Physical Chemistry: Chemical Equilibrium

Chemical Equilibrium Basics

Equilibrium refers to the state in which both reactants and products are present in concentrations which have no further tendency to change with time.
A dynamic equilibrium is established when the rates of forward and backward reactions are equal.
Equilibrium does not mean the concentrations of reactants and products are equal, rather, their rate of formation is equal.
A reaction in equilibrium can still be manipulated with changes in concentration, pressure, temperature, or by the use of a catalyst.

Le Chatelier’s Principle

Le Chatelier’s Principle predicts changes in the position of equilibrium due to changes in conditions.
If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.
For an exothermic reaction, increasing the temperature will result in the equilibrium position moving in the direction of the endothermic reaction to try to decrease the temperature.

Graphical Representation of Equilibrium

The evolution of an equilibrium can be represented graphically.
Plots of concentration against time allow us to visualise how an equilibrium is reached.
The rates of the forward and reverse reactions eventually become equal. At that point, the quantities of chemicals stop changing - equilibrium is established.

Equilibrium Constant, Kc

The equilibrium constant, Kc, is a measure of the position of equilibrium.
The value of Kc does not change with changes in concentration, but does change with temperature.
In the expression for Kc, the concentrations of the products appear in the numerator and the concentrations of the reactants in the denominator.

Calculating Equilibrium Concentrations

If you know the balanced equation for a reaction, plus the equilibrium constant and the initial concentrations, you can calculate the equilibrium concentrations.
Perform all calculations to the correct number of significant figures.

Effects of Changing Conditions

Changing the conditions of a reaction in equilibrium can affect the position of equilibrium.
Possible changes include: altering concentrations, changing pressure or volume, modifying temperature, or adding a catalyst.
Each of these changes affect the system in different ways, and understanding this is important for predicting the results of such changes.

Remember to fully grasp these key concepts about chemical equilibrium which are crucial in understanding the specific behaviour of a system when equilibrium is involved.