IB Chemistry SLTopic 5 — The Rate of ReactionPaper 1 & 2Core idea~12 min read
Factors Affecting Reaction Rate
Five things change how fast a reaction goes, and every one of them works through the same two quantities: how often particles collide, and what fraction of those collisions succeeds. Get that framing right and the explanations write themselves.
📚 What you need to know
The five factors are concentration, pressure (for gases), temperature, surface area and a catalyst.
Concentration, pressure and surface area increase the collision frequency only.
Temperature increases both the frequency and the proportion of collisions with energy ≥ Ea.
A catalyst lowers Ea, increasing the proportion that succeed, without changing the frequency.
The temperature effect is dominated by the proportion, not the extra speed.
Pressure only matters where gases are involved; surface area only where a solid is.
The organising idea
Learn which column each factor belongs to and almost every explanation question in this topic becomes a one-liner.
Concentration and pressure
Nothing about the individual particles has changed. There are simply more of them in the way.
A more concentrated solution has more particles in the same volume, so particles are closer together and collide more often. More collisions per second means more successful collisions per second, even though the fraction that succeed is exactly the same.
Pressure does the identical thing to gases. Squeezing the same number of gas particles into a smaller volume raises the number per unit volume — it is concentration by another name, which is why the explanation is the same.
Be careful about which questions pressure applies to. Increasing the pressure above a reaction between two solutions does essentially nothing, because liquids are almost incompressible. Pressure is a gas lever.
Surface area
Particles buried inside a lump cannot react with anything. Only those on the surface are available to be collided with.
When one reactant is a solid, the reaction can only happen at its surface. Break the solid into smaller pieces and the same mass exposes far more surface, so more of its particles are accessible and the collision frequency rises.
This is why powdered magnesium reacts vigorously with acid where a ribbon fizzes gently, and it is also why fine dusts — flour, custard powder, coal — are an explosion hazard.
Temperature
Temperature is the strong lever, because it is the only one that pulls both. Heating a mixture:
gives the particles more kinetic energy, so they move faster and collide more often;
and, far more importantly, means a greater proportion of particles have energy ≥ Ea, so a greater proportion of collisions succeed.
WORKED EXAMPLE
Raising the temperature from 300 K to 310 K roughly doubles the rate of many reactions. Yet the average particle speed increases by only about 1.7%. Explain the discrepancy.
Step 1 — the collision frequency effectSpeed scales with the square root of temperature: √(310/300) = 1.017, so particles collide only about 1.7% more often.frequency effect ≈ +2%Step 2 — the energy effectThe fraction of particles with energy ≥ Eₐ grows very steeply with temperature. For a typical Eₐ of 50 kJ mol⁻¹, that fraction is about 1.9 times larger at 310 K than at 300 K.proportion effect ≈ +90%Step 3 — combine1.02 × 1.9 ≈ 2, so the rate doublesAlmost all of the effect comes from the proportion, not the speed. An answer that mentions only “particles move faster” is describing the smaller half of the reason.
Examiners often ask about lowering the temperature rather than raising it, precisely because it catches students reciting a memorised answer. The theory is identical, applied backwards: fewer collisions, and a much smaller proportion with energy ≥ Ea, so the rate falls sharply.
Catalysts
A catalyst provides an alternative reaction pathway with a lower activation energy, and is chemically unchanged at the end of the reaction. Because the bar is lower, a greater proportion of the collisions already happening now qualify as successful.
Notice what a catalyst does not do. It does not make particles move faster, it does not increase the collision frequency, and it does not change ΔH or the amount of product you eventually get. The next two pages take this apart properly.
WORKED EXAMPLE
Marble chips react with excess dilute hydrochloric acid. Explain, in terms of collisions, the effect on the rate of (a) using the same mass of powdered marble, and (b) diluting the acid to half its concentration.
(a) powdering the marbleSame mass, much greater surface area, so more carbonate particles are exposed to the acid.collision frequency ↑ → successful collisions per second ↑faster, but the same total volume of CO₂(b) halving the acid concentrationFewer H⁺ ions per unit volume, so collisions with the marble surface are less frequent.collision frequency ↓ → rate ↓slower, and still the same total volume of CO₂The acid is in excess in both cases, so the marble is limiting and decides the yield. Rate changes; amount does not.
WORKED EXAMPLE
Which of these would increase the rate of a reaction between a gas and a solid? I increasing the pressure of the gas II using larger lumps of the solid III lowering the temperature A. I only B. I and II C. II and III D. I, II and III
Work through themI ✓ more gas particles per unit volume, so more collisionsII ✗ larger lumps mean LESS surface area, so fewer collisionsIII ✗ lower temperature means fewer collisions and a smaller proportion reaching EₐAOptions II and III are both the right factor applied in the wrong direction — the standard trap in these questions.
💡 Exam tip
Structure every explanation the same way: what changes → effect on frequency and/or proportion → effect on successful collisions per second → rate.
For temperature, give both reasons and make clear the proportion with energy ≥ Ea is the dominant one.
Do not offer pressure for a solution-only reaction or surface area where no solid is present.
Separate rate from yield. Changing the rate changes how long it takes, not how much you get.
Use the phrase “greater proportion of particles with energy greater than or equal to Ea“ — it is what the mark scheme is looking for.
⚠️ Common mix-up
Saying temperature works only by making particles move faster. That is the small part of the effect.
Saying there are “more collisions” without saying per unit time. More collisions overall would also happen if you simply waited longer.
Claiming a catalyst increases the collision frequency. It changes the proportion that succeed.
Thinking a faster reaction produces more product. It produces the same amount, sooner.
Confusing concentration with amount. Adding water increases the volume but lowers the concentration.
Up next: Activation Energy — the quantity every one of these explanations has been leaning on, drawn properly on an energy profile.
Want this explained one-to-one?
Book a free session with an experienced IB Chemistry tutor and get your trickiest topics made simple.
