AHL: Catalysis and reaction mechanisms

IB Chemistry HL· R3.6· 20 min read

1. Fundamentals of Catalytic Action★★☆☆☆⏱ 5 min

📘 Definition

Catalyst

A substance that increases the rate of a chemical reaction, without being consumed in the overall reaction, remaining chemically unchanged at the end.

Example:

Manganese(IV) oxide in the decomposition of aqueous hydrogen peroxide.

Catalysts work by providing an entirely new reaction mechanism for the reaction, with a lower overall activation energy ( $E_{a}$ ) than the uncatalyzed pathway. Because $E_{a}$ is lower, a greater proportion of particles have energy equal to or greater than $E_{a}$ at a given temperature, leading to a faster rate. From the Arrhenius equation, a lower $E_{a}$ gives a larger rate constant $k$ :

k = A e^{-E_a/RT}

📐 Worked Example

An uncatalyzed reaction has an activation energy of 120 kJ mol⁻¹. A catalyst lowers $E_{a}$ by 30 kJ mol⁻¹ at 298 K. Calculate the factor by which the rate constant increases, assuming the pre-exponential factor $A$ is unchanged. ( $R = 8.31$ J K⁻¹ mol⁻¹)

1
Take the ratio of the catalyzed and uncatalyzed rate constants, and simplify:
2
$\ln\left(\frac{k_{cat}}{k_{uncat}}\right) = \frac{E_{a,uncat} - E_{a,cat}}{RT}$
3
Convert the activation energy difference to J mol⁻¹ and substitute values:
4
$E_{a,uncat} - E_{a,cat} = 30000 \text{ J mol}^{-1} \\ \ln\left(\frac{k_{cat}}{k_{uncat}}\right) = \frac{30000}{8.31 \times 298} \approx 12.13$
5
Exponentiate both sides to get the ratio:
6
$\frac{k_{cat}}{k_{uncat}} = e^{12.13} \approx 1.8 \times 10^5$

Exam tip:

Always draw reaction profile diagrams with the same starting and ending enthalpy for catalyzed and uncatalyzed reactions, only the peak height changes.

2. Homogeneous Catalysis★★☆☆☆⏱ 6 min

📘 Definition

Homogeneous Catalysis

Catalysis where the catalyst is in the same physical phase (state) as all reactants.

Example:

Acid catalysis of ester hydrolysis, where all species are liquid.

In homogeneous catalysis, the catalyst reacts with reactants to form unstable reaction intermediates. These intermediates then react further to form the final products and regenerate the original catalyst, which is therefore not consumed overall. Common examples include acid-base catalysis, chlorine radical catalysis of ozone depletion, and enzyme catalysis in aqueous biological systems.

📐 Worked Example

The oxidation of sulfur dioxide by oxygen follows this two-step mechanism catalyzed by nitrogen(II) oxide (NO): Step 1: $NO (g) + O_{2} (g) \to NO_{2} (g) + O (g)$ Step 2: $NO_{2} (g) + SO_{2} (g) \to NO (g) + SO_{3} (g)$ Explain why NO is the catalyst, and identify the reaction intermediate.

1
Add the two step equations to get the overall reaction, canceling species that appear on both sides:
2
$\begin{align*} \cancel{\text{NO}(g)} + \text{O}_2(g) &\rightarrow \cancel{\text{NO}_2(g)} + \text{O}(g) \\ \cancel{\text{NO}_2(g)} + \text{SO}_2(g) &\rightarrow \cancel{\text{NO}(g)} + \text{SO}_3(g) \\ \hline \text{O}_2(g) + \text{SO}_2(g) &\rightarrow \text{O}(g) + \text{SO}_3(g) \end{align*}$
3
NO is consumed in the first step and regenerated in the second step, and does not appear in the overall reaction. This confirms it is the catalyst. $NO_{2}$ is produced in the first step and consumed in the second, so it is the reaction intermediate.

3. Heterogeneous Catalysis★★★☆☆⏱ 6 min

📘 Definition

Heterogeneous Catalysis

Catalysis where the catalyst is in a different physical phase from the reactants, most commonly a solid catalyst with gaseous or liquid reactants.

Example:

Solid iron catalyst in the Haber process for ammonia synthesis.

Heterogeneous catalysis proceeds via these key steps: 1. Reactant molecules diffuse to the catalyst surface, 2. Reactant molecules adsorb (bind) to active sites on the catalyst surface, 3. Adsorption weakens bonds within reactant molecules and orients them favorably for reaction, 4. Products form from the reacted species, 5. Product molecules desorb from the surface, freeing active sites for new reactants.

📐 Worked Example

Outline why the iron catalyst in the Haber process ( $N_{2} (g) + 3 H_{2} (g) ⇌ 2 NH_{3} (g)$ ) is classified as heterogeneous, and describe its mechanism of action.

