Rates of Reaction and Energy Changes
MYP Year 4 Sciences — Rates of Reaction and Energy Changes
Two things are always being tested in this topic: how fast a reaction goes, and which way energy moves while it happens. Examiners love giving you a table of temperatures or a graph and asking you to read it correctly — most marks are lost not from bad chemistry but from quoting the wrong number (final instead of peak temperature, wrong subtraction order for , reading a curve's average slope instead of its tangent). Get the reading habits right and the theory underneath is genuinely simple.
Overview — how this topic fits together
The two questions this topic answers
Question 1: does the surroundings get hotter or colder while this reaction happens (energy changes)? Question 2: what makes the reaction go faster or slower (rates)? Every subtopic below is really just building the vocabulary and diagrams to answer one of those two questions properly.
- Energy changes are explained at the particle level using bond breaking (costs energy) and bond forming (releases energy).
- Rate is explained using collision theory — particles have to meet, hard enough, the right way round.
- Energy-profile diagrams and rate-time graphs are the two pieces of 'graph literacy' examiners test hardest in this unit.
- Reversible reactions and equilibrium sit at the end because they combine both ideas: a system where forward and reverse rates matter, and where energy still flows both ways.
The shape of the chapter
Command terms this topic is actually examined with
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Justify | Give reasons/evidence for a stated claim, tied to the specific data given | Criterion A/C | Quote the actual number from the table (e.g. the exact ΔT) — a correct label with no number typically loses a mark. |
| Deduce | Reach a conclusion using a visible logical or numerical step | Criterion C | Show the calculation/reasoning step before the final answer — a bare conclusion loses the reasoning mark even if correct. |
| Explain | State what happens AND why, using the underlying mechanism | Criterion A | Observation alone (e.g. 'it got hot') without bond energy/collision reasoning caps the mark. |
| State | A short, precise answer, no reasoning required | Criterion A | Missing the exact term or value costs the mark; extra explanation isn't penalised but wastes time. |
| Identify | Name the specified number of relevant points from context | Criterion A | Only the number of points asked for is credited — pick the clearest one if only one is required. |
| Formulate | Construct a testable prediction with a scientific reason | Criterion B | A prediction with no reason usually earns only half the available marks. |
| Calculate | Produce a numerical answer with correct sign, working and unit | Criterion A | Right number, wrong sign (e.g. −30 instead of +30) can still lose the final mark. |
Key point
Overview