# Chemical energetics

> CIE A-Level Chemistry · CIE 9701
> Source: https://www.owlsprep.com/study/cie-9701-u5-overview/
> Weight: n/a

This unit introduces core thermochemical concepts for chemical reactions, covering enthalpy changes, Hess' law, and bond enthalpies, foundational for understanding energy changes across all chemistry topics.

**Prerequisites:** AS-level basic energy changes and chemical bonding concepts

## Learning objectives

- Define, classify, and calculate different types of enthalpy change from experimental data
- Apply Hess' law to construct enthalpy cycles and calculate unknown enthalpy changes
- Relate average bond enthalpy values to overall reaction enthalpy and relative bond strength

## Unit at a glance

This unit builds step-by-step understanding of energy changes that occur during chemical reactions. You will start by learning how to define and measure different types of enthalpy change, before moving to the core principle of Hess' law, which lets you calculate unknown enthalpy values even when they cannot be measured directly. The final topic connects bond breaking and bond forming to overall reaction enthalpy, explaining why some reactions release energy and others absorb it.

This unit contains the following core sub-topics:
- [Enthalpy changes](https://www.owlsprep.com/study/cie-9701-u5-enthalpy-changes/) — Learn to define, classify, and calculate different types of standard enthalpy change from experimental data.
- [Hess' law](https://www.owlsprep.com/study/cie-9701-u5-hess-law/) — Apply Hess' law to construct enthalpy cycles and calculate unknown enthalpy changes for reactions.
- [Bond enthalpies](https://www.owlsprep.com/study/cie-9701-u5-bond-enthalpies/) — Use average bond enthalpy values to calculate reaction enthalpy and compare relative bond strengths.

## Common pitfalls

- **Wrong:** Reversing the sign convention for bond breaking and making
  - Why it fails: This leads to incorrect final values for reaction enthalpy calculated from bond enthalpies
  - Correct: Remember: bond breaking is endothermic ($+\Delta H$), bond making is exothermic ($-\Delta H$)
- **Wrong:** Ignoring reaction stoichiometry in Hess' law calculations
  - Why it fails: Enthalpy change scales with moles of reactant, so unadjusted values give incorrect results
  - Correct: Always multiply enthalpy values by the molar coefficients in your target reaction equation

## Cheatsheet

| Concept | Key Formula/Rule |
| --- | --- |
| Experimental enthalpy change | $q = mc\Delta T$, $\Delta H = -q/n$ (per mole of limiting reactant) |
| Hess' Law Statement | Total enthalpy change is independent of the reaction path taken |
| Enthalpy from formation values | $\Delta H_r = \sum \Delta H_f(\text{products}) - \sum \Delta H_f(\text{reactants})$ |
| Enthalpy from combustion values | $\Delta H_r = \sum \Delta H_c(\text{reactants}) - \sum \Delta H_c(\text{products})$ |
| Enthalpy from bond enthalpies | $\Delta H_r = \sum(\text{bonds broken}) - \sum(\text{bonds formed})$ |

## What's next

Begin your study of this unit by working through the first sub-topic on enthalpy changes, which lays the foundational definitions and calculations needed for the rest of the unit. After you complete all three sub-topics here, you can move on to the next unit in the CIE A-Level Chemistry syllabus.

- [Enthalpy changes (first sub-topic of this unit)](https://www.owlsprep.com/study/cie-9701-u5-enthalpy-changes/)
- [Hess' Law](https://www.owlsprep.com/study/cie-9701-u5-hess-law/)
- [Bond Enthalpies](https://www.owlsprep.com/study/cie-9701-u5-bond-enthalpies/)

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