AQA GCSE Chemistry Chemistry Paper 2 (Higher), November 2020: Question 10

✅ Correct Answer

Water (or H₂O )

💡 Key Knowledge

The state symbol (aq) stands for aqueous, meaning a substance is dissolved in water. Water is therefore the solvent.

✅ Correct Answer

• The mixture becomes (more) red [1]
• (because the position of) equilibrium moves to the right / forward reaction is favoured [1]
• to reduce / counteract the increase in thiocyanate ion concentration [1]

🧠 Exam Technique (3-Step Formula)

1. Observation: Name the new colour clearly (it shifts towards the product side, which is red).
2. Direction of Shift: State whether equilibrium shifts to the left or right.
3. Reasoning: State that the system acts to oppose/counteract the change (Le Chatelier's Principle).

❌ Common Errors

• Stating only that "it turns red" without explaining the shift in equilibrium.
• Writing that the colour is "colourless" because SCN⁻ is colourless, ignoring the reversible equilibrium shift.
• Vague explanations like "because more reactants are added" without referring to counteracting the change.

✅ Correct Answer

• (The position of) equilibrium moves to the left / reverse reaction is favoured [1]
• to reduce / counteract the increase in temperature [1]
• (therefore) the forward reaction is exothermic [1]

💡 Equilibrium Temperature Rules

• An increase in temperature favours the endothermic direction (to take in heat).
• Heating caused the mixture to become yellow (Fe³⁺ side = left), so the reverse reaction must be endothermic.
• Because the reverse reaction is endothermic, the forward reaction must be exothermic.

❌ Common Errors

• Concluding the forward reaction is endothermic because the temperature was increased. Remember: look at which way it actually shifted!
• Forgetting to mention the direction of shift (moving left / towards the yellow side) and jumping straight to an unjustified conclusion.

✅ Correct Answer

• There is no change in the equilibrium position [1]
• because no gases are present / only aqueous solutions/liquids are present [1]

💡 Key Knowledge

Pressure only alters the position of equilibrium in reactions involving gases. In aqueous solutions or liquid systems, particles are already closely packed, so changing pressure has negligible effect on concentrations.

❌ Common Errors

• Stating "there are equal moles on both sides" — this rule only applies when comparing moles of gas.
• Forgetting to state the first marking point: explicitly confirming that there is no change.

✅ Correct Answer

☑ Co²⁺

💡 Transition Metals vs Group Metals

• Cobalt (Co): A transition element. A characteristic property of transition elements is that they form coloured compounds.
• Al³⁺, Mg²⁺, Na⁺: Main-group metals (Groups 3, 2, and 1). Their ions typically form white solids and colourless solutions.