Brønsted-Lowry acids are proton donors, bases are proton acceptors

According to the Brønsted-Lowry theory:

• An acid is defined as a substance that donates a proton (H⁺).
• A base is defined as a substance that accepts a proton.

For instance, when a Brønsted-Lowry acid (HA) is dissolved in water, it donates a proton to a water molecule, forming a hydronium ion (H₃O⁺):

HA_(aq) + H₂O_(l) ⇌ H₃O⁺_(aq) + A^-_(aq)

Conversely, when a Brønsted-Lowry base (B) is mixed with water, it accepts a proton from a water molecule, forming a hydroxide ion (OH^-):

B_(aq) + H₂O_(l) ⇌ BH⁺_(aq) + OH^-_(aq)

Strength depends on extent of dissociation

The strength of an acid or a base is determined by its ability to dissociate in water. This dissociation process can be reversible, as shown in the equations below:

• Acid dissociation: HA + H_2_O ⇌ H_3_O^+^ + A^-^
• Base dissociation: B + H_2_O ⇌ BH^+^ + OH^-^

Strong acids and bases:

Strong acids, such as hydrochloric acid (HCl), and strong bases, like sodium hydroxide (NaOH), dissociate completely in water. This results in a significant release of H_3_O^+^ and OH^-^ ions, respectively, with the forward reaction being predominantly favoured:

• HCl_(aq)_ ➔ H^+^(aq) + Cl^-^(aq)
• NaOH_(aq)_ ➔ Na^+^(aq) + OH^-^(aq)

Weak acids and bases:

Conversely, weak acids and bases, like ethanoic acid (CH_3_COOH) and ammonia (NH_3_), only partially dissociate in water, releasing fewer H_3_O^+^ and OH^-^ ions. In these cases, the reverse reaction is favoured:

• CH_3_COOH_(aq)_ ⇌ H^+^(aq) + CH_3_COO^-^(aq)
• NH_3(aq)_ + H_2_O_(l)_ ⇌ NH_4_^+^(aq) + OH^-^(aq)

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.

Key points:

• 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: