# R1: What drives chemical reactions?

> IB Chemistry HL · IB Chemistry HL (2025 syllabus)
> Source: https://www.owlsprep.com/study/ib-chemistry-hl-u4-overview/
> Weight: n/a

This unit explores the energy and disorder changes that determine whether a chemical reaction proceeds spontaneously. You will learn core and advanced methods to calculate energy changes and predict reaction direction, answering the core question of what drives chemical change.

**Prerequisites:** Basic stoichiometry; Introduction to chemical bonding

## Learning objectives

- Explain how energy and disorder changes determine the direction of spontaneous chemical reactions
- Calculate enthalpy changes for reactions using multiple experimental and theoretical methods
- Predict reaction spontaneity using entropy and Gibbs free energy concepts
- Solve extended AHL calculation problems involving enthalpy, entropy and free energy changes

## Unit at a Glance

This unit follows a logical progression from experimental measurement of heat changes to theoretical prediction of reaction spontaneity. We start with core concepts of enthalpy and experimental calorimetry, then build up to indirect methods for calculating enthalpy when experiments are not possible.

After covering enthalpy calculation methods, we introduce entropy as a measure of disorder, then extend to Gibbs free energy (the key predictor of spontaneity) for HL students. Every concept builds on the previous to answer the unit's core question.

This unit includes the following core and AHL sub-topics:
- [Enthalpy change and calorimetry](https://www.owlsprep.com/study/ib-chemistry-hl-u4-enthalpy-change-and-calorimetry/) — Define enthalpy change and learn to measure heat transfer in reactions via experimental calorimetry.
- [Hess's law](https://www.owlsprep.com/study/ib-chemistry-hl-u4-hess-s-law/) — Calculate overall enthalpy change for reactions using enthalpy values of intermediate steps.
- [Entropy and reaction spontaneity](https://www.owlsprep.com/study/ib-chemistry-hl-u4-entropy-and-reaction-spontaneity/) — Introduce entropy as a measure of disorder and explain its role in determining spontaneity.
- [Bond enthalpies](https://www.owlsprep.com/study/ib-chemistry-hl-u4-bond-enthalpies/) — Estimate reaction enthalpy from average bond enthalpies of broken and formed bonds.
- [Energy cycles](https://www.owlsprep.com/study/ib-chemistry-hl-u4-energy-cycles/) — Use energy cycles to calculate enthalpy changes of formation and combustion.
- [AHL: Extended enthalpy and entropy calculations](https://www.owlsprep.com/study/ib-chemistry-hl-u4-ahl-extended-enthalpy-and-entropy/) — Practice advanced calculations involving standard states and entropy changes of reaction.
- [AHL: Gibbs free energy and reaction spontaneity](https://www.owlsprep.com/study/ib-chemistry-hl-u4-ahl-gibbs-free-energy-and/) — Combine enthalpy and entropy to calculate Gibbs free energy and predict reaction spontaneity.

## Common pitfalls

- **Wrong:** Confusing endothermic/exothermic sign conventions for enthalpy change
  - Why it fails: Different conventions across sources often lead to flipped signs in final calculations
  - Correct: Use IB convention: $\Delta H < 0$ for exothermic (heat released), $\Delta H > 0$ for endothermic (heat absorbed)
- **Wrong:** Flipping the bond enthalpy calculation formula
  - Why it fails: Forgetting that bond breaking is endothermic and bond forming is exothermic flips the final sign of $\Delta H$
  - Correct: Always use: $\Delta H = \sum (E_{\text{bonds broken}}) - \sum (E_{\text{bonds formed}})$
- **Wrong:** Assuming negative enthalpy means a reaction is always spontaneous
  - Why it fails: Enthalpy alone does not determine spontaneity; entropy and temperature also play critical roles
  - Correct: Always use Gibbs free energy change ($\Delta G$) to confirm reaction spontaneity

## Cheatsheet

| Concept | Key Formula/Relationship |
| --- | --- |
| Calorimetry enthalpy change | $q = mc\Delta T$, $\Delta H = -q/n$ (constant pressure) |
| Hess's Law | $\Delta H_{\text{total}} = \sum \Delta H_{\text{individual steps}}$ |
| Reaction enthalpy from bond enthalpies | $\Delta H = \sum E_{\text{bonds broken}} - \sum E_{\text{bonds formed}}$ |
| Standard entropy change of reaction | $\Delta S^\circ = \sum S^\circ_{\text{products}} - \sum S^\circ_{\text{reactants}}$ |
| Gibbs Free Energy Change | $\Delta G = \Delta H - T\Delta S$ |
| Standard Gibbs Free Energy of reaction | $\Delta G^\circ = \sum \Delta G^\circ_f_{\text{products}} - \sum \Delta G^\circ_f_{\text{reactants}}$ |
| Spontaneity condition | Reaction is spontaneous if $\Delta G < 0$ |

## What's next

Begin this unit with the first sub-topic, which introduces core concepts of enthalpy change and experimental measurement that all later topics in this unit rely on. After you complete all sub-topics in this unit and master the concepts of reaction spontaneity, you can proceed to the first topic of the next unit on reaction kinetics.

- [Enthalpy change and calorimetry](https://www.owlsprep.com/study/ib-chemistry-hl-u4-enthalpy-change-and-calorimetry/)
- [Hess's Law](https://www.owlsprep.com/study/ib-chemistry-hl-u4-hess-s-law/)
- [Entropy and reaction spontaneity](https://www.owlsprep.com/study/ib-chemistry-hl-u4-entropy-and-reaction-spontaneity/)

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