IB Chemistry SL & HLTopic 5 — The Rate of Chemical ChangePaper 1 & 2Core idea~11 min read
Factors Affecting Reaction Rate
There are five things you can change to speed a reaction up. That sounds like five explanations to memorise, but it is really only two — every one of them works by changing how often particles collide, or what fraction of those collisions actually works.
📘 What you need to know
The five factors: concentration, pressure, temperature, surface area and catalysts.
Concentration, pressure and surface area work only by increasing collision frequency.
Temperature does both — and the important effect is the bigger fraction of particles with energy above Ea.
A catalyst provides an alternative pathway with a lower activation energy, so a bigger fraction of collisions succeeds.
Pressure only matters if there are gases involved. Surface area only matters if there is a solid.
Rate roughly doubles for every 10 °C rise, as a rough rule of thumb only.
The two levers
Before the details, get this framework straight. Rate is the number of successful collisions per second, and that number is made of two parts:
The whole topic in one line
successful collisions per second = (collisions per second) × (fraction that succeed)
So there are exactly two ways to speed a reaction up: increase the first bracket, or increase the second. Every factor below does one or both.
Factor
What to do
More collisions per second?
Bigger fraction with E ≥ Ea?
Concentration
Use a more concentrated solution
Yes — particles are closer together
No change
Pressure (gases)
Squeeze the gas into a smaller volume
Yes — same particles, less space
No change
Surface area (solids)
Break the solid into smaller pieces
Yes — more of the solid is exposed
No change
Temperature
Heat the mixture
Yes — but only slightly
Yes — this is the main effect
Catalyst
Add a catalyst
No change
Yes — a lower Ea route is available
If you can fill in that table from memory, you can answer almost any “explain why the rate increases” question. Just name the lever, then say what it does to successful collisions per second.
Concentration and pressure
These two are the same idea wearing different clothes. Concentration is about particles per unit volume in a solution; pressure is about particles per unit volume in a gas. Increase either and the particles are simply closer together, so they run into each other more often.
Yellow haloes mark places where two particles are meeting. Nothing about the particles themselves has changed — there are just more chances for a collision.
The fraction does not change here. Packing particles closer together does not make any individual particle more energetic. So the proportion of collisions that succeed is exactly the same — there are simply more attempts per second.
Surface area
When one reactant is a solid, the reaction can only happen on the outside of it. Particles buried in the middle of a lump are doing nothing until the layers above them have dissolved away.
Break the lump up and you expose surfaces that were previously hidden inside. The amount of solid is identical — but the number of places a collision can happen goes up a lot.
The orange dots are acid particles. On the right they can get into the gaps as well as around the outside, and those gaps are surface that did not exist before.
Temperature
Temperature is the odd one out, because it pulls both levers at once. Almost every student names the first effect and stops, and that is where marks go missing.
Effect 1: particles move faster, so they collide more often
True, but small. Going from 25 °C to 35 °C (298 K to 308 K) speeds the particles up by only about 2%, because average speed depends on the square root of the temperature. On its own that would barely change the rate at all.
Effect 2: a much bigger fraction of particles has energy above Ea
This is the real story. Those same ten degrees can almost double the fraction of collisions carrying enough energy to react. That is where the familiar “rate doubles for every 10 °C” rule comes from — not from particles moving faster.
Put numbers on it. For a reaction with Ea of about 50 kJ mol−1, warming from 298 K to 308 K increases collision frequency by roughly 2%, but increases the fraction of collisions with enough energy by a factor of about 1.9. The second effect does essentially all the work.
If a question is worth 2 or 3 marks, give both effects but make clear which dominates. Something like: “particles collide slightly more often, but far more importantly a much greater proportion of collisions now has energy greater than or equal to Ea.”
One warning about the doubling rule: it is a rough guide, not a law. Depending on the reaction it might be every 9 °C or every 11 °C, and the interval drifts as the temperature climbs. Do not use it in a calculation unless a question specifically tells you to.
Catalysts
A catalyst speeds a reaction up without being used up itself. It is chemically unchanged by the end, which is why a tiny amount can process an enormous quantity of reactant.
