4.4.2.6 - Strong and weak acids

Concentration and acid strength (Higher)

Concentration describes the amount of acid dissolved per unit volume, while strength describes the degree of ionisation.

A concentrated acid solution contains a relatively large amount of acid per unit volume; a dilute one contains less. These words describe relative concentration and need a stated comparison. For instance, an acid solution at $1.0\,\mathrm{mol\,dm^{-3}}$ is more concentrated than a solution of the same acid at $0.10\,\mathrm{mol\,dm^{-3}}$. Dilution adds water, increasing volume without adding acid solute.

A strong acid is completely ionised in aqueous solution. A weak acid is only partially ionised. Ionisation produces ions, including hydrogen ions; it is not the same as dissolving. A weak acid can be well dissolved while most of its particles remain as un-ionised molecules.

Adding water to hydrochloric acid makes it more dilute but does not turn it into a weak acid. Adding more ethanoic acid can make its solution more concentrated but does not make that acid completely ionised. A pH reading reflects hydrogen-ion concentration, which depends on both the acid concentration and how it ionises. It does not by itself tell you both properties separately.

Complete and partial ionisation (Higher)

The proportion of acid particles that ionise determines how many hydrogen ions a given amount of acid supplies.

Hydrochloric, nitric and sulfuric acids are examples of strong acids. Ethanoic, citric and carbonic acids are examples of weak acids. Strong acids are completely ionised in aqueous solution; weak acids contain a mixture of un-ionised acid molecules and ions.

For hydrochloric acid, ionisation can be represented by:

For ethanoic acid, the reversible arrow represents partial ionisation:

Hydrogen ions and ethanoate ions can recombine, while other acid molecules ionise. The solution does not simply consist of permanently “active” and “inactive” molecules. At any instant only a proportion is ionised.

Compare solutions at the same acid concentration, under the same conditions. The stronger acid produces a greater hydrogen-ion concentration, so it has a lower pH. In a controlled comparison with the same clean metal pieces, this greater hydrogen-ion concentration can also produce a faster initial reaction. Reaction rate is supporting evidence under stated controls; strength itself is defined by ionisation, not by how violently an uncontrolled experiment appears to react.

Worked example 1 — complete versus partial ionisation

Illustrative data: two acids each release one hydrogen ion per ionised molecule and have concentration $0.10\,\mathrm{mol\,dm^{-3}}$. One is fully ionised and the other is 2% ionised. Convert percentage to a fraction and multiply:

The fully ionised acid gives more hydrogen ions and has lower pH. The supplied 2% belongs to this dataset; it is not a constant percentage for every weak-acid solution.

Worked example 2 — using a different supplied fraction

An acid at $0.080\,\mathrm{mol\,dm^{-3}}$ releases one hydrogen ion per ionised molecule and is 4% ionised under the stated conditions:

The result is smaller than the acid concentration, as partial ionisation requires. A completely ionised acid at the same concentration gives $0.080\,\mathrm{mol\,dm^{-3}}$, so its hydrogen-ion concentration is $0.080/0.0032=25$ times greater.

pH and hydrogen-ion concentration (Higher)

Each whole-unit decrease in pH means a tenfold increase in hydrogen-ion concentration.

The pH scale is not a linear count of hydrogen ions. A solution at pH 3 has ten times the hydrogen-ion concentration of a solution at pH 4, and one hundred times that of a solution at pH 5. A difference of two pH units therefore means $10\times10=100$, not twice as much.