Edexcel A-Level Chemistry AS Paper 1, November 2020: Question 3

✅ Correct Answer

An electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms.

💡 Key Knowledge

• Must mention both the electrostatic attraction and the shared pair of electrons.
• The attraction acts between the positive nuclei and the negatively charged shared electrons.

❌ Common Errors

Students often lose marks by stating that atoms "share electrons to gain a full outer shell" rather than defining what holds the bond together physically (the electrostatic attraction).

✅ Correct Answer

Each nitrogen atom has 5 outer electrons. They share one pair between the two N atoms, one pair with each of the two H atoms per nitrogen, leaving one lone pair on each nitrogen atom.

🧠 Exam Technique

• Nitrogen electrons = crosses (×), Hydrogen electrons = dots (•).
• Show the N-N single covalent bond clearly with one cross and one dot (or two crosses depending on atom choice).
• Do not forget the non-bonding lone pairs on both nitrogen atoms!

❌ Common Errors

Failing to include the lone pairs on the nitrogen atoms, or drawing a double bond between the two nitrogen atoms resulting in pentavalent nitrogen.

✅ Correct Answer

107° (Acceptable range: 105° to 108°)

💡 Key Knowledge

Hydrazine has a tetrahedral electron-pair geometry around each nitrogen atom (3 bonding pairs, 1 lone pair). Because lone pairs repel more than bonding pairs, the normal 109.5° tetrahedral angle is compressed down to approximately 107° (similar to ammonia).

✅ Correct Answer

Hydrazine is very soluble because it forms hydrogen bonds with water molecules between the lone pairs on nitrogen/oxygen and the hydrogen atoms bonded to electronegative atoms (N or O).

🧠 Exam Technique

• Labelled Diagram Description: Draw a water molecule interacting with hydrazine via a dashed line labelled "hydrogen bond" involving a H attached to N or O.
• Include partial charges (δ+ and δ-) if helpful, and show the linear alignment of the hydrogen bond with the lone pair (approx. 180°).

❌ Common Errors

Labelling a covalent bond inside the molecule as a hydrogen bond, or failing to show the hydrogen bond connecting a hydrogen atom to a lone pair on a separate molecule.

✅ Correct Answer

Any two of the following valid reasons:

• Large quantities of gases are produced from liquids, creating high pressure for thrust.
• Formation of the exceptionally strong triple bond in N₂ releases a massive amount of energy (highly exothermic).
• The reaction is very fast / self-igniting.
• Hot gases expand rapidly.

💡 Key Knowledge

Look closely at the state symbols in the equation: N₂H₄(l) + 2H₂O₂(l) → N₂(g) + 4H₂O(g) . Going from liquids to exclusively gas products creates an enormous volumetric expansion.

❌ Common Errors

Simply stating "it is exothermic" without linking it to the specific thermodynamics (like the strong triple bond in nitrogen gas forming) or failing to reference the production of gases from liquids.