Less energy is required to remove outer electrons (first ionisation energy decreases).

🧠 Exam Technique

This is a classic 3-mark explanation question. Ensure you hit the logical chain in order: Structure (shells/radius) → Electrostatics (shielding/nuclear attraction) → Energetics (ionisation energy/ease of losing electrons).

❌ Common Errors

• Vague statements like "shielding increases" without linking it to the decreasing pull from the nucleus.
• Mentioning "nuclear charge increases down the group, so reactivity increases" — examiners penalise this because increased nuclear charge alone would pull electrons *closer*, missing the dominant effect of extra shells and shielding.

✅ Correct Answer (i)

Definition: Oxidation and reduction of the same element in a single reaction.

Oxidation number changes for Cl in 4KClO₃ → KCl + 3KClO₄ :

• Chlorine is oxidised from +5 (in KClO₃ ) to +7 (in KClO₄ ).
• Chlorine is reduced from +5 (in KClO₃ ) to -1 (in KCl ).

✅ Correct Answer (ii)

potassium chlorate(VII)

💡 Key Knowledge

Disproportionation requires a single species to undergo simultaneous oxidation (loss of electrons / increase in oxidation number) and reduction (gain of electrons / decrease in oxidation number).

❌ Common Errors

Forgetting to include the Roman numerals in brackets for systematic names (e.g., writing just "potassium chlorate"), or mixing up oxidation state signs (omitting the + sign for positive numbers).

✅ Correct Answer

Equation: Ba(NO₃)₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaNO₃(aq)

Entropy change: Entropy decreases / entropy change is negative ( ΔS < 0 ).

Explanation: Aqueous ions form a solid precipitate ( BaSO₄ ), resulting in less disorder, fewer ways of arranging energy, and a more ordered system.

🧠 Exam Technique

Mark 2 (the explanation) is dependent on identifying the formation of a solid or a decrease in disorder. Always explicitly contrast the state change (aqueous/ions to solid precipitate) to secure the mark.

❌ Common Errors

Omitting state symbols or getting them wrong. Note that state symbols are explicitly requested in the prompt for part (d).

✅ Correct Answer

Equation: ½ I₂(s) → I(g)

Entropy change: Entropy increases / entropy change is positive ( ΔS > 0 ).

Explanation: A solid changes into a gas, which has more disorder, greater freedom of movement, and more ways of dispersing energy.

💡 Key Knowledge

The standard enthalpy change of atomisation is the enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions.

❌ Common Errors

Writing I₂(s) → 2I(g) instead of using a fraction ( ½ ), which violates the per-mole-of-gaseous-atoms definition for atomisation. Also forgetting state symbols ( (s) and (g) ).