AQA GCSE Chemistry Chemistry Paper 1 (Higher), June 2025: Question 7

✅ Model Answer & Key Comparison Points

• Storage & Transport: Hydrazine is a liquid, making it significantly easier and safer to transport and store than compressed hydrogen gas (takes up less volume, no heavy high-pressure tanks).
• Hazards: Both pose safety hazards; hydrogen is highly explosive/flammable, whereas hydrazine is toxic.
• Products: Both reactions produce non-polluting, harmless products (hydrogen makes only H₂O; hydrazine makes non-toxic N₂ and H₂O).
• Electrical Performance: Hydrazine produces a higher voltage (1.61 V vs 1.23 V), meaning fewer cells are needed to achieve the required output, leading to lighter battery packs.
• Cost / Materials: Hydrazine requires no catalyst, reducing manufacturing costs; hydrogen requires an expensive, scarce platinum catalyst.
• Judgement: "Overall, hydrazine is better for portable transport because it produces a higher voltage, is cheaper due to requiring no catalyst, and being a liquid makes it far easier to store than pressurised hydrogen gas, despite its toxicity."

🧠 Exam Technique: Securing Level 2 (3–4 Marks)

• Always include both sides: You must state advantages and disadvantages or compare both fuels directly.
• Add explanatory links: Do not just copy the table! Don't just say "hydrazine is a liquid"; explain "so it takes up less space and is easier to store". Don't just say "needs no platinum"; add "so it is cheaper".
• Give a final judgement: Level 2 strictly requires a concluding sentence stating which is better and why.

✅ Correct Diagram Description

The diagram has 5 overlapping bond regions (four N–H overlaps and one central N–N overlap):

• Shared pairs (bonds): Place 1 pair of electrons (one dot, one cross) in each of the four N–H overlaps, and 1 pair in the central N–N overlap.
• Lone pairs: Place 1 non-bonding pair (2 dots or 2 crosses) on the outer ring of each Nitrogen atom.
• Hydrogen shells: Hydrogen must contain only the 2 shared electrons in its overlap (no non-bonded electrons).

❌ Common Errors to Avoid

• Adding electrons to Hydrogen: Hydrogen only needs 2 electrons to fill its outer shell. Adding extra lone pairs to H forfeits Mark 2!
• Forgetting Nitrogen's lone pairs: Nitrogen is in Group 5. It uses 3 electrons to form single bonds, leaving 2 non-bonding valence electrons (1 lone pair).
• Double bonding the N–N: Hydrazine contains a single N–N bond, not a double bond.

✅ Accepted Answers

• It shows the 3D arrangement / 3D shape of the molecule.
• It shows the bond angles / spatial orientation in space.

💡 Examiner Tip

Dot-and-cross diagrams are 2D flat projections and incorrectly make molecules look planar. Ball-and-stick models show realistic spatial orientations (such as the non-planar, pyramidal arrangement around each nitrogen atom).

❌ Calculation Traps to Watch Out For

• Counting O–H bonds as 2 instead of 4: Notice the big stoichiometric balancing number: 2 H–O–H means there are 2 molecules of water, giving 4 O–H bonds in total!
• Bracket sign error: Subtracting (2062 + X) means both terms become negative: 2801 − 2062 − X . Students who write 2801 − 2062 + X end up with completely wrong answers.
• Exothermic sign confusion: Because it is exothermic, ΔH is −581 . If using Broken − Formed = ΔH , you must equate to −581 , not +581 .

💡 Quick Self-Check

Is your answer physically sensible? Single covalent bonds between period 2 elements (like N–N or C–C) generally range between 150 to 350 kJ/mol. An answer of 158 kJ/mol fits right in line with expectations.