3.94CH1 · Section 3: Physical chemistry

Rates of reaction

Key learnings

Collision theory drives every rate change
The rate is how quickly reactants are used up or products form. Particles must collide with at least the activation energy to react. Collisions that are more frequent or more energetic speed it up: higher concentration, pressure or surface area → more frequent collisions; higher temperature → both; a catalyst → more successful collisions. Temperature has two effects, the energy one larger — quoting only "faster collisions" drops a mark.
Catalysts: lower activation energy, unchanged
A catalyst increases the rate but is chemically unchanged at the end (3.12). It works by providing an alternative pathway with a lower activation energy (3.13), so more colliding particles have enough energy to react. On a reaction profile the catalysed route has a lower peak but the same reactant and product levels — a catalyst does not change $\Delta H$ . $E_a$ is the gap from the reactant level to the top of the curve.
Measuring rate: gradient of the graph = rate
Rate is followed by a measurable change over time — the volume of gas (gas syringe) or the loss of mass as gas escapes. Average rate $= \dfrac{\text{change in quantity}}{\text{time taken}}$ . On a graph of product against time the gradient is the rate: steepest at the start and flattening to horizontal when a reactant runs out. Faster conditions give a steeper initial gradient but, with the same amounts of reactant, the same final plateau value.

Common mistakes

Drawn from real examiner reports.

Describing rate without a comparison

When asked how a change affects the rate, candidates write vague phrases such as "it fizzes quickly" with no comparison to the other condition. The mark needs a comparative statement — e.g. "the reaction is faster than at the lower temperature" or "more gas is produced per second". Always compare explicitly: faster/slower, more/less per unit time.

November 2024 Paper 2C — a rate item credited "it moves faster" (a comparison) but rejected "it fizzes quickly" because it was not compared to the other condition.

Naming only one of temperature's two effects

For temperature you must mention BOTH effects to score full marks — particles move faster so collide more frequently, AND a greater proportion of particles have energy $\ge$ the activation energy (the energy effect is the larger one). Writing only "the particles move faster" is the classic dropped mark. Explanations must use collision-theory language.

November 2024 Paper 2C examiner report — explanation marks were lost for vague answers that did not reference the underlying mechanism.

A catalyst is not 'used up'

A catalyst is chemically unchanged at the end — it is not "used up". Nor does it "lower the temperature needed": it lowers the activation energy. Answers that say a catalyst is used up, or that it reduces the temperature required, score zero. It provides an alternative pathway so more collisions succeed, but the catalyst itself is recovered unchanged.

(common Edexcel chemistry error)

Drawing reaction profiles freehand

Energy (reaction profile) diagrams must be drawn with a ruler, with the activation-energy arrow long and clear, the reactant/product levels correct (products lower for exothermic) and a $\Delta H$ arrow in the right direction. Sketchy freehand diagrams, or arrows too short, lose marks — a catalysed route is shown with a lower peak but the same start and end levels.

June 2024 Paper 2C examiner report — candidates lost marks on the energy diagram for not using a ruler and for activation-energy arrows being too short to be acceptable.

Mass falls because gas leaves the flask

In a mass-loss experiment the balance reading falls because carbon dioxide gas is produced and leaves the open flask — not because the marble chips shrink, and not because "acid escaped". The mass lost equals the mass of gas given off. Say that the gas is produced AND leaves the flask: " $\text{CO}_2$ is given off" alone does not explain the falling reading.

November 2024 Paper 1C Q8(a) — the correct reason is that CO2 gas is produced and leaves the open flask; "CO2 given off" without stating it left the flask did not score.

Say 'successful' and 'kinetic', not 'chance'

Explanations need precise words: say the collisions are more frequent and that a greater proportion of particles have enough kinetic energy, giving more successful collisions per second. Mark schemes want the frequency of successful collisions — "probability" or "chance" of a collision is not credited, and the word "kinetic" is often missing.

November 2024 Paper 1C Q8(d) — marks need "frequency of successful collisions" (not "probability"/"chance"), and the word "kinetic" is often missing.

Exam tips

Rate graphs: smooth curve, show working
Draw a single smooth curve through the points — never "dot to dot" lines. Show working on the graph when reading a value. Read the value actually asked for (e.g. where two curves cross). Steepest = fastest (the start); horizontal = finished.
Name apparatus; give the unit cm3/s
Name apparatus precisely: a gas syringe and a conical flask (not "beaker"). Give the rate unit as $\text{cm}^3/\text{s}$ — "m" for minutes is rejected. For the rate at an instant, draw a tangent and use its gradient, not total ÷ total time.
Explain rate in three linked steps
Structure a rate explanation: (1) state the rate changes; (2) give the collision reason — more frequent collisions and/or more particles with energy $\ge$ the activation energy; (3) conclude more successful collisions per second. Compare conditions explicitly.

Revision notes