3.1.5 – The ion product constant of water
📌 Ionisation of water
- Ionising water produces hydrogen ions and hydroxide ions (H+and OH–)
H2O(l) ⇌ H+ (aq) + OH– (aq)
- The constant of water KW can be defined using this formula
- KW = [H+][OH–] which is fixed at 1.00 x 10-14 at STP
- In pure water (distilled) we know that H+ = OH– therefore giving us the concentration of H+ ions as 1.00 x 10-7
- Knowing this, and applying the previously learnt formula, we can determine the pH of pure distilled water to be 7.0
📌 Inverse relationships between ions
- The relationship between hydrogen ions and hydroxide ions is inverse
- Thus, as hydrogen ions increase in concentration, hydroxide ions decrease the substance becomes more acidic
- Since KW is temperature dependent, we can use the concentration of either hydrogen or hydroxide ions to calculate the value of KW
📌 The water constant
- Given that KW is temperature dependent and dissociation of water is endothermic then the followiung two statements musrt be true :
- 1. Increasing the temperature causes equilibrium to shift right, thus pH decreases as concentration of both ions increases
- 2. Decreasing the temperature causees equilibrium to shift left, thus pH increases as concentration of both ions decreases
- However, regardless of this change, the concentration of hydrogen ions is always equal to hydroxide ions, therefore despite the pH change, water does not become either acidic or basic