pH is a scale used to measure the concentration of H₃O⁺ ions in a solution. A solution is considered acidic when pH is less than 7 and basic when pH is greater than 7.

Self-ionization of water

The self-ionization of water:  ,ΔH=positive

• The self-ionization of water is an endothermic reaction. Energy is absorbed by the reactants in order for the forward reaction to proceed.

K_w is the ionisation constant of water and is a constant. In pure water and when conditions are set to 25°C, [H₃O⁺] and [OH^–] are at concentrations of 10^-7M. Therefore, the value of K_w at 25°C is 10^-14.

@25℃

pH
pH is given by the formula:     
or it can be transposed as:       

pH neutral is when the concentration of H₃O⁺ is equal to OH^–. A common misconception is that pH neutral is always at the pH 7. This is only the case at 25°C. If the temperature of the solution is greater than 25°C, neutral pH would be less than 7. Similarly, if the temperature of the solution is less than 25°C, neutral pH would be greater than 7.

Take the equation for the self-ionization of water for example. If the temperature of water were to increase from 25°C to 30°C, since it is an endothermic reaction, the system will favour a net forward reaction, increasing the amount/concentration of both H₃O⁺ and OH^–. A higher concentration of H₃O⁺ means that the pH will be lower than the starting pH of 7. However since hydroxide ions and hydronium both share the same concentration, the solution is considered pH neutral.

Acidity Constant (K_a)

K_a measures the extent of a reaction between water and an acid. K_a is used to identify between strong and weak acids. The higher the K_a value, the stronger the acid. It is given by the formula:

If K_a were to be deduced from a reaction involving a weak acid, its concentration can be assumed to be its original starting concentration. This is based on the assumption that not much of the reactant will be reacted because it is a weak acid.

Percentage hydrolysis is often used in conjunction with K_a and it measures the extent of the reaction by calculating the amount of acid that has been ionised. It is given by the equation:

Buffers

Buffers are solutions that a weak acid/base and its conjugate acid/base.

Buffers are solutions that can absorb the addition of acids or bases and maintain a stable pH. They are essential for keeping systems within a certain range of pH without it exceeding a certain limit. Body systems in particular need to be maintained under a certain range of pH and buffers are important at doing so.

A common buffer is a solution of CH₃COOH/CH₃COO^–.

The addition of a strong acid to this solution will increase the concentration of H₃O⁺ in the solution. The system of the buffer counteracts this by favouring a net backward reaction in an attempt to decrease the concentration of H₃O⁺ and thus lessen the effect of the addition of the strong acid.

The addition of a strong base to this solution will decrease the concentration of H₃O⁺ in the solution. The system will then favour a net forward reaction and thus lessen the effect of the addition of the strong base.

See Also