Acids and bases as proton transfer

The definitions

The standard defines acids and bases in terms of proton transfer:

• An acid is a proton donor — it gives away an $H^+$ ion.

• A base is a proton acceptor — it takes an $H^+$ ion.

• A proton here means a hydrogen ion, $H^+$ — a hydrogen atom that has lost its electron is just a proton.

• Every acid–base reaction is therefore an $H^+$ moving from one species to another.

An acid needs a base

• An acid can only donate a proton if something is there to accept it.
• So acids and bases always act together — you never have one acting alone.

When an acid dissolves in water, water acts as the base:

$$HCl + H_2O \rightarrow Cl^- + H_3O^+$$

• HCl donates a proton → it is the acid.

• $H_2O$ accepts it → it is the base.

• $H_3O^+$ is the hydronium ion (also called the oxonium ion).

• You will often see this written more simply as $HCl \rightarrow H^+ + Cl^-$. Both are acceptable; the full version shows what the proton actually attaches to.

Conjugate pairs

• When an acid donates its proton, what is left is its conjugate base.
• When a base accepts a proton, what it becomes is its conjugate acid.

$$\underset{\text{acid}}{HCl} + \underset{\text{base}}{H_2O} \rightleftharpoons \underset{\text{conj. base}}{Cl^-} + \underset{\text{conj. acid}}{H_3O^+}$$

• A conjugate pair differs by exactly one $H^+$.
• Here the pairs are HCl / $Cl^-$ and $H_2O$ / $H_3O^+$.

Finding a conjugate

• Conjugate base — remove one $H^+$: take away one H and make the charge one more negative.

  • $$HNO_3 \rightarrow NO_3^-$$
  • $$H_2SO_4 \rightarrow HSO_4^-$$
  • $$NH_4^+ \rightarrow NH_3$$

• Conjugate acid — add one $H^+$: add one H and make the charge one more positive.

  • $$NH_3 \rightarrow NH_4^+$$
  • $$OH^- \rightarrow H_2O$$
  • $$CH_3COO^- \rightarrow CH_3COOH$$

• Watch the charge, not just the hydrogen. Removing $H^+$ from a neutral molecule leaves a 1− ion.

Species that can do both

• Some species can donate or accept a proton, depending on what they meet.
• Water is the important example:
  • With an acid it accepts a proton, acting as a base: $HCl + H_2O \rightarrow Cl^- + H_3O^+$
  • With a base it donates one, acting as an acid: $NH_3 + H_2O \rightarrow NH_4^+ + OH^-$

Bases and alkalis

• A base is any proton acceptor.
• An alkali is a base that is soluble in water, producing $OH^-$ ions in solution.
• All alkalis are bases; not all bases are alkalis.