Proton transfer and conjugate pairs

The definitions

• An acid is a proton donor — it gives away an $\text{H}^+$.

• A base is a proton acceptor — it takes an $\text{H}^+$.

• Every acid–base reaction is therefore a transfer of a proton from one species to another.

• The standard describes aqueous systems as those "in which proton transfer occurs", so this is the language to use throughout.

Conjugate pairs

• When an acid donates its proton, what remains is its conjugate base.
• When a base accepts a proton, what it becomes is its conjugate acid.
• The two members of a conjugate pair differ by exactly one $\text{H}^+$.

$$\underset{\text{acid}}{\text{CH}_3\text{COOH}} + \underset{\text{base}}{\text{H}_2\text{O}} \rightleftharpoons \underset{\text{conjugate acid}}{\text{H}_3\text{O}^+} + \underset{\text{conjugate base}}{\text{CH}_3\text{COO}^-}$$

• There are two conjugate pairs in every such equation: $\text{CH}_3\text{COOH}/\text{CH}_3\text{COO}^-$ and $\text{H}_3\text{O}^+/\text{H}_2\text{O}$.

Finding a conjugate

• Conjugate base: remove one $\text{H}^+$ — take away a hydrogen and make the charge one more negative.

  • $\text{HF} \rightarrow \text{F}^-$; $\text{NH}_4^+ \rightarrow \text{NH}_3$

• Conjugate acid: add one $\text{H}^+$ — add a hydrogen and make the charge one more positive.

  • $\text{NH}_3 \rightarrow \text{NH}_4^+$; $\text{CH}_3\text{COO}^- \rightarrow \text{CH}_3\text{COOH}$

• Change the charge, not just the hydrogen. Writing $\text{CH}_3\text{COO}$ without its negative charge is the commonest slip here.

Water is amphiprotic

• Water can act as either an acid or a base, depending on what it meets.
• With an acid it acts as a base, accepting the proton to become $\text{H}_3\text{O}^+$.
• With a base it acts as an acid, donating a proton to become $\text{OH}^-$:

$$\text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-$$

• This is why the same substance appears on both sides of the story throughout this topic.

The species you must recognise

The specification names these, and you are expected to know them without being told.

	Examples
strong acids	$\text{HCl}$, $\text{HBr}$, $\text{HNO}_3$, $\text{H}_2\text{SO}_4$
strong bases	$\text{KOH}$, $\text{NaOH}$
weak acids	$\text{HF}$, $\text{CH}_3\text{COOH}$, $\text{NH}_4^+$
weak bases	$\text{NH}_3$, $\text{CH}_3\text{NH}_2$, $\text{CH}_3\text{COO}^-$

• Note that $\text{NH}_4^+$ and $\text{NH}_3$ are a conjugate pair, and so are $\text{CH}_3\text{COOH}$ and $\text{CH}_3\text{COO}^-$. Recognising an ion as a weak acid or base is what lets you predict whether a salt solution is acidic or basic.
• Less familiar weak acids and bases may appear, but the question will supply the information you need.

Writing $\text{H}_3\text{O}^+$ rather than $\text{H}^+$

• A bare proton does not exist in water — it is always attached to a water molecule as the hydronium ion, $\text{H}_3\text{O}^+$.
• The booklet's relationships all use $[\text{H}_3\text{O}^+]$, so use that notation in your answers.