Conjugate Acids and Conjugate Bases

This lesson covers: 

1. Conjugate acid-base pairs
2. Reactions of acids with metals and bases
3. The ionic product of water, K_w

Acids and bases form conjugate pairs

When Brønsted-Lowry acids and bases react together, they form conjugate acid-base pairs on opposite sides of the reaction equation:

A conjugate acid-base pair consists of two species that are interconverted by the transfer of a proton (H⁺).

• In the forward reaction, HA acts as an acid, donating a proton to form its conjugate base (A^-).
• In the reverse reaction, A^- acts as a base, accepting a proton from BH⁺ to reform the acid (HA).
• Similarly, B and BH⁺ form another conjugate pair, with B being the base (proton acceptor) and BH⁺ its conjugate acid.

For example, when ethanoic acid (CH₃COOH) reacts with water:

In this reaction:

• CH₃COOH and CH₃COO^- are a conjugate pair. CH₃COOH is the acid (proton donor) and CH₃COO^- is its conjugate base.
• H₂O and H₃O⁺ form the other conjugate pair, with H₂O acting as the base (proton acceptor) and H₃O⁺ as its conjugate acid.

Remember:

• The conjugate base always has one less H⁺ than its conjugate acid.
• The conjugate acid always has one more H⁺ than its conjugate base.

Water can act as an acid or a base

Water is an amphiprotic substance, meaning it can behave as both an acid and a base depending on the reaction.

1. Water as a base - When reacting with acids, water accepts a proton to form a hydronium ion (H₃O⁺), which is water's conjugate acid. For example:

2.   Water as an acid - When interacting with bases, water donates a proton to form a hydroxide ion (OH^-), which is water's conjugate base. For example:

Acids react with metals and bases

1. Acids react with reactive metals to produce a salt and hydrogen gas:

• Metal + acid ➔ salt + hydrogen
• Example:  Mg_(s) + 2HCl_(aq) ➔ MgCl_2(aq) + H_2(g)

2. Acids react with metal carbonates to produce a salt, water, and carbon dioxide gas:

• Metal carbonate + acid ➔ salt + water + carbon dioxide
• Example:  CaCO_3(s) + 2HCl_(aq) ➔ CaCl_2(aq) + H₂O_(l) + CO_2(g)

3. Acids react with alkalis (soluble bases) to yield a salt and water:

• Acid + alkali ➔ salt + water
• Example:  HCl_(aq) + NaOH_(aq) ➔ NaCl_(aq) + H₂O_(l)

4. Acids react with insoluble bases (usually metal oxides) to form a salt and water:

• Acid + metal oxide ➔ salt + water
• Example:  2HCl_(aq) + CuO_(s) ➔ CuCl_2(aq) + H₂O_(l)

The ionic product of water (K_w)

A small fraction of water molecules spontaneously transfer protons between themselves, a process called autoionisation:

2H₂O_(l) ⇌ H₃O⁺_(aq) + OH^-_(aq)

Or, more simply:

H₂O_(l) ⇌ H⁺_(aq) + OH^-_(aq)

The equilibrium constant (K_c) for this reaction is:

Kc​=[H2​O][H+][OH−]​

However, because water is present in vast excess compared to H⁺ and OH^-, its concentration [H₂O] is essentially constant.

Multiplying both sides of the K_c expression by [H₂O] gives the ionic product of water (K_w):

K_w = [H⁺][OH^-]

At 298 K, Kw​=1.00×10−14 mol2 dm−6

The value of K_w varies with temperature.

H⁺ and OH^- concentrations in pure water

In pure water, the concentrations of H⁺ and OH^- are equal:

Kw​=[H+]2=[OH−]2=1.00×10−14 mol2 dm−6

Therefore, in pure water at 298 K:

[H+]=[OH−]=√(1.00×10−14)​=1.00×10−7 mol dm−3