1
Classification: The reactants ( $N_{2}$ and $H_{2}$ ) are gaseous, while the iron catalyst is solid. Since they are in different phases, the catalysis is heterogeneous.
2
Mechanism steps:
1. Gaseous $N_{2}$ and $H_{2}$ diffuse to the solid iron surface
2. Molecules adsorb onto active sites on the iron surface
3. Adsorption weakens the strong covalent bonds in $N_{2}$ and $H_{2}$ , breaking them into N and H atoms
4. N and H atoms react to form $NH_{3}$ molecules
5. $NH_{3}$ molecules desorb from the iron surface and diffuse away as gas

4. Industrial and Biological Catalysis★★★☆☆⏱ 3 min

Catalysis is the foundation of most industrial chemical processes, as it allows reactions to proceed at viable rates at lower temperatures and pressures, reducing energy costs and carbon emissions. Biological catalysts are called enzymes: globular proteins that are highly specific to their substrate, and work under mild physiological conditions.

Industrial Process	Catalysis Type	Catalyst Used
Haber process (ammonia)	Heterogeneous	Solid iron
Contact process (sulfuric acid)	Heterogeneous	Vanadium(V) oxide
Cracking of alkanes	Heterogeneous	Zeolite
Esterification	Homogeneous	Hydrogen ions ( $H^{+}$ )
Catalytic converter (cars)	Heterogeneous	Platinum/palladium

✓ Quick check

Which of the following is a property of all catalysts?
- They are consumed in the overall reaction
- They lower the activation energy of the reaction
- They change the equilibrium yield of product
- They are in a different phase to reactants
Reveal answer
1 —
Correct! Only this statement is true for all catalysts.

5. Common Pitfalls

Wrong move:

Claiming catalysts never participate in the reaction at all.

Why:

Catalysts do participate in the reaction, forming intermediates, but are regenerated unchanged at the end.

Correct move:

State that catalysts are not consumed in the overall reaction, and remain chemically unchanged.

Wrong move:

Claiming catalysts change the position of equilibrium or the enthalpy change of a reaction.

Why:

Catalysts speed up forward and reverse reactions equally, so they do not affect equilibrium position or $Δ H$ .

Correct move:

State that catalysts only lower activation energy and increase the rate at which equilibrium is reached.

Wrong move:

Confusing catalysts and intermediates in multi-step mechanisms.

Why:

The order of appearance/production is reversed for the two species.

Correct move:

Catalyst: consumed first, regenerated later. Intermediate: produced first, consumed later.

Wrong move:

Using "absorption" instead of "adsorption" for heterogeneous catalysis.

Why:

Absorption describes molecules penetrating into the bulk of a material, while adsorption describes molecules binding to the surface.

Correct move:

Always use adsorption when referring to reactants binding to a solid catalyst surface.

Wrong move:

Assuming all biological catalysts are heterogeneous.

Why:

Most enzymes are dissolved in the same aqueous phase as their substrates, so they are homogeneous catalysts.

Correct move:

Classify catalysis based on phase of catalyst vs reactants, not whether it is biological or industrial.

6. Quick Reference Cheatsheet

Catalysis Type	Phase Relationship	Key Features	Common Example
Homogeneous	Same phase as reactants	Forms reaction intermediates, catalyst regenerated	Acid-catalyzed ester hydrolysis
Heterogeneous	Different phase to reactants	Adsorption to surface active sites, desorption of products	Iron catalyst for Haber process
All Catalysts	Any	Lower $E_{a}$ via new pathway, not consumed overall	Any catalytic reaction
Key Exam Fact	Any	No effect on $Δ H$ or equilibrium position	All exam questions on catalysis

7. Frequently Asked

Do catalysts change the position of equilibrium?

No. Catalysts speed up both the forward and reverse reactions equally, so equilibrium is reached faster, but the position of equilibrium and final yield do not change.

What is the difference between a catalyst and a reaction intermediate?

A catalyst is consumed in an early step of the mechanism and regenerated in a later step. A reaction intermediate is produced in an early step and consumed in a later step. Neither appear in the overall reaction equation.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2025 · Paper 2
Compare catalysis types
2024 · Paper 1
Identify catalyst in mechanism
2023 · 2
Explain heterogeneous catalysis

What's Next

This sub-topic builds on your foundational knowledge of kinetics and multi-step reaction mechanisms, and connects to core topics including equilibrium, industrial chemistry, and organic reaction mechanisms. Understanding how catalysts work is also critical for exploring biological topics like enzyme action, a common IB Chemistry option. Catalysis is a frequent topic in both Paper 1 and Paper 2 exam questions, so mastering the distinction between homogeneous and heterogeneous catalysis and how to identify catalysts in mechanisms is key for achieving a high score. Next, you can explore more advanced concepts in reaction kinetics or apply your knowledge to organic reaction mechanisms.