Catalysts feel like the cheat code of IB Chemistry: same reactants, same products, just faster. Then you meet a question where the rate suddenly drops and everyone in the diagram looks innocent. That’s the moment catalyst poisoning matters. Not because it’s complicated, but because it’s realistic -- in labs, in cars, and in the big industrial processes the IB loves to mention.
In this guide, you’ll learn catalyst poisoning in plain language, keep the definitions exam-ready for IB Chemistry, and connect it to the kinds of explanations that earn marks.
A catalyst chair with “reserved” poisons on the cushions
Catalyst poisoning (IB Chemistry definition you can actually use)
In IB Chemistry, catalyst poisoning is when a catalyst becomes less effective because another substance binds to its surface and blocks the active sites.
That’s the entire idea. Once active sites are blocked:
reactant particles can’t adsorb
fewer successful surface reactions happen per second
the reaction rate decreases
yield and efficiency often drop (especially in industry)
A heterogeneous catalyst (usually a solid) works because its surface has active sites: specific places where reactants stick, bonds weaken, and reactions become easier.
A clean way to describe heterogeneous catalysis (and set up poisoning) is:
adsorption of reactants onto the surface
reaction on the surface (often with weakened bonds)
desorption of products
When a poison takes up those surface locations, it’s not that the catalyst “stops existing.” It’s that the catalyst can’t do its job.
In IB Chemistry, you’ll usually classify poisoning as reversible or irreversible. The distinction is all about how strongly the poison sticks.
Reversible catalyst poisoning
Reversible poisoning happens when the poison binds weakly enough that it can be removed by changing conditions (for example, heating or altering the mixture). The catalyst can often be regenerated.
A common example students meet is carbon monoxide (CO) acting as a poison for some metal catalysts under certain conditions.
Irreversible catalyst poisoning
Irreversible poisoning is when the poison binds strongly and essentially “permanently” blocks the surface. In practice, the catalyst must be replaced or undergo heavy industrial regeneration.
Classic examples include:
lead compounds damaging catalytic converter catalysts
sulfur compounds poisoning industrial metal catalysts
heavy metals binding to enzyme active sites
Reversible vs irreversible poisoning joke
Common catalyst poisons you should recognize
The specific poison depends on the catalyst and conditions, but in IB Chemistry these are the names worth remembering:
Metal (industrial) catalysts
sulfur compounds
lead compounds
phosphorus compounds
chlorine and chlorinated hydrocarbons
carbon monoxide (sometimes)
Enzymes (biological catalysts)
heavy metal ions (e.g., Hg²⁺, Pb²⁺)
cyanide
certain inhibitors/drugs
The exam move is to say what they do: they bind to active sites and reduce adsorption of reactants.
Real examples that make IB Chemistry questions feel easier
Industrial contexts show up in IB Chemistry because they force you to talk about efficiency, purification, and economics.
Haber process (ammonia)
In the Haber process, the iron catalyst can be poisoned by impurities such as sulfur compounds and carbon oxides (CO, CO₂). This is why reactants are purified before entering the reactor. In an exam, that sentence is basically the whole markscheme.
Catalytic converters
Catalytic converters use Pt, Pd, and Rh catalysts to convert harmful exhaust gases into less harmful products. They are poisoned by lead and sulfur compounds, which is one reason unleaded petrol became essential.
Hydrogenation reactions
Nickel catalysts used in hydrogenation can be poisoned by sulfur impurities in oils and fats. It’s a neat reminder that “poisoning” isn’t only about gases.
Use this when you see poisoning in a structured response:
Define poisoning: blocked active sites
Mention adsorption decreases
State rate decreases, catalyst activity drops
Classify reversible vs irreversible
Give a relevant example (Haber, converter, hydrogenation, enzyme)
Add one prevention method (purification or regeneration)
If you want to drill the wording with real prompts, the RevisionDojo Questionbank is ideal for kinetics-style explanations under time pressure.
Using RevisionDojo to master IB Chemistry catalysis faster
Catalyst poisoning is small, but it connects to a lot: kinetics, adsorption, industrial chemistry, and enzyme inhibition. That’s why it’s worth revising in a connected way.
On RevisionDojo, you can tighten this topic quickly using:
Is catalyst poisoning the same as the catalyst being “used up”?
In IB Chemistry, catalysts are not consumed overall, because they are regenerated in the mechanism. But poisoning is a different idea: the catalyst becomes inactive because active sites are blocked, not because the catalyst was converted into products. That’s why you can have a catalyst present in the reactor but still see a lower reaction rate. In exam terms, say: “the catalyst is not consumed chemically but can be deactivated by poisoning.” This is also why industrial plants invest in purification systems. The catalyst still exists, it just can’t provide the same alternative pathway effectively.
Why do poisons bind so strongly to catalysts?
Many poisons form strong interactions with the catalyst surface, especially with transition metals that adsorb species efficiently. In IB Chemistry, the key language is that poisons “bind to active sites,” which prevents reactants from adsorbing. Strong binding can be due to favourable bonding between the poison and the metal surface or active site geometry. The practical consequence is that reactants lose access to the sites where bonds would normally weaken and react. When that happens, the surface mechanism slows dramatically. So the “strength” matters because it determines whether poisoning is reversible or irreversible.
Can a poisoned catalyst be recovered?
Sometimes, yes -- if the poisoning is reversible. In that case, heating, changing conditions, or industrial regeneration steps can remove the poison and reopen active sites. But in IB Chemistry, you must also be ready to state the opposite: irreversible poisoning often requires catalyst replacement. The difference is whether the poison is weakly adsorbed or strongly bound. A good exam answer explicitly names “regeneration” and connects it to reversible poisoning. If you’re unsure, state what you’d expect: stronger binding usually means harder to remove.
Conclusion: the simplest way to remember catalyst poisoning
Catalyst poisoning is just this: in IB Chemistry, a poison blocks the catalyst’s active sites, so reactants can’t adsorb and the reaction rate falls. From there, everything else is detail -- reversible vs irreversible, real industrial examples, and prevention by purification or regeneration.
If you want this topic to feel automatic under exam time pressure, use RevisionDojo’s Study Notes, Flashcards, and Questionbank to rehearse the exact wording IB Chemistry rewards -- then test it with Mock Exams and Predicted Papers until it sticks.
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