# AHL: Catalysis and reaction mechanisms

> IB Chemistry HL · IB Chemistry HL (2025 syllabus)
> Source: https://www.owlsprep.com/study/ib-chemistry-hl-u6-ahl-catalysis-and-reaction-mechanisms/

This sub-topic explores how catalysts accelerate reactions by providing alternative reaction pathways with lower activation energy. You will learn to classify catalysis types, identify catalysts in mechanisms, and describe their roles in industrial and biological systems, a frequently tested HL topic.

**Prerequisites:** [Activation energy and collision theory](https://www.owlsprep.com/study/ib-chemistry-hl-kinetics-collision-theory/); [Multi-step reaction mechanisms](https://www.owlsprep.com/study/ib-chemistry-hl-reaction-mechanisms-multi-step/)

## Learning objectives

- Explain how catalysts increase reaction rate via alternative reaction pathways
- Distinguish between homogeneous and heterogeneous catalysis
- Identify catalysts and intermediates in multi-step reaction mechanisms
- Describe examples of catalysis in industrial and biological systems

## Fundamentals of Catalytic Action

**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. $$\frac{k_{cat}}{k_{uncat}} = e^{12.13} \approx 1.8 \times 10^5$$

> **tip**
>
> Catalysts do not change the overall enthalpy change ($\Delta H$) of the reaction, only the activation energy of the pathway.

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

## Homogeneous Catalysis

**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: $\text{NO}(g) + \text{O}_2(g) \rightarrow \text{NO}_2(g) + \text{O}(g)$
Step 2: $\text{NO}_2(g) + \text{SO}_2(g) \rightarrow \text{NO}(g) + \text{SO}_3(g)$
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. $\text{NO}_2$ is produced in the first step and consumed in the second, so it is the reaction intermediate.

## Heterogeneous Catalysis

**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.

> **warning**
>
> Catalyst poisoning occurs when impurities bind permanently to active sites, blocking reactants and reducing catalytic activity. For example, lead poisons the platinum catalyst in car catalytic converters.

**Worked example:** Outline why the iron catalyst in the Haber process ($\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)$) is classified as heterogeneous, and describe its mechanism of action.

1. Classification: The reactants ($\text{N}_2$ and $\text{H}_2$) are gaseous, while the iron catalyst is solid. Since they are in different phases, the catalysis is heterogeneous.
2. Mechanism steps:
1. Gaseous $\text{N}_2$ and $\text{H}_2$ diffuse to the solid iron surface
2. Molecules adsorb onto active sites on the iron surface
3. Adsorption weakens the strong covalent bonds in $\text{N}_2$ and $\text{H}_2$, breaking them into N and H atoms
4. N and H atoms react to form $\text{NH}_3$ molecules
5. $\text{NH}_3$ molecules desorb from the iron surface and diffuse away as gas

## Industrial and Biological Catalysis

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 ($\text{H}^+$) |
| Catalytic converter (cars) | Heterogeneous | Platinum/palladium |

**Check your understanding**

1. 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

   *Answer:* They lower the activation energy of the reaction

   *Why:* Correct! Only this statement is true for all catalysts.

## Common pitfalls

- **Wrong:** Claiming catalysts never participate in the reaction at all.
  - Why it fails: Catalysts do participate in the reaction, forming intermediates, but are regenerated unchanged at the end.
  - Correct: State that catalysts are not consumed in the overall reaction, and remain chemically unchanged.
- **Wrong:** Claiming catalysts change the position of equilibrium or the enthalpy change of a reaction.
  - Why it fails: Catalysts speed up forward and reverse reactions equally, so they do not affect equilibrium position or $\Delta H$.
  - Correct: State that catalysts only lower activation energy and increase the rate at which equilibrium is reached.
- **Wrong:** Confusing catalysts and intermediates in multi-step mechanisms.
  - Why it fails: The order of appearance/production is reversed for the two species.
  - Correct: Catalyst: consumed first, regenerated later. Intermediate: produced first, consumed later.
- **Wrong:** Using "absorption" instead of "adsorption" for heterogeneous catalysis.
  - Why it fails: Absorption describes molecules penetrating into the bulk of a material, while adsorption describes molecules binding to the surface.
  - Correct: Always use adsorption when referring to reactants binding to a solid catalyst surface.
- **Wrong:** Assuming all biological catalysts are heterogeneous.
  - Why it fails: Most enzymes are dissolved in the same aqueous phase as their substrates, so they are homogeneous catalysts.
  - Correct: Classify catalysis based on phase of catalyst vs reactants, not whether it is biological or industrial.

## 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 $\Delta H$ or equilibrium position | All exam questions on 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.

- [AHL: Spectroscopic identification of organic compounds](https://www.owlsprep.com/study/ib-chemistry-hl-u6-ahl-spectroscopic-identification-of-organic/)
- [Practical and investigative skills](https://www.owlsprep.com/study/ib-chemistry-hl-u7-overview/)
- [Experimental Design](https://www.owlsprep.com/study/ib-chemistry-hl-u7-experimental-design/)

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