AQA GCSE Chemistry Chemistry Paper 1 (Higher), June 2025: Question 3

✅ Correct Answer

Substance reduced: SnO₂ (or tin oxide) [1 mark]

Reason: It loses oxygen [1 mark]

💡 Key Knowledge

• Oxidation = gain of oxygen.
• Reduction = loss of oxygen.
• Carbon ( C ) gains oxygen to form CO₂ (oxidised).
• Tin oxide ( SnO₂ ) loses oxygen to form tin metal ( Sn ) (reduced).

🧠 Exam Technique

Always name the reactant that starts with oxygen as the substance reduced (e.g. tin oxide or SnO₂). Note that Mark 2 depends on having Mark 1 correct.

❌ Common Errors

Writing just "tin" or "Sn" as the substance reduced. Tin is the product; the substance that actually loses oxygen during the reaction is tin oxide ( SnO₂ ).

✅ Correct Answer

Carbon is more reactive than tin
(or tin is less reactive than carbon).

💡 Key Knowledge

Metals less reactive than carbon can be extracted from their oxides by reduction with carbon. A more reactive element displaces a less reactive element from its compound.

🧠 Exam Technique

When asked why a non-metal/metal can extract another, the answer is almost always a direct comparative statement about their relative reactivity.

❌ Common Errors

Stating only that "carbon is reactive" without comparing it to tin. It must be a comparative statement: "more reactive than tin".

✅ Final Answer

80%

❌ Common Errors

• Forgetting to include carbon in the total reactant mass (using 128 instead of 140).
• Dividing by product mass instead of reactant mass (though here both sum to 140, forming good habits is essential).
• Confusing atom economy with percentage yield. Atom economy is theoretical and based entirely on the balanced equation.

✅ Correct Differences (any two)

Tungsten (compared to potassium):

• Has a higher melting / boiling point
• Is denser / has higher density
• Is harder / stronger (potassium is soft and can be cut with a knife)
• Is less reactive (potassium reacts violently with water)
• Can form ions with different charges (variable oxidation states)
• Forms coloured compounds
• Can act as a catalyst

🧠 Exam Technique

Always state which metal has which property! You can phrase it in terms of tungsten (e.g., "tungsten has a higher melting point") or potassium (e.g., "potassium is less dense"). Avoid vague words like "different strength" without qualifying which is higher.

💡 Key Knowledge

Transition elements are typical strong, dense engineering metals with high melting points. Group 1 alkali metals are uniquely soft, have low densities (lithium, sodium, and potassium float on water), and have low melting points.

❌ Common Errors

Giving electronic structure differences (e.g. "potassium has 1 outer electron"). The question asks for differences in properties, not atomic structure!

🧠 Exam Technique

Remember the big balancing numbers! The ' 3 ' in front of H₂ and H₂O means you must multiply their formula masses by 3. Show both left and right totals clearly.

❌ Common Errors

Forgetting balancing coefficients: calculating 232 + 2 = 234 and 184 + 18 = 202 , which leads to unequal totals and loss of marks.

✅ Correct Answer (any one)

• For sustainable development
• For economic reasons / to reduce costs
• To minimise waste produced
• To minimise use of limited resources / conserve raw materials
• To minimise energy use in processing waste

❌ Common Errors

Confusing atom economy with percentage yield. Do NOT write "gives a higher yield" or "produces more product" – atom economy is about the proportion of reactant atoms that become useful products, not reaction efficiency.