Chemical Equilibria (Oxford AQA International A Level Chemistry)
Revision Note
Written by: Richard Boole
Reviewed by: Stewart Hird
Reversible Reactions & Dynamic Equilibrium
Reversible reaction
Some reactions go to completion where the reactants are used up to form the products
The reaction stops when all of the reactants are used up
In reversible reactions the products can react to reform the original reactants
To show a reversible reaction, two opposing half arrows are used: ⇌
Reversible reaction example
Dynamic equilibrium
In a dynamic equilibrium the reactants and products are dynamic (they are constantly moving)
In a dynamic equilibrium:
The rate of the forward reaction is the same as the rate of the backward reaction in a closed system
The concentrations of the reactants and products are constant
Sketch graphs of reactant and product concentrations as a reaction approaches equilibrium
Open and closed systems
A closed system is one in which none of the reactants or products escape from the reaction mixture
In an open system, matter and energy can be lost to the surroundings
When a reaction takes place entirely in solution, equilibrium can be reached in open flasks as a negligible amount of material is lost through evaporation
If the reaction involves gases, equilibrium can only be reached in a closed system
Thermal decomposition of calcium carbonate in a closed system
Thermal decomposition of calcium carbonate in an open system
Examiner Tips and Tricks
A common misconception is to think that the concentrations of the reactants and products are equal.
However, they are not equal but they remain constant at dynamic equilibrium (i.e. the concentrations are not changing).
The concentrations will change as the reaction progresses, only until the equilibrium is reached.
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