Edexcel A-Level Chemistry AS Paper 1, June 2016: Question 1

✅ Correct Answers

• (i) Chlorine is a gas and iodine is a solid.
• (ii) Astatine is a solid. Justification: As you go down Group 7, the number of electrons increases, leading to stronger London forces (instantaneous dipole-induced dipole forces) requiring more energy to overcome.

💡 Key Knowledge

• Halogens show a clear trend in physical state down the group: F₂ (gas), Cl₂ (gas), Br₂ (liquid), I₂ (solid), At₂ (solid).
• Boiling and melting temperatures increase down the group due to larger molecular size and more electrons.

❌ Common Errors

• Referring to "ions" instead of molecules or atoms when describing halogen states.
• Vague references to "molecular weight" instead of explicitly stating "more electrons leading to stronger London forces".

✅ Correct Answers

Equation: Cl₂ + 2NaOH → NaCl + NaClO + H₂O (or ionic equivalent: Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O )

Type of reaction: Disproportionation

🧠 Exam Technique

• Make sure chemical equations are fully balanced with correct stoichiometry (notice the 2 in front of NaOH).
• Memorise the definition of disproportionation: a reaction in which the same element is simultaneously oxidized and reduced. Chlorine goes from oxidation state 0 in Cl₂ to -1 in NaCl and +1 in NaClO.

❌ Common Errors

• Writing NaClO₃ instead of NaClO (remember this is cold, dilute NaOH used to make bleach, whereas hot alkali produces chlorate(V)).

✅ Correct Answers

Correct Option: D (white smoke)

💡 Key Knowledge

• Hydrogen halides (like HBr) react with ammonia gas (NH₃) in an acid-base neutralisation reaction to form white solid ammonium salts ( NH₄Br ).
• The solid particles disperse in air, appearing as dense white smoke.

❌ Common Errors

• Confusing "steamy fumes" (which hydrogen halides give off alone in moist air) with the "white smoke" produced specifically when they meet ammonia gas.

✅ Correct Answers

• Point 1: Electronegativity is the relative ability of an atom to attract the bonding electrons in a covalent bond.
• Point 2: Fluorine is more electronegative than chlorine.
• Point 3: Therefore, fluorine is δ- (delta minus) and chlorine is δ+ (delta plus).

🧠 Exam Technique

• Use precise chemical terminology: state "bonding electrons in a covalent bond", not just "electrons".
• Explicitly link the electronegativity difference to the partial charges (δ+ and δ-) on the specific atoms in ClF₃.

❌ Common Errors

• Using the word "species" or "molecule" instead of "atom" when defining electronegativity.
• Reversing the partial charges due to confusing periodic trends in electronegativity (fluorine is the most electronegative element in the periodic table).

✅ Correct Answers

Correct Option: C ( 1.81 × 10²³ )

📐 Step-by-Step Calculation

1. Find the molar mass of MgCl₂:
   Ar(Mg) = 24.3, Ar(Cl) = 35.5
   Molar mass = 24.3 + (2 × 35.5) = 95.3 g mol⁻¹
2. Calculate moles of MgCl₂ units:
   Moles = Mass / Molar Mass = 9.53 g / 95.3 g mol⁻¹ = 0.100 mol
3. Calculate total moles of ions:
   Each formula unit of MgCl₂ dissociates into 3 ions (one Mg²⁺ ion and two Cl⁻ ions).
   Total moles of ions = 0.100 mol × 3 = 0.300 mol
4. Calculate total number of ions using Avogadro's constant:
   Number of ions = 0.300 mol × 6.02 × 10²³ mol⁻¹ = 1.806 × 10²³ ≈ 1.81 × 10²³

❌ Common Errors

• Forgetting to multiply by 3 to account for the total number of ions (Mg²⁺ + 2Cl⁻), which leads to option B ( 1.20 × 10²³ ) as a distractor trap.