IMAT Chemistry · Acids, Bases and pH

What Makes an Acid or a Base: Three Definitions

Arrhenius looks at ions in water, Brønsted-Lowry at proton transfer, and Lewis at electron pairs; each definition is wider than the one before.

All Acids, Bases and pH notes

Why this matters

Five past questions test the definitions. The ministry papers asked what role nitric acid plays when it meets a stronger acid (2023), what ammonia is when it gives its lone pair to a boron compound (2024), and which statement fits the Brønsted-Lowry theory (2024).

Concept 1 of 3: Arrhenius, Brønsted-Lowry and Lewis definitions of acids and bases

Chemists widened the idea of an acid three times. Arrhenius only counted substances that make ions in water. Brønsted and Lowry noticed that the real event is a proton (an H⁺ ion) moving from one particle to another, in water or not. Lewis went one step further: the proton is accepted because the base has a lone pair to give it, so any electron-pair donor is a base and any acceptor is an acid.

Definition

Arrhenius: an acid releases $\mathrm{H^+}$ (really $\mathrm{H_3O^+}$ ) in water; a base releases $\mathrm{OH^-}$ in water.
Brønsted-Lowry: an acid is a proton donor; a base is a proton acceptor. Water is not needed.
Lewis: an acid is an electron-pair acceptor; a base is an electron-pair donor. The bond formed is a dative (coordinate) bond.
Every Arrhenius acid is a Brønsted-Lowry acid, and every Brønsted-Lowry base is a Lewis base (it uses a lone pair to hold the proton).
A Lewis acid does not need hydrogen: $\mathrm{BF_3}$ , $\mathrm{AlCl_3}$ and metal ions such as $\mathrm{Fe^{3+}}$ are Lewis acids.

Theory	Acid	Base	Example
Arrhenius	Gives $\mathrm{H^+}$ in water	Gives $\mathrm{OH^-}$ in water	$\mathrm{HCl}$ and $\mathrm{KOH}$ in water
Brønsted-Lowry	Proton donor	Proton acceptor	$\mathrm{NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-}$ : water donates, ammonia accepts
Lewis	Electron-pair acceptor	Electron-pair donor	$\mathrm{AlCl_3 + Cl^- \rightarrow AlCl_4^-}$ : $\mathrm{AlCl_3}$ accepts the pair A Lewis acid often has an incomplete octet (boron, aluminium) or is a positive metal ion.

Each definition includes the one above it, so a reaction can be acid-base in the Lewis sense even when no proton moves.

Practice this conceptself-check · 4 quick reps

A Lewis acid does not have to contain hydrogen

Students often assume every acid has an H to give. In the Lewis sense,

\mathrm{BF_3}

\mathrm{AlCl_3}

and metal ions are acids because they accept an electron pair. The base is the species with the lone pair, such as

\mathrm{NH_3}

\mathrm{H_2O}

or a halide ion.

Concept 2 of 3: Conjugate acid-base pairs in a proton transfer

When an acid gives away a proton, what is left behind could take a proton back: it is a base. That leftover is the acid's conjugate base. In the same way, a base that has taken a proton becomes a conjugate acid. So every Brønsted-Lowry reaction has two pairs, and the two members of a pair differ by exactly one

\mathrm{H^+}

Definition

The conjugate base of an acid is the acid minus one $\mathrm{H^+}$ : $\mathrm{HNO_2 \rightarrow NO_2^-}$ .
The conjugate acid of a base is the base plus one $\mathrm{H^+}$ : $\mathrm{NH_3 \rightarrow NH_4^+}$ .
Adding or removing $\mathrm{H^+}$ changes the charge by one: a neutral acid gives a 1− conjugate base.
Whether a substance acts as an acid or a base depends on its partner. Nitric acid is an acid in water, but it accepts a proton (acts as a base) from an even stronger acid.

Two conjugate pairs in every proton transfer

\underbrace{\mathrm{HA}}_{\text{acid}} + \underbrace{\mathrm{B}}_{\text{base}} \rightleftharpoons \underbrace{\mathrm{A^-}}_{\text{conj. base}} + \underbrace{\mathrm{HB^+}}_{\text{conj. acid}}

$\mathrm{HA / A^-}$ the first pair: the acid and what is left when it loses $\mathrm{H^+}$
$\mathrm{B / HB^+}$ the second pair: the base and what it becomes after gaining $\mathrm{H^+}$

Worked example

In pure ethanoic acid, perchloric acid reacts:

\mathrm{HClO_4 + CH_3COOH \rightarrow ClO_4^- + CH_3COOH_2^+}

. Identify the acid, the base and the two conjugate pairs.

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A conjugate pair differs by H⁺, not by OH⁻

The conjugate base is what remains after an acid loses a proton. It is never formed by adding

\mathrm{OH^-}

, and a conjugate acid is never formed by removing

\mathrm{OH^-}

. Options that describe conjugates in terms of hydroxide ions mix up Brønsted-Lowry with Arrhenius.

An acid can act as a base

A substance's role depends on its partner. Nitric or ethanoic acid accepts a proton from a much stronger acid, such as sulfuric or perchloric acid, and in that reaction it is the base.

