Topic 7.1: Dynamic equilibrium

7.1.1 Outline the characteristics of chemical and physical systems in a state of equilibrium

Physical systems

As bromine is a volatile liquid, with a boiling point close to room temperature, a significant number of particles will have enough energy to escape from the liquid state and form vapour in the process known as evaporation. At the same time, some of these vapour molecules will collide witht he surface of the liquid, lose energy and become liquid in the process known as condensation.

There will however come a time when the rate of evaporation is equal to the rate of condensation and at this point there will be no net change in the amounts of liquid and gas present. We say that the system has reached equilibrium. This will only occur in a closed system, where the gas can't escape.

Chemical systems

Consider the reaction of dissociation between hydrogen iodide HI and its elements hydrogen and iodine.

If were to analyse the contents of the flask at this point, we could find HI, hydrogen and iodine would all be present and that if there were no change in conditions, their concentration would remain constant over time.


Characteristics of equilibrium state

Equilibrium is dynamic - The reaction has not stopped but both forward and backward reactions are occuring at the the same rate

Equilibrium is achieved in a closed system - A closed system prevents exchange of matter with the surroundings, so equilibrium is achieved where both reactants and products can react and recombine with each other.

The concentration of reactants and products remain constant at equilibrium - They are being produced and destroyed at an equal rate.

At equilibrium there is no change in marcoscopic properties - This refers to observable properties such as colour and density. These do not change as they depend on the concentrations of the components of the mixture

Equilibrium can be reached from either direction - The same equilibrium mixture will result under the same conditions, no matter whether the reaction is started with all reactants, all products or a mixture of both.