IB Chemistry: Catalysts & Reaction Pathways — Every Question Answered

This topic covers how a catalyst speeds up a reaction by opening an alternative pathway with lower activation energy, without being consumed itself. According to the IB Chemistry guide (first assessment 2025), it sits inside Reactivity 2.2, and it's tested through multiple-choice items on Paper 1, structured questions on Paper 2, and data-based analysis on Paper 3.

At SL you explain qualitatively why a catalyst lowers activation energy and sketch the enthalpy profile change. HL students go further, using the Arrhenius equation to show quantitatively how a lower Ea increases the rate constant k, and interpreting catalytic effects on reaction rate with actual numbers rather than description alone.

A homogeneous catalyst is in the same phase as the reactants — nitrogen monoxide catalysing ozone breakdown in the gas-phase atmosphere is the classic IB example. A heterogeneous catalyst is in a different phase, usually a solid speeding up a gas or liquid reaction, like the iron catalyst in the Haber process. Exams often ask you to identify which type applies to a given industrial reaction.

A catalyst provides an alternative reaction pathway with a lower activation energy, so a greater proportion of colliding particles have enough kinetic energy to react at a given temperature. It isn't consumed in the overall reaction and doesn't change the enthalpy change, ΔH, between reactants and products — only the height of the energy barrier changes.

Plot enthalpy on the y-axis and reaction progress on the x-axis, showing the uncatalysed pathway as one hump peaking at its activation energy. Add a second, lower hump for the catalysed pathway that starts and finishes at exactly the same reactant and product enthalpy levels — only the peak height changes, never the overall ΔH.

Enzymes are biological catalysts, mostly proteins, that lower activation energy for biochemical reactions by binding a substrate at their active site. The IB treats this as a specific case of the same catalysis principle — an alternative lower-Ea pathway — but with added specificity from the enzyme's shape and marked sensitivity to temperature and pH that inorganic catalysts generally lack.

The Arrhenius equation, k = Ae^(−Ea/RT), shows that lowering Ea via a catalyst increases the rate constant k at a fixed temperature, since a smaller exponent term gives a larger overall value. HL students are expected to compare catalysed and uncatalysed Ea values and calculate the resulting ratio of rate constants directly.

No — a catalyst never shifts the position of equilibrium or changes the equilibrium constant, Kc. It speeds up the forward and reverse reactions equally, so equilibrium is reached faster but at exactly the same position. Examiners specifically target the 'faster equals more product' confusion in Paper 2 mark schemes.

The most common mistake is drawing a catalysed energy profile that shifts the reactant or product enthalpy level instead of only lowering the activation-energy peak — examiners deduct marks whenever ΔH visibly changes. A close second is claiming the catalyst 'gets used up' or 'shifts equilibrium', both of which contradict how catalysis actually works.

'Sketch' asks for a labelled diagram without exact numerical values — axes, general shape, key labels matter more than precision. 'Explain' demands reasoning linking cause and effect, such as why a lower Ea increases rate. 'Compare and contrast' expects both similarities and differences, for example between homogeneous and heterogeneous catalysis, and loses marks if you only cover one.

Yes — catalysed reaction rates are a genuinely popular IA choice, from the decomposition of hydrogen peroxide with manganese(IV) oxide or catalase, to iodine-clock variations at different catalyst concentrations. Examiners favour these investigations because they generate clear quantitative data for the assessment criteria, provided temperature and concentration are controlled carefully across trials.

Catalysts and reaction pathways is one of the more approachable kinetics topics — most students find the ideas intuitive once they've drawn a proper energy profile diagram a few times. The harder part at HL isn't the concept itself, it's applying the Arrhenius equation accurately under exam time pressure, especially when the question gives awkward units.

The most effective approach combines drawing energy profile diagrams from memory, working through past-paper data-based questions on rate and catalysis, and testing recall of exact definitions like activation energy and catalyst. Short, spaced sessions across several weeks beat one long cramming block, especially since this topic rewards diagram accuracy over rote memorisation.