Concept 3 of 3: Amphoteric and amphiprotic substances

Some substances can go either way. A species that holds an H it can lose and also has a lone pair to take an H⁺ can donate or accept a proton, depending on what it meets. Water is the everyday example: with an acid it accepts a proton, with a base it gives one.

Definition

An amphoteric substance can react both as an acid and as a base.
An amphiprotic species can both donate and accept a proton. All amphiprotic species are amphoteric.
Typical amphiprotic species: water, and ions that still carry an acidic H, such as $\mathrm{HCO_3^-}$ , $\mathrm{HPO_4^{2-}}$ and $\mathrm{H_2PO_4^-}$ .
Amphoteric oxides and hydroxides: $\mathrm{Al_2O_3}$ , $\mathrm{Al(OH)_3}$ , $\mathrm{ZnO}$ , $\mathrm{Zn(OH)_2}$ . They dissolve in strong acid and in strong alkali.
Amino acids carry an acidic $\mathrm{-COOH}$ and a basic $\mathrm{-NH_2}$ group, so they are amphoteric too.

Substance	Acting as an acid	Acting as a base
Water	$\mathrm{H_2O \rightarrow OH^-}$ (gives $\mathrm{H^+}$ to $\mathrm{NH_3}$ )	$\mathrm{H_2O \rightarrow H_3O^+}$ (takes $\mathrm{H^+}$ from $\mathrm{HCl}$ )
Hydrogencarbonate ion	$\mathrm{HCO_3^- \rightarrow CO_3^{2-}}$	$\mathrm{HCO_3^- \rightarrow H_2CO_3}$
Aluminium hydroxide	$\mathrm{Al(OH)_3 + OH^- \rightarrow [Al(OH)_4]^-}$	$\mathrm{Al(OH)_3 + 3H^+ \rightarrow Al^{3+} + 3H_2O}$
Amino acid (glycine)	$\mathrm{-COOH \rightarrow -COO^-}$	$\mathrm{-NH_2 \rightarrow -NH_3^+}$

Practice this conceptself-check · 3 quick reps

Amphoteric does not mean neutral

An amphoteric substance reacts with acids AND with bases. That says nothing about the pH of its own solution:

\mathrm{NaHCO_3}

solution is slightly basic,

\mathrm{NaH_2PO_4}

solution slightly acidic. Do not pick pH 7 just because a species is amphoteric.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Conjugate acid-base pairs in a proton transfer
Two conjugate pairs in every proton transfer
$\underbrace{\mathrm{HA}}_{\text{acid}} + \underbrace{\mathrm{B}}_{\text{base}} \rightleftharpoons \underbrace{\mathrm{A^-}}_{\text{conj. base}} + \underbrace{\mathrm{HB^+}}_{\text{conj. acid}}$

Reference tables (2)

Arrhenius, Brønsted-Lowry and Lewis definitions of acids and bases3 rows

Theory	Acid	Base	Example
Arrhenius	Gives $\mathrm{H^+}$ in water	Gives $\mathrm{OH^-}$ in water	$\mathrm{HCl}$ and $\mathrm{KOH}$ in water
Brønsted-Lowry	Proton donor	Proton acceptor	$\mathrm{NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-}$ : water donates, ammonia accepts
Lewis	Electron-pair acceptor	Electron-pair donor	$\mathrm{AlCl_3 + Cl^- \rightarrow AlCl_4^-}$ : $\mathrm{AlCl_3}$ accepts the pair A Lewis acid often has an incomplete octet (boron, aluminium) or is a positive metal ion.

Each definition includes the one above it, so a reaction can be acid-base in the Lewis sense even when no proton moves.

Amphoteric and amphiprotic substances4 rows

Substance	Acting as an acid	Acting as a base
Water	$\mathrm{H_2O \rightarrow OH^-}$ (gives $\mathrm{H^+}$ to $\mathrm{NH_3}$ )	$\mathrm{H_2O \rightarrow H_3O^+}$ (takes $\mathrm{H^+}$ from $\mathrm{HCl}$ )
Hydrogencarbonate ion	$\mathrm{HCO_3^- \rightarrow CO_3^{2-}}$	$\mathrm{HCO_3^- \rightarrow H_2CO_3}$
Aluminium hydroxide	$\mathrm{Al(OH)_3 + OH^- \rightarrow [Al(OH)_4]^-}$	$\mathrm{Al(OH)_3 + 3H^+ \rightarrow Al^{3+} + 3H_2O}$
Amino acid (glycine)	$\mathrm{-COOH \rightarrow -COO^-}$	$\mathrm{-NH_2 \rightarrow -NH_3^+}$

Watch out for (4)

A Lewis acid does not have to contain hydrogen
→ Arrhenius, Brønsted-Lowry and Lewis definitions of acids and bases
A conjugate pair differs by H⁺, not by OH⁻
→ Conjugate acid-base pairs in a proton transfer
An acid can act as a base
→ Conjugate acid-base pairs in a proton transfer
Amphoteric does not mean neutral
→ Amphoteric and amphiprotic substances

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