Molecular and Ionic Equilibria

The section on equilibrium is one of the more difficult sections of the course. It demands a good working knowledge of mathematics, though not specifically at higher level. What is important is to have a feeling for values and to be good at manipulating figures and solving equations. It is important to know and understand the terms and definitions of this subject.


It is important to know the language of the subject, i.e., to know and understand the definitions, viz. the terms:

Predicting Changes in Equilibrium Position

This involves applying Le Chatelier’s Principle. The factors that can change a position of equilibrium are (i) temperature, (ii) pressure (for gaseous reactions), and (iii) concentrations (for substances in solution).

Solving Equilibrium Problems

The way to go about solving problems is to adopt a standard approach, and to use it all the time. Then if you encounter a difficult problem you have a much greater chance of either solving it, or at least getting most of it done.

1. First write the expression for K.
(i) Write the balanced equation for the reaction (if not already given),
(ii) Write the K expression by putting the concentrations of the substances on the right-hand side of the equation on top of the expression, and putting the concentrations of the substances on the left-hand side on the bottom of the K expression.
(iii) If there is more than one mole of any substance in the balanced equation, raise the concentration to a power equal to the number of moles of it in the equation.
2. Make out a grid, showing:
(i) the starting concentrations, or amounts, of all relevant substances,
(ii) the changes that take place,
(iii) the final concentrations, or amounts.
3. Sufficient information will be given about some of these to work out the required values.

Ionic Equilibria

Definitions to be known:

Calculating pH Values

The pH of a solution is minus the log of the H+ ion concentration, and most pH calculations involve two stages:
(i) finding the value of the hydrogen ion concentration, i.e., [H+], and
(ii) calculating minus the log of that value.
For bases, the first step is to calculate the concentration of hydroxide ion, i.e., [OH], then the pOH, and finally the pH.


The table list the three most common indicators with their colours and uses.

Indicator Colour change between pH values Titration use
Methyl orange
Red — yellow
Red — blue
Colourless — violet
2.9 — 4.6
5.0 — 8.0
8.3 — 10.0
Strong acid + weak base
Strong acid + strong base
Weak acid + strong base

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