The Characteristics of Dynamic Equilibrium (DP IB Chemistry): Revision Note
The Characteristics of Dynamic Equilibrium
What are reversible reactions?
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: ⇌
A reversible reaction

What is 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
There is no change in macroscopic properties such as colour and density as they depend on the concentration
Dynamic equilibrium between hydrogen, iodine and hydrogen iodide

Graph of concentration against time

Graph of concentration against time

Examiner Tips and Tricks
Dynamic equilibrium can also be established in physical systems, for example, in a bottle of ethanol
Some liquid ethanol will evaporate and some ethanol vapour will condense
An equilibrium exists between the two phases as the rate of evaporation = the rate of condensation.
C2H5OH (l) ⇌ C2H5OH (g)
What is a closed system?
A closed system is one in which none of the reactants or products escape from the reaction mixture
In an open system some matter is lost to the surroundings
When a reaction takes place entirely in solution, equilibrium can be reached in open flasks
If the reaction involves gas, equilibrium can only be reached in a closed system
A closed system

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 constant (the concentrations are not changing)
Stating that the concentrations are equal will lose a mark in an exam
The dynamic equilibrium can be reached by starting either with the reactants or products
In both cases, the concentrations of the reactants and products remain constant once dynamic equilibrium has been reached
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