Dynamic Equilibrium in Reversible Reactions (Edexcel International AS Chemistry): Revision Note
Exam code: XCH11
Dynamic Equilibrium in Reversible Reactions
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: ⇌
The diagram shows an example of a forward and backward reaction that can be written as one equation using two half arrows
Dynamic equilibrium
In a dynamic equilibrium, the forward and backward reactions are dynamic (they are both still taking place)
In a dynamic equilibrium, the rate of the forward reaction is the same as the rate of the backward reaction in a closed system, and the concentrations of the reactants and products are constant
The diagram shows a snapshot of a dynamic equilibrium in which molecules of hydrogen iodide are breaking down to hydrogen and iodine at the same rate as hydrogen and iodine molecules are reacting together to form hydrogen iodide
The diagram shows that the concentration of the reactants and products does not change anymore once equilibrium has been reached (equilibrium was approached using reactants)
The diagram shows that the concentration of the reactants and products does not change anymore once equilibrium has been reached (equilibrium was approached using products)
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
The diagram shows a closed system in which no carbon dioxide gas can escape and the calcium carbonate is in equilibrium with the calcium oxide and carbon dioxide
The diagram shows an open system in which the calcium carbonate is continually decomposing as the carbon dioxide is lost causing the reaction to eventually go to completion
Examiner Tips and Tricks
A common misconception is to think that the concentrations of the reactants and products are equal.
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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