IB Chemistry HLTopic 5 — How Fast? The Rate of ReactionPaper 1 & 2HL only | Core idea~8 min read
Molecularity
Molecularity is just a head count: how many particles take part in one elementary step. It sounds almost too simple to be worth a page — until you realise how easily it gets confused with order. They are different ideas, found in different ways, and mixing them up is one of the most reliable ways to lose marks in this topic.
📘 What you need to know
Molecularity = the number of particles taking part in a single elementary step.
Unimolecular: one particle reacts on its own.
Bimolecular: two particles collide and react. This is by far the most common.
Termolecular: three particles must collide at the same moment. Very rare.
Molecularity is always a whole number: 1, 2 or 3.
Molecularity applies to one step. Order applies to the overall reaction.
For an elementary step only, the order matches the molecularity — the coefficients become the powers.
You cannot work out molecularity from the overall balanced equation.
The three types
Count the particles on the left-hand side of the step. That number is the molecularity. Nothing else to it.
Almost every elementary step you meet will be bimolecular. If a proposed mechanism needs a termolecular step, that is usually a hint the mechanism should be split into two simpler steps instead.
🤔 Why are termolecular steps so rare?
Two particles meeting is already a demanding thing to ask: they have to arrive at the same place, at the same time, with enough energy, and facing the right way. For three particles you need all of that to happen simultaneously for every pair at once. In a gas or a solution, particles are mostly far apart and moving randomly, so the chance of a genuine three-way meeting is tiny. Nature’s workaround is to do it in two stages: two particles join first, then the result meets the third.
Molecularity is not the same as order
This is the whole reason the page exists. The two words sound like they should mean the same thing, and in one special case they give the same number — but they come from completely different places.
Question
Molecularity
Order
What does it count?
Particles taking part in one step
How strongly the rate depends on a concentration
What does it apply to?
One elementary step
The overall reaction
How do you find it?
Read it straight off the step
Measure it by experiment
What values can it take?
Only 1, 2 or 3
0, 1, 2, fractions, even negative
Can you get it from the balanced equation?
Yes, if that equation is a single elementary step
No, never
Is it a real physical count?
Yes — actual particles in a collision
No — it describes measured behaviour
ALLOWED: powers from an elementary step
Elementary step: 2NO2 → NO3 + NO
This is a real bimolecular collision between two NO2 particles, so:
rate = k[NO2]2
The 2 in front of NO2 is a genuine particle count, so it becomes the power.
NOT ALLOWED: powers from an overall equation
Overall equation: 2N2O5 → 4NO2 + O2
Experiment shows this is first order:
rate = k[N2O5]
The 2 in front of N2O5 is bookkeeping. Writing a power of 2 here would be wrong.
Before you turn any coefficient into a power, stop and ask one question: is this line a single step, or a summary? If the question calls it an elementary step, you are allowed. If it is the overall equation, you are not.
Worked examples
WORKED EXAMPLE
Classifying the steps of a mechanism
Dinitrogen pentoxide decomposes: 2N2O5 → 4NO2 + O2. A proposed mechanism is:
Step 1: N2O5 → NO2 + NO3
Step 2: NO2 + NO3 → NO + O2 + NO2
Step 3: NO + NO3 → 2NO2
State the molecularity of each step.
Count the particles on the left of each stepStep 1One N₂O₅ particle, falling apart with nothing else involved.UnimolecularStep 2One NO₂ and one NO₃ — two particles.BimolecularStep 3One NO and one NO₃ — two particles.Bimolecularonly the left-hand side counts – step 3 makes 2NO₂, but that is not molecularity
WORKED EXAMPLE
When you may use the coefficients as powers
Using the same reaction, step 1 is the slow step.
(a) Write the rate equation.
(b) A student writes rate = k[N2O5]2 because the overall equation starts with 2N2O5. Explain the error.
(a) Use the slow step, which is elementaryStep 1 is unimolecular: one N₂O₅ particle takes part, so the power is 1.rate = k[N2O5] — first order(b) Where the student went wrongThe 2 in the overall equation is there to balance atoms, not to describe a collision. Two N₂O₅ molecules do not have to meet for the reaction to start — they break up one at a time.Powers may only be taken from an elementary step, never from the overall equationthe give-away is that the experimental order (1) does not match the coefficient (2)
WORKED EXAMPLE
Judging a proposed mechanism
A student proposes that 2NO(g) + O2(g) → 2NO2(g) happens in one step. State the molecularity this would need, and explain why a chemist would be doubtful.
Step 1: Count the particlesTwo NO particles plus one O₂ particle, all in the same step.It would have to be termolecularStep 2: Why that is doubtfulAll three would need to arrive at the same point, at the same instant, with enough energy and the correct orientation. In a gas, that combination is extremely unlikely.A two-step mechanism with two bimolecular steps is far more believable“unlikely” is the right word here, not “impossible” – termolecular steps are rare, not banned
🧠 One word each
Molecularity = particles.Order = powers. Molecularity is something you can picture happening; order is something you can only measure. They agree only when the equation in front of you is a single elementary step, which is exactly the case examiners test.
💡 Exam tip
Count only the left-hand side of the step. Products play no part in molecularity.
Use the proper words — unimolecular, bimolecular, termolecular — not “one-particle” or “two-particle”.
If a question hands you an elementary step and asks for the rate equation, the coefficients become the powers. That is the one time you may do it.
Molecularity is never zero and never a fraction. If your answer is either, you have found an order instead.
Spotting a termolecular step in a proposed mechanism is usually the criticism the question is fishing for.
Watch the spelling: it is termolecular, not tri-molecular.
⚠ Common mix-up
Treating molecularity and order as the same word. One is a particle count, the other is measured behaviour.
Taking powers from the overall equation. Only elementary steps allow it.
Counting the products. Molecularity looks at what goes in, not what comes out.
Giving a molecularity of 0. A step with no particles cannot happen.
Calling a termolecular step impossible. It is rare, not forbidden.
Assuming molecularity tells you the speed. It tells you how many particles, not how fast — a unimolecular step can be the slow one.
Up next: The Arrhenius Equation — the equation that finally puts numbers on the link between temperature, activation energy and the rate constant.
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