The Characteristics of Dynamic Equilibrium

What are reversible reactions?

Some reactions go to completion where the reactants are used up to form the products and 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 half arrows are used: ⇌

A reversible reaction

The forward reaction is the dehydration of a hydrated salt, the reverse reaction is the hydration of the anhydrous salt

The diagram shows an example of a forward and backward reaction that can be written as one equation using two half arrows

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

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

Graph of concentration against time

Concentration of reactants decreases and products increases until equilibrium is reached and their concentrations remain constant

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)

Graph of concentration against time

Concentration of reactants increases and products decreases until equilibrium is reached and their concentrations remain constant

The same equilibrium can be approached starting with the products

Exam Tip

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

In this closed system, no particles can escape, an equilibrium exists between calcium carbonate, calcium oxide and carbon dioxide

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

An open system

In this open system, gas particles escape and only the forward reaction occurs

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

Exam Tip

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

The Characteristics of Dynamic Equilibrium (HL IB Chemistry)

Revision Note