# Chemical Reactions

> CIE IGCSE Chemistry · CIE IGCSE Chemistry (0620)
> Source: https://www.owlsprep.com/study/cie-0620-u6-overview/
> Weight: 12-15% of total exam marks across MCQ and structured sections

This unit explores core rules governing how chemical reactions behave, including reaction speed, reversible system dynamics, and electron transfer in redox processes, a foundational base for all higher-level chemistry topics.

**Prerequisites:** [Writing balanced chemical equations](https://www.owlsprep.com/study/cie-0620-u4-chemical-formulae-equations/); [Particle theory](https://www.owlsprep.com/study/cie-0620-u1-states-of-matter/)

## Learning objectives

- Calculate and interpret reaction rates using experimental mass, volume, and concentration data
- Explain dynamic equilibrium and apply Le Chatelier’s Principle to predict changes to equilibrium systems
- Identify redox reactions using electron transfer, oxidation state changes, and oxygen/hydrogen gain/loss rules
- Link reaction principles to real-world industrial processes including the Haber and Contact processes

## Unit at a glance

You will start by learning to measure reaction rates and apply collision theory to explain how temperature, concentration, surface area, and catalysts change reaction speed. Next, you will explore reversible reactions and dynamic equilibrium, including how external changes shift equilibrium position in industrial applications. You will finish by mastering redox classification, a core reaction type critical to corrosion, energy production, and biological systems.

Work through the following sub-topics in order to build a complete understanding of this unit:
- [Rate of Reaction](https://www.owlsprep.com/study/cie-0620-u6-rate-of-reaction/) — Learn to calculate reaction rates and explain how key variables change reaction speed using collision theory.
- [Reversible Reactions and Equilibrium](https://www.owlsprep.com/study/cie-0620-u6-reversible-reactions-and-equilibrium/) — Explore dynamic equilibrium, Le Chatelier’s Principle, and applications to high-yield industrial chemical production.
- [Redox](https://www.owlsprep.com/study/cie-0620-u6-redox/) — Identify redox reactions using three complementary classification systems: electron transfer, oxidation state changes, and oxygen/hydrogen gain/loss.

## Common pitfalls

- **Wrong:** Confusing dynamic equilibrium with reaction completion
  - Why it fails: At equilibrium, forward and reverse reaction rates are equal, not zero, so reactant and product concentrations stay constant but reactions are still active.
  - Correct: Always link equilibrium to equal forward/reverse reaction rates, not zero reaction activity.
- **Wrong:** Using total product volume to compare reaction rates
  - Why it fails: Reaction rate measures change in reactant/product per unit time, not total amount produced over the full reaction.
  - Correct: Divide changes in mass, volume, or concentration by time elapsed to calculate valid reaction rates.
- **Wrong:** Assuming only metals undergo oxidation/reduction
  - Why it fails: Non-metals including chlorine, sulfur, and nitrogen regularly change oxidation states in common reactions.
  - Correct: Calculate oxidation states for all elements in a reaction to accurately identify oxidizing and reducing agents.

## Cheatsheet

| Key Concept/Formula | Definition/Unit | Related Sub-topic |
| --- | --- | --- |
| Rate of reaction | $\frac{\text{Change in mass/volume/concentration}}{\text{Time taken}}$ | Rate of Reaction |
| Collision theory requirement | Particles must collide with minimum activation energy and correct orientation to react | Rate of Reaction |
| Dynamic equilibrium conditions | Closed system, equal forward/reverse reaction rates, constant species concentrations | Reversible Reactions and Equilibrium |
| Le Chatelier’s Principle | Equilibrium systems shift to oppose external changes to concentration, temperature, or pressure | Reversible Reactions and Equilibrium |
| Oxidation (electron transfer definition) | Loss of electrons | Redox |
| Reduction (oxidation state definition) | Decrease in oxidation number | Redox |
| Catalyst effect on equilibrium | Speeds up rate of reaching equilibrium, no change to final equilibrium position | Reversible Reactions and Equilibrium |

## What's next

Begin your study of this unit with the Rate of Reaction sub-topic, where you will practice calculating rates from experimental data and apply collision theory to explain real-world reaction speed changes. Once you have worked through all three sub-topics in this unit, you will be ready to progress to the next unit on Acids, Bases and Salts, which builds directly on reaction principle knowledge from this section.

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