Chemistry 2nd Paper · Classes 11-12 · Chapter 1 · 1.15

Brønsted-Lowry Theory and Conjugate Pairs.

There is not a single OH in an NH₃ molecule, yet chemistry calls it a base. Arrhenius's theory has no room for that. In 1923 two men asked the question differently: who gave the proton, and who took it? That one question settles acids, bases, amphoteric species and conjugate pairs together.

1. Who gave the proton, who took it

In 1923 Johannes Brønsted of Denmark and Thomas Lowry of England, working separately, gave the protonic theory of acids and bases. That is why the name comes as a pair: Brønsted-Lowry.

The definition is short. An acid is a proton (H⁺) donor and a base is a proton acceptor. What a substance is does not hang on its name; what matters is whether it gives or takes a proton in the reaction.

In water an acid hands over a proton and the water takes it:

HCl + H₂O → H₃O⁺ + Cl⁻
H₂SO₄ + H₂O → H₃O⁺ + HSO₄⁻
HSO₄⁻ + H₂O → H₃O⁺ + SO₄²⁻
H₂PO₄⁻ + H₂O → H₃O⁺ + HPO₄²⁻

In all four H₂O takes the proton, so here water is the base.

Now put HCl against other partners: NH₃ + HCl → NH₄⁺ + Cl⁻ and OH⁻ + HCl → H₂O + Cl⁻. Both times HCl gives the proton and NH₃ and OH⁻ take it, so they are the bases.

The link to Arrhenius runs one way only. Every Arrhenius acid is a Brønsted-Lowry acid, but not every Brønsted-Lowry base is an Arrhenius base. F⁻, CO₃²⁻ and NH₃ have a lone pair and can take a proton, yet they are not Arrhenius bases. OH⁻ is a base in both theories.

So the idea of a base has widened. And proton transfer is not tied to water: it can happen in a homogeneous gas, in aqueous and non-aqueous solutions and in heterogeneous mixtures.

2. The ones that go both ways

In HCl + H₂O → Cl⁻ + H₃O⁺ water takes a proton: it is a base. In NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ water gives a proton: it is an acid. The same water in two roles. This is called being amphoteric.

Water does it to itself. In the self-ionisation 2H₂O ⇌ H₃O⁺ + OH⁻ one molecule gives a proton (the acid) and another takes it (the base). So H₃O⁺ is the conjugate acid of H₂O and OH⁻ is its conjugate base.

Water is not alone. A compound that acts as an acid or a base depending on the situation is amphoteric. HCO₃⁻ as an acid gives CO₃²⁻ + H₂O; as a base it gives H₂CO₃ + HO⁻.

HSO₄⁻ is the same. As an acid it gives SO₄²⁻ + H₃O⁺; as a base (with N₂H₆²⁺) it gives H₂SO₄ + N₂H₅⁺. More amphoteric ions: HS⁻, H₂PO₄⁻, HPO₄²⁻, H₂PO₃⁻. Below are ten chips: the first seven belong to four particular reactions, the last three are ions on their own. Sort them.

3. Conjugate pairs: one H⁺ apart

Look at H₂S + NH₃ ⇌ HS⁻ + NH₄⁺. It holds two pairs: H₂S and HS⁻ are one conjugate pair, NH₃ and NH₄⁺ another.

The conjugate base is what an acid becomes after giving a proton: one H⁺ less, one more negative charge. The conjugate acid is what a base becomes after taking a proton: one H⁺ more. For example HCl → Cl⁻; NH₄⁺ ⇌ NH₃; H₂S → HS⁻.

The general form is acid1 + base2 ⇌ base1 + acid2. Written separately: acid ⇌ H⁺ + conjugate base, and base + H⁺ ⇌ conjugate acid.

Table 1.9 finds pairs by this rule. (1) HF + H₂O ⇌ F⁻ + H₃O⁺: pairs HF/F⁻ and H₃O⁺/H₂O. (2) H₂PO₄⁻ + OH⁻ ⇌ HPO₄²⁻ + H₂O: pairs H₂PO₄⁻/HPO₄²⁻ and H₂O/OH⁻. (3) HPO₄²⁻ + SO₃²⁻ ⇌ PO₄³⁻ + HSO₃⁻: pairs HPO₄²⁻/PO₄³⁻ and HSO₃⁻/SO₃²⁻. Two of them are amphoteric here, H₂O and HPO₄²⁻, because each is a base in one reaction and an acid in another.

Solved problem 1.68 does the same for one acid. By Arrhenius, HNO₂ → H⁺ + NO₂⁻, an acid. By Brønsted-Lowry, HNO₂ + H₂O ⇌ NO₂⁻ + H₃O⁺, and the conjugate base is NO₂⁻.

In the drawing, pick a box on the left, then touch its partner on the right. HCO₃⁻ and HSO₄⁻ appear in both modes because they are an acid once and a base once; read the role from the column heading.

4. A strong acid has a weak partner

The two members of a pair pull opposite ways. The conjugate base of a strong acid is weak; that of a weak acid is strong. Likewise the conjugate acid of a strong base is weak, and that of a weak base is strong. HCl is a strong acid, so Cl⁻ is a weak base. HCO₃⁻ is a weak acid, so its CO₃²⁻ is a strong base. OH⁻ → H₂O and NH₃ → NH₄⁺ follow the same rule.

In the book's table, strong acid with weak conjugate base: HCl/Cl⁻, HNO₃/NO₃⁻, H₂SO₄/HSO₄⁻, HSO₄⁻/SO₄²⁻. Weak acid with strong conjugate base: HNO₂/NO₂⁻, HCO₃⁻/CO₃²⁻. The weak acids: H₂CO₃, HCOOH, CH₃COOH, HCN, HF, H₂S, H₂O. Among bases, OH⁻ is strong (its conjugate acid H₂O is weak), NH₃ is weak (NH₄⁺ is strong), Cl⁻ is weak (HCl is strong).

The whole thing sits in one row. Order of acid strength: HClO₄ > H₂SO₄ > HI > HBr > HCl > HNO₃ > H₃O⁺ > HSO₄⁻ > H₂SO₃ > H₃PO₄ > HNO₂ > HF > HCOOH > CH₃COOH > H₂CO₃ > H₂S > NH₄⁺ > HCN > HCO₃⁻ > H₂O.

The order of conjugate base strength is exactly the reverse: ClO₄⁻ < HSO₄⁻ < I⁻ < Br⁻ < Cl⁻ < NO₃⁻ < H₂O < SO₄²⁻ < HSO₃⁻ < H₂PO₄⁻ < NO₂⁻ < F⁻ < HCOO⁻ < CH₃COO⁻ < HCO₃⁻ < HS⁻ < NH₃ < CN⁻ < CO₃²⁻ < OH⁻. So among Cl⁻, NO₂⁻, CH₃COO⁻ and CN⁻ the strongest base is CN⁻, because its acid HCN is the weakest. Now take ten acids from the book's row, in its own order, and arrange them yourself.

5. Check yourself

The first question is on the heart of the theory.

In the second, count one H⁺ to find the pair.

In the last, use the strong-weak rule.

The whole note on seven cards.

Source: Chemistry 2nd Paper, Classes 11-12 (NCTB), Chapter 1 “Environmental Chemistry”, section 1.15.