It works by offering the reactants a different route to the products — one with a lower activation energy. Because the bar is lower, a much larger fraction of the ordinary collisions already clears it.
Say it this way
a catalyst provides an alternative pathway of lower Ea
Two other points examiners like:
Homogeneous catalysts are in the same phase as the reactants (for example, all in solution). Heterogeneous catalysts are in a different phase (for example, a solid catalyst with gaseous reactants).
Catalysts cut the environmental cost of industry: reactions can run at lower temperatures and pressures, less energy is needed, and side reactions can be suppressed so less waste is made.
Rate is not yield. A catalyst gets you to the finishing line faster. It does not move the finishing line. For a reversible reaction it speeds up both directions equally, so the position of equilibrium is unchanged.
Worked examples
WORKED EXAMPLE
Which changes speed the reaction up?
Zinc granules react with dilute sulfuric acid. Which of these would increase the rate? I. Using the same mass of zinc as a powder II. Diluting the acid with water III. Warming the mixture from 20 °C to 40 °C A. I only B. I and III only C. II and III only D. I, II and III
I: powder instead of granules
More surface area, so more collisions per second. Speeds it up.
II: diluting the acid
Lower concentration, so fewer collisions per second. Slows it down.III: warming it
More collisions, and a much bigger fraction above Ea. Speeds it up.
Answer: B“same mass as a powder” is the giveaway — the amount is unchanged, only the surface
WORKED EXAMPLE
Explaining the temperature effect properly
Explain why raising the temperature by 10 °C roughly doubles the rate of a reaction, even though the particles only speed up by about 2%. (3 marks)
Mark 1: the small effect
Faster particles collide a little more often, so collision frequency rises slightly.
Mark 2: the big effect
The spread of energies shifts, so a much greater proportion of particles has energy greater than or equal to Ea.
Mark 3: the comparisonAlmost all of the increase in rate comes from the bigger proportion of successful collisions, not from the faster movement.the word “proportion” is what earns the second mark — “more collisions” alone does not
WORKED EXAMPLE
Putting a number on surface area
A single cube of calcium carbonate has sides of 2.0 cm. Calculate its total surface area. It is then cut into eight identical 1.0 cm cubes. Calculate the new total surface area, and comment on the effect on rate.
Step 1: the single cube (6 faces)6 × (2.0 × 2.0) = 24 cm²Step 2: one small cube6 × (1.0 × 1.0) = 6 cm²Step 3: all eight together8 × 6 = 48 cm²Surface area doubles, from 24 cm² to 48 cm²Commentthe volume is still 8 cm³, so the same amount of solid now has twice as many sites for collisions — the reaction goes faster
💡 Exam tip
Never stop at “more collisions”. Finish the chain: more collisions per second → more successful collisions per second → faster rate.
For temperature, always use the word proportion (or fraction) alongside Ea. That is the marking point.
Check the state symbols before choosing a factor. No gas means pressure is irrelevant; no solid means surface area is irrelevant.
For catalysts, write alternative pathway with lower Ea. Saying it “lowers the activation energy” is loosely marked at best.
Watch for questions that change the amount rather than the concentration. Doubling the volume of the same solution does not change the rate at the start.
If asked for a factor to keep constant in a rate experiment, temperature is nearly always the answer.
⚠ Common mix-up
Thinking temperature works mainly by making particles move faster. It works mainly by increasing the fraction with enough energy.
Applying pressure to a reaction with no gases. Squeezing a liquid or solid does essentially nothing to its rate.
Confusing amount with concentration. Adding more of the same solution gives you more product in the end, but not a faster start.
Saying a catalyst is used up. It is regenerated, which is why so little is needed.
Saying a catalyst increases the yield. It changes how fast, not how far.
Thinking a powder contains more solid than a lump. Same mass, same moles — just more surface.
Treating “rate doubles every 10 °C” as exact. It is a rough rule, and it does not hold for every reaction.
Up next: Activation Energy — where Ea actually sits on an energy profile, how to read it off for exothermic and endothermic reactions, and how to find Ea for the reverse reaction.
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