# Chemical equilibria (AS)

> CIE A-Level Chemistry · AS Level
> Source: https://www.owlsprep.com/study/cie-9701-u7-overview/
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This unit introduces core equilibrium concepts for reversible chemical reactions, foundational to understanding industrial processes, acid-base chemistry, and solubility in later A-Level topics.

**Prerequisites:** Understanding of reversible reactions and mole calculations; Basic fundamentals of reaction rates

## Learning objectives

- Explain the core characteristics of dynamic equilibrium in reversible reactions
- Predict shifts in equilibrium position using Le Chatelier's principle for changing reaction conditions
- Write expressions for and calculate equilibrium constants, and interpret their physical meaning

## Unit at a glance

Many chemical reactions do not go to completion, instead reaching a steady balanced state called equilibrium. This unit follows a logical learning arc: you will first build the conceptual foundation of what dynamic equilibrium is, then learn to predict how equilibrium responds to changing conditions, and finally quantify equilibrium positions using mathematical constants.

This unit is split into 3 connected sub-topics:
- [Dynamic equilibrium characteristics](https://www.owlsprep.com/study/cie-9701-u7-dynamic-equilibrium-characteristics/) — Learn the defining properties of dynamic equilibrium in closed reversible systems.
- [Le Chatelier's principle](https://www.owlsprep.com/study/cie-9701-u7-le-chatelier-s-principle/) — Predict equilibrium shifts from changes in concentration, temperature, pressure and catalyst presence.
- [Equilibrium constants](https://www.owlsprep.com/study/cie-9701-u7-equilibrium-constants/) — Write $K_c$ and $K_p$ expressions, calculate their values, and interpret what they indicate about equilibrium position.

## Common pitfalls

- **Wrong:** Claiming dynamic equilibrium can form in an open system
  - Why it fails: Open systems allow reactants or products to escape, so a steady state cannot be maintained
  - Correct: Only recognize dynamic equilibrium in closed systems for CIE exam questions
- **Wrong:** Including solid or pure liquid concentrations in equilibrium constant expressions
  - Why it fails: The concentration of solids and pure liquids is constant at a fixed temperature
  - Correct: Omit all solids and pure liquids from $K_c$ and $K_p$ expressions

## Cheatsheet

| Concept / Formula | Key Unit Rule |
| --- | --- |
| Dynamic equilibrium | Rate of forward reaction = Rate of reverse reaction; concentrations are constant |
| Le Chatelier's principle | Equilibrium shifts to counteract any external change in reaction conditions |
| Equilibrium constant general form | $K = \frac{[products]^{stoichiometric \ coefficients}}{[reactants]^{stoichiometric \ coefficients}}$ |
| Effect of catalyst | Catalysts speed up reaching equilibrium, do not change equilibrium position or $K$ value |
| Effect of temperature | Temperature changes alter the value of $K$, shifting equilibrium in the endothermic direction for a temperature increase |

## What's next

Begin this unit with the first sub-topic on dynamic equilibrium characteristics to build the core conceptual foundation required for later topics in this unit. Once you complete all three sub-topics here, you will move on to the next AS unit covering reaction kinetics, which connects equilibrium concepts to the study of reaction rates.

- [Dynamic equilibrium characteristics](https://www.owlsprep.com/study/cie-9701-u7-dynamic-equilibrium-characteristics/)
- [Le Chatelier's principle](https://www.owlsprep.com/study/cie-9701-u7-le-chatelier-s-principle/)
- [Equilibrium constants](https://www.owlsprep.com/study/cie-9701-u7-equilibrium-constants/)

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