Topic 14 · page 1 of 18

Acids, bases and conjugate pairs

An acid gives away a proton and a base takes one, so every acid-base reaction is a proton transfer, and every reaction contains two conjugate pairs.

12 min read14.114.2

Proton donors and proton acceptors 14.1

A hydrogen atom has one proton and one electron. Remove the electron and you have an H⁺ ion, which is just a proton. This is why chemists use "proton" and "H⁺" for the same thing.

An acid-base reaction is a proton transfer: the acid gives up an H⁺ and the base takes it. Neither can do its job alone. An acid only behaves as an acid when something is there to accept its proton.

Hydrogen chloride dissolved in water is the standard example:

HCl(g)+HX2O(l)→HX3OX+(aq)+ClX−(aq) \ce{HCl(g) + H2O(l) -> H3O+(aq) + Cl-(aq)}

HCl donates a proton, so it is the acid. Water accepts it, so water is the base. The base needs a lone pair of electrons to form the new bond to the proton, and the water molecule has two.

An H⁺ ion does not exist free in water. It joins a water molecule to form the oxonium ion, H₃O⁺. Chemists write H⁺(aq) as a shorthand for H₃O⁺(aq), and both are accepted. The shorthand makes equations shorter, but the proton transfer is still there.

Water can be either 14.1

Water accepted a proton above. It can also donate one. When ammonia dissolves, some of it takes a proton from water:

NHX3(aq)+HX2O(l)⇌NHX4X+(aq)+OHX−(aq) \ce{NH3(aq) + H2O(l) <=> NH4+(aq) + OH-(aq)}

Now ammonia is the base (it accepts H⁺) and water is the acid (it donates H⁺). A substance that can act as an acid or as a base depending on the other reactant is called amphoteric. Water is the commonest example.

Reaction Acid Base
HCl + H₂O HCl H₂O
NH₃ + H₂O H₂O NH₃
CH₃COOH + H₂O CH₃COOH H₂O

Conjugate acid-base pairs 14.2

Look at what each particle becomes once the proton has moved. The acid has lost an H⁺, so it is now a species that could accept one back: a base. The base has gained an H⁺, so it is now a species that could give one away: an acid.

Acid Conjugate base
HCl Cl⁻
H₃O⁺ H₂O
CH₃COOH CH₃COO⁻
NH₄⁺ NH₃
H₂O OH⁻
HCO₃⁻ CO₃²⁻

The stronger the acid, the weaker its conjugate base. Chloride is such a weak base that it hardly accepts a proton from water at all, which is why HCl is a strong acid. You will use this idea on the page about salts.

Every reaction has two pairs 14.2

In any acid-base reaction one species acts as an acid and another as a base. After the reaction there are two new species. That gives two conjugate pairs, and each pair has one reactant and one product.

Proton transfer in the reaction of ethanoic acid with ammonia. Ethanoic acid, acid 1, and ammonia, base 2, are the reactants. The ethanoate ion, base 1, and the ammonium ion, acid 2, are the products. A curved arrow shows H plus moving from ethanoic acid to ammonia, and dashed lines join each conjugate pair.
Two pairs in one reaction. Pair 1: CH₃COOH and CH₃COO⁻. Pair 2: NH₄⁺ and NH₃.
CHX3COOH+NHX3⇌CHX3COOX−+NHX4X+ \ce{CH3COOH + NH3 <=> CH3COO- + NH4+}

MethodIdentifying the pairs

  1. Find the reactant that loses an H⁺. That is acid 1. The product it becomes is base 1.
  2. Find the reactant that gains an H⁺. That is base 2. The product it becomes is acid 2.
  3. Check that each pair differs by exactly one H⁺.

Worked exampleMethanoic acid with ammonia

Identify the conjugate pairs in HCOOH+NHX3⇌HCOOX−+NHX4X+\ce{HCOOH + NH3 <=> HCOO- + NH4+}.

  1. Methanoic acid, HCOOH\ce{HCOOH}, loses an H⁺ to become HCOOX−\ce{HCOO-}. So HCOOH\ce{HCOOH} is acid 1 and HCOOX−\ce{HCOO-} is base 1.
  2. NH₃ gains an H⁺ to become NH₄⁺. So NH₃ is base 2 and NH₄⁺ is acid 2.
  3. Pair 1: HCOOH\ce{HCOOH} / HCOOX−\ce{HCOO-}. Pair 2: NH₄⁺ / NH₃. Each differs by one H⁺.

Check yourself

  1. State what is meant by a Brønsted–Lowry acid and a Brønsted–Lowry base.
  2. Write the formula of the conjugate base of (a) HNO₃, (b) HSO₄⁻, (c) H₃O⁺, and the conjugate acid of (d) NH₃, (e) OH⁻, (f) CO₃²⁻.
  3. In HCl+HX2O→HX3OX++ClX−\ce{HCl + H2O -> H3O+ + Cl-}, identify the two conjugate acid-base pairs.
  4. Write an equation to show that water can act as a base, and another to show that it can act as an acid. Name the term for a substance that can do both.
  5. In CHX3COOX−+HX2O⇌CHX3COOH+OHX−\ce{CH3COO- + H2O <=> CH3COOH + OH-} identify the acid and the base on each side, and the two conjugate pairs.
  6. A student says that in NHX3+HCl→NHX4Cl\ce{NH3 + HCl -> NH4Cl} the NH₃ and HCl form a conjugate pair. Explain what is wrong.

Answers cover these until you have tried the questions

  1. An acid is a proton donor (H⁺ donor). A base is a proton acceptor (H⁺ acceptor).
  2. Remove one H⁺ for a conjugate base: (a) NO₃⁻, (b) SO₄²⁻, (c) H₂O. Add one H⁺ for a conjugate acid: (d) NH₄⁺, (e) H₂O, (f) HCO₃⁻.
  3. HCl loses H⁺ to become Cl⁻: pair 1 is HCl (acid) / Cl⁻ (base). H₂O gains H⁺ to become H₃O⁺: pair 2 is H₃O⁺ (acid) / H₂O (base).
  4. Water as a base: HCl+HX2O→HX3OX++ClX−\ce{HCl + H2O -> H3O+ + Cl-} (it accepts a proton). Water as an acid: NHX3+HX2O⇌NHX4X++OHX−\ce{NH3 + H2O <=> NH4+ + OH-} (it donates a proton). A substance that can do both is amphoteric.
  5. Left side: H₂O donates H⁺, so it is the acid; CH₃COO⁻ accepts H⁺, so it is the base. Right side: CH₃COOH is the acid and OH⁻ is the base. Pair 1: H₂O / OH⁻. Pair 2: CH₃COOH / CH₃COO⁻.
  6. A conjugate pair differs by one H⁺ and is made of an acid and its own base. HCl and NH₃ are the two reactants: HCl is the acid and NH₃ is the base. The pairs are HCl / Cl⁻ and NH₄⁺ / NH₃.