AQA A-Level Chemistry AS Paper 2, June 2025: Question 2

✅ Model Answer

• M1: Provides an alternative reaction pathway / route / mechanism.
• M2: With a lower activation energy (E_a).

🧠 Exam Technique & Examiner Insight

This is a classic textbook definition question that should guarantee 2 marks in under 60 seconds.

• Always state both distinct parts: "alternative pathway" AND "lower activation energy".
• Do not stop at saying "it lowers the activation energy" without mentioning that it offers an alternative pathway.

💡 Key Knowledge

• A catalyst remains chemically unchanged at the end of the reaction.
• By providing an alternative pathway with a lower E_a, a greater fraction of reacting molecules possess energy equal to or greater than the activation energy at any given temperature.

❌ Common Errors

• Writing simply: "It speeds up the reaction without being used up" (describes what it does, not how it increases rate).
• Claiming that the catalyst "gives energy to molecules" or "increases kinetic energy" (false: only temperature increases average kinetic energy).

✅ Model Answer

• M1: Particles / molecules / reactants have more kinetic energy / move faster.
• M2: Significantly more molecules / particles have energy greater than or equal to the activation energy (E ≥ E_a).
• M3: Therefore, successful collisions are more frequent (or more successful collisions per unit time).

🧠 Exam Technique: "Rate" vs "Frequency"

• The word "frequent" is vital: Stating "more successful collisions" is not enough; examiners require frequency or per unit time.
• Emphasise M2: While particles do move faster, the dominant reason for the rate increase is the exponential increase in the fraction of molecules with E ≥ E_a.

💡 Key Knowledge

Refer to the Maxwell–Boltzmann distribution:

• Higher temperature broadens the distribution curve and shifts its peak to the right at a lower height.
• The total area under the curve to the right of E_a increases considerably, showing a much greater proportion of molecules with sufficient energy to react upon collision.

❌ Common Misconceptions to Avoid

• Writing "atoms" instead of molecules/particles (examiner specifically instructs: "Ignore atoms").
• Mentioning "vibrations" (applies to solids/IR spectroscopy, not gas-phase kinetics).
• Omitting the word successful or effective before "collisions". A simple increase in collision rate contributes only a small fraction to the overall rate increase.

✅ Marking Criteria & Full Marks Checklist

• M1: Partial charges and lone pairs: Both oxygen atoms show two lone pairs (:). Charges δ- on oxygen and δ+ on the hydrogen directly bonded to oxygen must be shown: O^δ-—H^δ+ ···· :O^δ- .
• M2: Hydrogen bond representation: A dashed or dotted line drawn precisely from a lone pair on an oxygen atom to the δ+ hydrogen of the other methanol molecule.
• M3: Geometry: The hydrogen bond angle is linear (180°) along the O—H····O sequence.

📐 Structural Diagram Walkthrough

Exact drawing notes: Draw one methanol molecule showing H₃C—O—H . Put 2 lone pairs on O and add δ- to O, δ+ to H. Draw a dashed/dotted line from one of the lone pairs directly to the H of the second methanol molecule. Ensure the O—H····O grouping forms a straight line.

🧠 Exam Technique

• Use a ruler: The sequence of atoms O—H····:O must be drawn in a straight line (180° bond angle) around the bridging hydrogen atom.
• Target the lone pair: Start or end your dotted/dashed line explicitly at an electron pair on oxygen, not at the letter 'O' itself.
• Lone pairs can be represented as pairs of dots, pairs of crosses, or orbital lobes containing dots.

❌ Frequent Mark Drop Traps

• Non-linear arrangement: Drawing the hydrogen bond at an arbitrary angle to the O—H covalent bond loses M3.
• Missing lone pairs: Forgetting to draw both lone pairs on the acceptor oxygen atom (or drawing only one).
• Wrong hydrogen atom: Hydrogen bonding to one of the C—H hydrogens instead of the O—H hydrogen receives zero marks for the interaction.
• Solid line instead of dashed: A solid line represents a covalent bond, not an intermolecular hydrogen bond.