IB Chemistry SL & HLTopic 5 — The Rate of Chemical ChangePaper 1 & 2Core skill~10 min read
Energy Profiles With and Without Catalysts
A catalyst does not shove the reactants over the hill. It shows them a lower path around it. Once you picture it that way, every catalyst question on the paper — including the tricky ones about ΔH and yield — answers itself.
📘 What you need to know
A catalyst speeds up a reaction and is chemically unchanged at the end.
It works by providing an alternative pathway with a lower activation energy.
On a profile, the catalysed route has a lower peak but the same reactants and products levels.
ΔH is completely unchanged by a catalyst.
Homogeneous catalyst = same phase as the reactants. Heterogeneous = different phase.
Because both directions get the lower route, a catalyst does not shift equilibrium or change the yield.
Catalysts cut industrial energy use, waste and unwanted side products.
What the profile looks like
Draw both routes on the same axes and the picture is very clear. Two humps, two different heights, but they start and finish at exactly the same levels.
The single ΔH arrow is the whole point of this diagram. Both curves leave the same reactants line and land on the same products line, so the energy released is identical either way.
Say it carefully. The catalysed route has its own lower activation energy — the original pathway still exists with its original Ea. That is why the accepted wording is “provides an alternative pathway of lower activation energy” rather than “lowers the activation energy”.
How a catalyst actually pulls it off
The profile tells you what happens but not how. For a solid catalyst working on gases, the trick is that the reactants stick to the surface, and being stuck there weakens their bonds before they even meet each other.
Two jobs are being done at once here: the reactants are held in the right orientation, and their bonds are pre-weakened. Both make a successful collision far more likely.
This picture also explains why surface area matters so much for solid catalysts, and why they are usually spread thinly over a honeycomb support — more surface means more places for the reactants to stick.
Homogeneous and heterogeneous
The words look intimidating but the split is simple: is the catalyst in the same physical state as the reactants, or not?
Type
Meaning
Example
Practical note
Homogeneous
Catalyst is in the same phase as the reactants
An acid catalyst in solution with dissolved reactants
Mixes perfectly, but can be awkward to separate from the product afterwards
Heterogeneous
Catalyst is in a different phase from the reactants
A solid metal catalyst with gaseous reactants
Easy to separate and reuse, so it suits continuous industrial processes
Where you meet them
Enzymes are biological catalysts. They let reactions in living cells run fast at body temperature, and industry borrows them to run processes at lower temperatures and pressures than would otherwise be needed.
Transition metals are common industrial catalysts because they can shift between several oxidation states. That lets them accept and give back electrons, opening up alternative low-energy routes for redox reactions.
Why catalysts matter beyond the exam
A lower activation energy means the reaction runs acceptably fast at a lower temperature. That has three knock-on effects industry cares about a great deal:
Less energy used — lower temperatures and pressures mean less fuel burnt and less carbon dioxide released.
Less waste — the catalyst is not consumed, and only a small amount is ever needed.
Better selectivity — a catalyst can favour the pathway you want and leave the side reactions alone, so more of your raw material ends up as useful product.
Rate versus yield, one more time. A catalyst gets you to equilibrium faster, but it speeds up the forward and reverse reactions by the same factor. The position of equilibrium, and therefore the yield, does not move. If a question asks how to increase the yield, a catalyst is the wrong answer.
Worked examples
WORKED EXAMPLE
Identifying the arrows on a profile
A profile shows an exothermic reaction with and without a catalyst. Arrow p runs from the reactants level to the higher peak, arrow q from the reactants level to the lower peak, and arrow r from the reactants level down to the products level. Which arrow is ΔH, and which is Ea for the catalysed reaction?
Step 1: ΔH connects reactants and products
That is arrow r — it ignores both peaks.
Step 2: the catalysed route is the lower peak
Its Ea runs from reactants up to that lower peak, which is arrow q.
ΔH = r, and Ea(catalysed) = qarrow p is the uncatalysed Ea — the biggest of the three
WORKED EXAMPLE
Working out both barriers
Without a catalyst, a reaction has Ea = 185 kJ mol−1. A catalyst lowers this to 95 kJ mol−1. The reaction has ΔH = −70 kJ mol−1. Find Ea for the reverse reaction on the catalysed route, and state the value of ΔH with the catalyst present.
Step 1: reverse barrier on the catalysed route
Ea(reverse) = Ea(forward) − ΔH
= 95 − (−70) = 165 kJ mol⁻¹Step 2: ΔH with the catalyststill −70 kJ mol⁻¹Ea(reverse, catalysed) = 165 kJ mol⁻¹; ΔH unchanged at −70 kJ mol⁻¹the catalyst lowered both barriers by the same 90 kJ mol⁻¹, which is why ΔH cannot move
WORKED EXAMPLE
Explaining why ΔH does not change
Explain why adding a catalyst changes the rate of a reaction but not its enthalpy change. (2 marks)
Mark 1: what the catalyst does
It provides an alternative pathway with a lower activation energy, so a greater proportion of collisions is successful and the rate increases.
Mark 2: what it does not doΔH depends only on the energies of the reactants and products, and the catalyst changes neither — it only alters the route between them.“only the route changes, not the start or the finish” is the sentence to remember
💡 Exam tip
Draw both curves from the same reactants line and to the same products line. If your two curves finish at different heights, you have implied a catalyst changes ΔH, and the diagram loses its marks.
Label the two curves catalysed and uncatalysed. An unlabelled pair of humps is ambiguous.
Use the phrase alternative pathway of lower Ea every time.
Say the catalyst is chemically unchanged at the end rather than “not involved” — it does take part, then gets regenerated.
If asked for a definition of homogeneous or heterogeneous, use the word phase, not “state” or “type”.
Watch for questions that quietly ask about yield or equilibrium position. A catalyst affects neither.
⚠ Common mix-up
Drawing the catalysed curve finishing lower. Very common, and it contradicts ΔH being unchanged.
Saying the catalyst “gives the particles more energy”. It does not add energy to anything; it lowers the bar they have to clear.
Saying it “lowers the activation energy of the reaction”. It offers a different reaction pathway that happens to have a lower one.
Thinking a catalyst increases yield. It speeds up both directions equally.
Confusing homogeneous and heterogeneous. Homo means same phase; hetero means different.
Claiming the catalyst is used up. If it were, it would be a reactant, not a catalyst.
Assuming a catalyst works for any reaction. Catalysts are specific — the pathway has to suit that particular reaction.
Up next: Maxwell-Boltzmann Distributions — the graph that shows exactly which particles can get over the hill, and the clearest way to explain both temperature and catalysts in one picture.
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