Overview
This lecture explains reversible reactions, equilibrium, and the energy changes involved, using clear examples relevant for GCSE chemistry.
Reversible and Irreversible Reactions
- Most chemical reactions in GCSE chemistry are irreversible; they go in one direction only.
- Example: Magnesium reacts with oxygen to form magnesium oxide; this reaction cannot be reversed.
- Some reactions can go both forward and backward, depending on conditions; these are reversible reactions.
- Example: Heating ammonium chloride produces ammonia and hydrogen chloride, which can recombine to form ammonium chloride if cooled.
Changing Reaction Direction
- Reversible reactions can be made to go forwards or backwards by changing conditions like temperature.
- Heating may drive the reaction forward, while cooling may drive it in reverse, but not all reactions respond the same way.
Energy Changes in Reversible Reactions
- When hydrated copper sulfate (blue) is heated, it forms anhydrous copper sulfate (white) and water, absorbing energy (endothermic).
- Adding water back to anhydrous copper sulfate releases energy (exothermic), making the reaction hot.
- If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction, with equal energy transfer.
Chemical Equilibrium
- In a sealed container, a reversible reaction eventually reaches a point where forward and reverse reactions occur at the same rate.
- This state is called equilibrium, and no overall change in amounts of reactants and products occurs.
Key Terms & Definitions
- Reversible Reaction — a reaction where products can reform reactants under certain conditions.
- Endothermic — a process that absorbs energy from its surroundings.
- Exothermic — a process that releases energy into its surroundings.
- Equilibrium — the state in a reversible reaction where forward and reverse reactions occur at the same rate in a closed system.
Action Items / Next Steps
- Learn and memorise the hydrated copper sulfate reversible reaction.
- Study the definition of equilibrium.
- Review reversible reaction questions in the revision workbook.