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Understanding Water Ionization and the pH Scale: Amphoteric Properties Explained

This overview explores the ionization of water, its amphoteric nature, and the pH scale. Water undergoes autoprotolysis, allowing it to act as both an acid and a base, leading to the formation of hydronium (H3O+) and hydroxide (OH-) ions. The ion product constant (Kw) at 25°C is 1.0 × 10^-14. The pH scale measures hydrogen ion concentration, with pure water being neutral at pH 7.0. As concentrations of H3O+ and OH- fluctuate, the pH changes, illustrating the dynamic balance of acidity and basicity in aqueous solutions.

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Understanding Water Ionization and the pH Scale: Amphoteric Properties Explained

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  1. pH Scale

  2. Ionization of Water • Water is amphoteric • 2H2O ↔ H3O+ + OH– • Acid + base • Kw = [H3O+][OH–] = 1.0 × 10–14 at 25°C • Acid: [H3O+] > [OH–] • Base: [H3O+] < [OH–] • Neutral: [H3O+] = [OH–]

  3. Water is Amphoteric • Amphoteric = both acid and base behavior • Compound can accept or donate H+

  4. Autoprotolysis Water undergoes self-ionisation  autoprotolysis, since H2O acts as an acid and a base. H2O + H2O H3O+ + OH- The extent of autoprotolysis is very small.

  5. [H3O+] [OH-] [H2O]2 Autoprotolysis The equilibrium constant expression for this reaction is: KC = The concentration of water is essentially constant. Therefore: [H2O]2 KC = [H3O+]  [OH-] constant=KW

  6. Ionization of Water • As usual the K value changes with temperature. • In pure water, what are the [H3O+] and [OH–]?

  7. (1.8 x 10-9)2 x 55.5 a2 x C Kd= = = 1 1-a Self Ionization of Water a25°C = 1.8 x 10-9 1.7982 x 10-16

  8. Self Ionization of Water Kd x 55.5 =Kw = [H+][OH-] =9.98 x 10-15 1 x 10-14 Kw = [H+][OH-] = 10-14 [H+] = [OH-] =10-7 mol dm-3

  9. Review of pH • pH = -log [H+] • pX = -log [anything X] • As [H+] goes down, pH goes up, and the opposite also occurs • Higher pH equals lower hydrogen or hydronium ion concentrations…

  10. pH Scale • Neutral: [H3O+] = [OH–] = 1.0 × 10-7 M • Acid: [H3O+] > [OH–] • Base: [H3O+] < [OH–] • Define pH = –log[H3O+] • Reverse calculation [H3O+] = 10–pH • For pure water, pH = –log[1.0 × 10-7] = 7.00

  11. Neutral, Acidic & Basic Conditions • Both ions are present in all aqueous solutions. • You’d think they would neutralize each other. • The do, but equilibrium keeps regenerating the ions. • Inversely proportional • As one goes up, the other goes down

  12. Temperature Changes Things

  13. Beer NaHCO3 Lemon Juice PureWater 0.1 M NaOH pH 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0.1 M HCl Milk 0.1 M NH3 Pickles Blood Acidity/Basicity of Solutions

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