IGCSE Chemistry 0620: Strong vs Weak Acids. Dissociation, pH and Conductivity

A campus safety office is comparing two spill-neutralising rinse solutions of the same concentration, $0.50\text{ mol/dm}^3$ : Rinse A is hydrochloric acid, and Rinse B is ethanoic acid. Both rinses are tested with a pH probe and a conductivity meter.

(a) Define the term proton donor, and use it to explain why both hydrochloric acid and ethanoic acid are classified as acids. [2]

(b) Write an equation to show hydrochloric acid dissociating in aqueous solution, and an equation to show ethanoic acid dissociating in aqueous solution. [2]

(c) The results show that Rinse A has a lower pH and a higher electrical conductivity than Rinse B, even though both rinses have the same concentration. Explain this observation in terms of dissociation. [3]

Part (a): Proton donor

A proton donor is a substance that donates (gives away) a hydrogen ion, $\text{H}^+$ , to another substance.

Both hydrochloric acid, $\text{HCl}$ , and ethanoic acid, $\text{CH}_3\text{COOH}$ , release $\text{H}^+$ ions when dissolved in water. Because each releases $\text{H}^+$ ions to the surrounding water molecules, both fit the definition of an acid as a proton donor. Regardless of how completely they do so.

Part (b): Dissociation equations

Hydrochloric acid is a strong acid. It is completely (fully) dissociated in aqueous solution, shown with a one-way arrow:

$\text{HCl(aq)} \rightarrow \text{H}^+\text{(aq)} + \text{Cl}^-\text{(aq)}$

Ethanoic acid is a weak acid. It is only partially dissociated in aqueous solution, shown with an equilibrium (reversible) arrow, since most of the acid remains as undissociated molecules:

$\text{CH}_3\text{COOH(aq)} \rightleftharpoons \text{H}^+\text{(aq)} + \text{CH}_3\text{COO}^-\text{(aq)}$

Part (c): Explaining the pH and conductivity results

Both rinses have the same concentration, $0.50\text{ mol/dm}^3$ , so the difference in results must come from how completely each acid dissociates, not from how much acid is dissolved.

Rinse A (hydrochloric acid) is a strong acid: essentially every $\text{HCl}$ molecule dissociates into ions. This produces a large concentration of free $\text{H}^+$ ions, which gives Rinse A its lower pH. It also produces a large concentration of free ions overall ( $\text{H}^+$ and $\text{Cl}^-$ ), which is why Rinse A conducts electricity more strongly.
Rinse B (ethanoic acid) is a weak acid: only a small proportion of $\text{CH}_3\text{COOH}$ molecules dissociate at any moment, while most remain as neutral, undissociated molecules. This produces far fewer free $\text{H}^+$ ions, giving Rinse B a higher pH, and far fewer free ions overall, so Rinse B conducts electricity more weakly.

Since a solution’s conductivity depends on the number of free ions present (not on the total number of acid molecules dissolved), the strong acid, despite having the same concentration, conducts far better than the weak acid.

Final answers

(a) A proton donor releases $\text{H}^+$ ions; both acids do this, so both are classified as acids.
(b) $\text{HCl(aq)} \rightarrow \text{H}^+\text{(aq)} + \text{Cl}^-\text{(aq)}$ (strong acid); $\text{CH}_3\text{COOH(aq)} \rightleftharpoons \text{H}^+\text{(aq)} + \text{CH}_3\text{COO}^-\text{(aq)}$ (weak acid).
(c) Rinse A dissociates fully, giving more free ions (lower pH, higher conductivity); Rinse B dissociates only partially, giving fewer free ions (higher pH, lower conductivity), despite equal concentration.