MYP Chemistry: Rates of Reaction FAQ

In my experience marking Criterion B and C work, the most common failure isn't understanding the theory — it's students describing what happens to a graph ("the line gets steeper") instead of why it happens ("more frequent successful collisions because average kinetic energy increased"). MYP examiners reward the causal link, not the observation.

Quick tip: whenever you describe a rate graph, force yourself to write the word "because" at least once per sentence.

Think of it as the master explanation the whole unit sits on. If a question asks "explain why increasing temperature increases rate," a full-mark answer links three ideas: particles move faster → more frequent collisions → more collisions exceed activation energy. Missing any one link usually costs a mark in the MYP achievement level descriptors for Criterion C (Communicating).

Factor-by-factor mechanism:

1. Temperature — particles move faster, so collisions are more frequent AND more energetic.
2. Concentration/pressure — more particles in the same volume, so collisions are more frequent only.
3. Surface area — more exposed particles, so collision frequency at the reacting surface increases.
4. Catalyst — provides an alternative pathway with lower activation energy, so more collisions succeed without needing more energy.

Common mistake: writing "catalysts speed up reactions by increasing collisions" — catalysts don't change collision frequency at all; they lower the energy barrier.

Worked example: A student collects 48 cm³ of gas in the first 20 seconds, then the curve levels off at 60 cm³ by 60 seconds.

• Initial rate ≈ 48 ÷ 20 = 2.4 cm³/s (steepest part of the curve).
• Rate at t = 40s is lower — draw a tangent and calculate its gradient rather than assuming a straight-line average.
• Reaction is complete once no more gas is produced, i.e. when the line becomes flat, not when it looks slow.

Examiners regularly dock marks for calculating rate using the total time instead of the tangent gradient at a specific point.

3 things to check before your next assessment:

1. Does every explanation mention collision frequency, collision energy, or both — explicitly?
2. Have you named the independent, dependent, and at least two controlled variables if it's an investigation task?
3. Have you distinguished correlation ("rate increased") from causation ("because more particles exceeded the activation energy")?

These three checks map directly onto MYP Sciences Criterion A (Knowing and Understanding) and Criterion C (Processing and Evaluating) descriptors for the highest achievement bands.

Worked example — investigating concentration:

Hypothesis: "As HCl concentration increases, reaction time will decrease because higher concentration means more frequent collisions per second." Marking against Criterion B rewards a hypothesis that names the mechanism, not just the trend.

Common mistake checklist:

• Mixing up "rate" (gradient) with "yield" (final height) on a graph.
• Saying a catalyst is "used up" — it isn't, it's unchanged at the end.
• Forgetting units when calculating rate (cm³/s, g/s, mol/s).
• Writing "more collisions" without saying whether frequency, energy, or both increased.

I see the catalyst mistake in almost every cohort I teach — worth double-checking on your next revision pass.

According to the IB's MYP: Sciences guide, all four sciences criteria are assessed on a 1–8 scale per year, and schools choose which criteria pair with which unit. A rates of reaction investigation is a very common vehicle for Criterion B and C because it lends itself naturally to variable identification and data evaluation.

A solid grip on MYP-level collision theory pays off directly in DP — the same explanation (frequency and energy of collisions) underpins the Maxwell-Boltzmann distribution you'll meet later. Students who treat MYP rates of reaction as memorisation rather than reasoning often struggle to adapt when DP asks them to interpret a rate graph mathematically instead of descriptively.

Parents often ask why their child "knew the theory but still lost marks" — this table is usually the answer: it's an application topic, not a recall one.

On revisionprep.com, MYP Chemistry Revision Notes and Topical Worksheets are built around exactly this gap — short concept summaries paired with graph-based and investigation-style practice questions, so students rehearse the application skill examiners actually reward, not just the definitions.