Edexcel A-Level Chemistry AS Paper 1, June 2019: Question 4

✅ Correct Answer

A (carbonates – increases, nitrates – increases)

💡 Key Knowledge

• Going down Group 2, the metal cations increase in ionic radius and charge density decreases.
• Larger cations have a lower polarizing effect on the large, complex anions (CO₃²⁻ and NO₃⁻).
• This results in less distortion of the anion's electron cloud, making the bonds within the anion more stable to heat.
• Therefore, thermal stability increases down the group for both carbonates and nitrates.

✅ Correct Answer & Marking Points

• Point 1 (Method & Comparison): A workable method to compare stability, such as timing how long it takes to produce brown NO₂ gas, relight a glowing splint, or measure volume of gas produced.
• Point 2 (Controlled Heating): The same heat applied (e.g., identical Bunsen burner flame or same distance from heat source).
• Point 3 (Fair Test / Quantities): Same amount (mass or moles) of each nitrate in separate test tubes.
• Point 4 (Safety): Use of a fume cupboard (due to toxic brown nitrogen dioxide gas, NO₂).

🧠 Exam Technique & Diagram Guidance

• Diagram: Draw two side-by-side test tubes containing solid nitrates connected to delivery tubes (or with glowing splints held at the mouth), heated by a Bunsen burner. Clearly label the "first nitrate", "second nitrate", and heat source.
• Workable alternatives: Mentioning a light sensor, gas syringe, or timing appearance of brown fumes all secure the method mark.

❌ Common Errors & Restrictions

• Generic Safety: Standard student safety gear like "gloves", "laboratory coat", "eye protection", or a "face mask" are explicitly excluded by the mark scheme. You must state a specific hazard precaution related to the chemicals used—in this case, a fume cupboard.
• Vague amounts like "a scoop" without implying equal quantities will lose the fair-testing mark.

✅ Correct Answer

B (hydroxides – increases, sulfates – decreases)

💡 Key Knowledge

• Hydroxides: Solubility increases down the group (e.g., Magnesium hydroxide is sparingly soluble, Barium hydroxide is much more soluble). Hydration enthalpy decreases less rapidly than lattice enthalpy.
• Sulfates: Solubility decreases down the group (e.g., Barium sulfate is insoluble, Magnesium sulfate is soluble). Lattice enthalpy changes very little down the group, but hydration enthalpy of the metal ion decreases significantly as ionic radius increases.