Strong and weak acids

What an acid gives away

A hydrogen atom is the simplest atom there is: one proton in the nucleus and one electron around it. Take the electron away and only the proton is left. So a hydrogen ion, H⁺, is just a proton.

When an acid dissolves in water, its molecules split up, and each gives away a hydrogen ion: a proton. Hydrogen chloride, HCl, splits into H⁺ and Cl⁻ ions. The splitting of a molecule into ions like this is called dissociation: the acid dissociates. The more of its molecules dissociate, the more H⁺ ions the solution has.

That gives a way to explain the two bottles. Perhaps both acids gave away all their protons, or perhaps one of them kept most of its own.

Your investigation

Back on the bench, the four bottles stand where you left them. Now a magnifier opens over bottles A and C: its left half shows the particles in A, its right half the particles in C, with the water left out. One of the two bottles is hydrochloric acid, HCl, and the other is ethanoic acid, CH₃COOH, the acid in vinegar.

Before you look, predict: which bottle will show more H⁺ ions, and what else will you see in the other one?

What the magnifier showed

Under the magnifier, bottle C held nothing but ions: H⁺ ions and Cl⁻ ions, and not one whole HCl molecule. Every hydrogen chloride molecule had dissociated. Bottle C is hydrochloric acid.

Bottle A looked quite different. Most of what it held were whole CH₃COOH molecules, with just a few H⁺ ions and ethanoate ions, CH₃COO⁻, among them. Only a small share of its molecules had dissociated. Bottle A is ethanoic acid. That is where its missing hydrogen ions went: they never left their molecules.

(enlarge)Open larger

Strong and weak acids

Hydrochloric acid is a strong acid: it is completely dissociated in water. Every molecule gives up its H⁺ ion, and the equation, with state symbols, has a one-way arrow:

HCl(aq) → H⁺(aq) + Cl⁻(aq)

Ethanoic acid is a weak acid: it is partially dissociated in water. Only a small share of its molecules give up their H⁺ ions at any moment, so its equation has a two-way arrow:

CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq)

The ⇌ arrow means the reaction is reversible: molecules keep splitting into ions, and ions keep joining back into molecules, so most of the acid stays as whole molecules. Sulfuric acid and nitric acid are strong acids too; ethanoic acid, citric acid and the acid in fizzy drinks are weak.

This explains part 1. At the same concentration, a strong acid has a higher concentration of H⁺ ions than a weak acid, so it has the lower pH: bottle C, pH 1, against bottle A, pH 3.

The H⁺ ions are also what react. Magnesium ribbon in an acid reacts with the H⁺ ions to make hydrogen gas. In a strong acid there are more H⁺ ions in each cm³ than in a weak acid at the same concentration, so more of them meet the magnesium each second, and it fizzes faster. For a fair comparison, keep everything else the same: the temperature, the volume of acid and identical pieces of magnesium.

Trap: "A weak acid has no H⁺ ions." It has some: bottle A turned universal indicator orange, and an acid's colour comes from its H⁺ ions. A weak acid has fewer H⁺ ions than a strong acid at the same concentration, because most of its molecules stay whole.

Strong is not the same as concentrated

A shop sells a very dilute hydrochloric acid, 0.0010 mol/dm³: a hundred times more dilute than bottle C. Its pH is 3, the same as bottle A, the ethanoic acid. Has hydrochloric acid become a weak acid?

No. Every one of its molecules is still dissociated: it is still a strong acid. There is simply far less acid in each dm³, so there are fewer H⁺ ions. Two different ideas are at work. Strong or weak says how completely an acid dissociates. Concentrated or dilute says how much acid is dissolved in each dm³. A strong acid can be dilute, and a weak acid can be concentrated.

Trap: "A strong acid is a concentrated acid." Strong is about the share of molecules that dissociate; concentrated is about how much acid is in the solution. To compare the strength of two acids by their pH, they must be at the same concentration.

Proton donors and proton acceptors

Hold an open bottle of concentrated hydrochloric acid next to an open bottle of ammonia solution, and a white smoke forms in the air between them. Hydrogen chloride gas meets ammonia gas, and they make tiny grains of ammonium chloride. What has passed from one molecule to the other?

The HCl molecule gives its H⁺ ion, a proton, to the ammonia molecule, NH₃, which becomes an ammonium ion, NH₄⁺:

HCl + NH₃ → NH₄⁺ + Cl⁻

So an acid is a proton donor: it gives away H⁺ ions. A base is a proton acceptor: it takes them. In neutralisation, the hydroxide ion is the proton acceptor: H⁺ + OH⁻ → H₂O. Ammonia has no hydroxide in it, yet it is still a base, because it accepts protons.

(enlarge)Open larger

Key idea: Acids are proton donors and bases are proton acceptors. A strong acid is completely dissociated in water, HCl(aq) → H⁺(aq) + Cl⁻(aq); a weak acid is partially dissociated, CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq). At the same concentration, a strong acid has more H⁺ ions and a lower pH than a weak acid.

Case closed

Why did two acids at the same concentration give different pH values? Because they don't give up their protons in the same way. Hydrochloric acid is a strong acid: every molecule dissociates, so its solution is full of H⁺ ions and has pH 1. Ethanoic acid is a weak acid: only a few of its molecules dissociate, so it has far fewer H⁺ ions and pH 3. Every acid is a proton donor, and every base a proton acceptor. Strong and weak describe how completely an acid dissociates, not how much of it is dissolved.