Static and Dynamic Equilibrium

Chemical Systems

Chemical systems are described in terms of the space or environment where a chemical reaction occurs and any possible interaction with external factors like temperature or changes to concentrations of reactants or products.

An open system is open to the exchange of both matter and energy with the external environment. Matter and energy are freely able to move into and out of the system.

A closed system is one where the exchange of energy is possible but the transfer of matter into and out of the system cannot occur.

An isolated system is one where the exchange of matter and energy into or out of the system does not occur.

In a school laboratory, these three systems may be modelled as follows:

  • Open system – reaction in an open test tube
  • Closed system – reaction in a test tube with a rubber stopper in the top
  • Isolated system – reaction in a test tube with a rubber stopper and insulation surrounding the test tube. Although, a perfect isolated system is still very difficult to achieve.

Key point: Reversible reactions can only occur in closed systems. 

Static and Dynamic Equilibrium

A chemical system is in equilibrium when the concentration of reactants and products is no longer changing over time. Forward and reverse reactions may still be occurring in a system which is at equilibrium, but these rates will be the same and no observable change in the properties of the system will be observed.

A reaction is in a state of static equilibrium when the reaction is not proceeding in either the forward or reverse direction. This also means that there is no exchange chemically between the reactants and products. A reaction which is in a state of static equilibrium has gone to completion and is considered irreversible. The image below illustrates a static equilibrium where all of the reactants are converted into products and the reaction cannot proceed in the reverse direction.

A reaction which is in dynamic equilibrium is still proceeding in both the forward and reverse directions at the same rate. The concentrations of the reactants and products remain the same. The image below illustrates a dynamic equilibrium where the forward and reverse reaction rates are occurring at the same rate.

Reaction rates and changes in concentrations can be illustrated using the following graphs:

Reaction Rate and Time

Initially the forward reaction rate is high and the reverse reaction is zero. Over time the rate of the forward reaction will decrease and the reverse reaction will increase. At some point the forward and reverse reaction rates will become equal. This is when equilibrium is established.

Concentration and Time

The initial concentration of reactants is high and the concentration of products is zero. Over time the concentration of reactants will decrease and the concentration of products will increase. At some point the reaction will reach equilibrium and the concentrations of reactants and products will remain constant.

Key point: Whilst the forward and reverse reaction rates at equilibrium will be equal, the concentrations and amounts of each reactant and product may not be. There could be lots of reactants and few products, or lots of products and few reactants. This all depends on the reaction type and conditions of the system.

Modelling Dynamic Equilibrium

Dynamic equilibrium can be modelled by using two measuring cylinders and a straw. One measuring cylinder is initially half full with liquid and the other empty. A straw is used to place at the bottom of the cylinder and transfer a volume of liquid to the other measuring cylinder. The straw is then used to transfer back a volume into the original cylinder. This is considered one interval of time.

This process continues until equal volumes are in each cylinder, at which point the system is at equilibrium. The following points can be noted:

  • The system may be considered closed as water is not likely to evaporate or leave the measuring cylinders during the experiment.
  • It illustrates a system at equilibrium as the straw can still transfer between the measuring cylinders but the volume does not change.
  • The volume of water transferred with the straw can represent the rate of the forward and reverse reaction.
  • The volume of water in the measuring cylinder can represent the concentration of reactants (1) and products (2)
  • Limitations of the model include that it may be argued that the system is not closed and that it does not illustrate changes that occur at the microscopic level.

Example 1:

In a hypothetical chemical reaction, A and B react to form CD. Draw a diagram to illustrate this system if the reaction was:

a) Irreversible and at static equilibrium

b) Reversible and at dynamic equilibrium

Answer:

a)

b)

Example 2:

The following image illustrates a chemical reaction that reaches equilibrium:

a) Write a balanced chemical equation for this reaction:

b) Is this a graph of reaction rate vs time or concentration vs time? Explain.

Answer:

a) 2SO3(g) ⇌ 2SO2(g) + O2(g)

b) Concentration vs time. The lines at equilibrium are not equal. Reaction rates will be equal at equilibrium but the concentrations may vary.