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Autoionization (Self-ionization) of Water

What is the Autoionization of Water

The autoionization or self-ionization of water is an endothermic ionization reaction where a molecule of pure water gets dissociated into hydrogen ions (H+) and hydroxyl ions (OH). As water is a very weak electrolyte, it gets ionized to a minimal extent.

When two water molecules collide, a hydrogen ion from one molecule gets transferred to the other, producing a positively charged hydronium ion (H3O+) and a negatively charged hydroxyl ion.

H2O (l) + H2O (l) ⇌ H3O+ (aq.) + OH (aq.)

We commonly use the simplified form of the reaction:

H2O (l) ⇌ H+ (aq.) + OH(aq.)

Autoionization of Water

Acid-Base Properties of Water

As it is amphoteric, water can act as both an acid and a base in solution. When mixed with a base, water donates hydrogen ions and acts as an acid. However, when water is mixed with an acid, it accepts hydrogen ions and acts as a base.

  • Acting as a base:

H2O (l) + CH3COOH (aq.) ⇌ H3O+ (aq.) + CH3COO (aq.)

  • Acting as an acid:

NH3 (aq.) + H2O (l) ⇌ NH4+ (aq.) + OH (aq.)

Water also undergoes autoionization, where it can be both an acid and a base to itself.

H2O (l) + H2O (l) ⇌ H3O+ (aq.) + OH (aq.)

Chemical Equation and Autoionization Constant

As mentioned during self-ionization, H2O splits into H+ and OH.

∴ Kc = [H+] [OH]/[H2O]; where ‘Kc’ is the dissociation constant of water at equilibrium.

As pure water is a poor electrolyte, it gets slightly ionized. So, the concentration of H2O is much greater than the concentrations of H+ and OH. Therefore, [H2O] can be regarded as constant.

∴ Kc [H2O] = [H+][OH]

As Kc and [H2O] are both constants, they are replaced by another constant, ‘Kw’.

∴ Kw = [H+][OH]; where Kw is called the autoionization constant of water.

From the above explanation, ‘Kw’ can be regarded as the ionic product of water, i.e., the product of the molar concentrations of H+ and OH ions.

As H+ ions exist as H3O+ ions in water, the ionic product can alternatively be stated as the product of molar concentrations of H3O+ and OH ions.

∴ Kw = [H3O+][OH]

Relationship among pH, pOH, and pKw

The autoionization constant (Kw) is temperature-dependent and increases with an increase in temperature. It has a fixed value of 1×10-14 at 25 ͦC.

The concentration of dissociated H+ and OH ions are equal. So, let the [H+] and [OH] be ‘x’.

By substituting the values in the equilibrium expression, we get,

[H+] [OH] = 1×10-14

x . x = 1×10-14

x2 = 1×10-14

x = 1×10-7

∴ [H+] = [OH] = 1×10-7

Now, we know, [H+] = 1×10-7

Taking negative logarithm on both sides

-log [H+] = -log (10-7)

∴ pH = – (-7) = 7 [pH of pure water is 7]

Similarly, pOH will also be 7.

Now, taking negative logarithm of Kw

-log (Kw) = -log (1×10-14)

∴ pKw = – (-14) = 14

Therefore, at 25 ͦC,

pKw = pH + pOH

References

  1. Water autoionization and Kw – Khanacademy.org
  2. Autoionization of Water – Courses.lumenlearning.com
  3. Self-Ionization of Water and the pH Scale – Chem.libretexts.org

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