Exercises: Models and conjugate pairs

Understanding of the behaviour of acids and bases developed decisively in the late 19th and early 20th centuries. Consider the substances $\mathrm{HCl}$ and $\mathrm{NH}_3$ at $25\,^{\circ}\mathrm{C}$.

1. a) Justify why hydrogen chloride behaves as an acid according to both Arrhenius's theory and the Brønsted-Lowry theory, writing the corresponding chemical equation in aqueous solution.
2. b) Explain, with reasoning, why a strict historical interpretation based only on the dissociation of hydroxides does not directly describe ammonia. Write its reaction with water and explain how proton transfer produces hydroxide ions, consistent with the extended aqueous definition of a base.

Two independent aqueous solutions of two monoprotic acids, $\mathrm{HA}$ and $\mathrm{HB}$, are prepared at the same initial analytical concentration $c_0 = 0.10\,\mathrm{mol\times L^{-1}}$ and at $25\,^{\circ}\mathrm{C}$. The diagram shows a microscopic sample of identical volume for both containers, in which a total equivalent of $6$ acid molecules was initially introduced into each solution. Observe the distribution of particles at equilibrium illustrated below:

1. a) From the count of species present at equilibrium (undissociated molecules and ions produced), determine the percentage or degree of dissociation of each acid.
2. b) Deduce, with reasoning, which of the two acids is stronger, justifying whether two solutions with exactly the same initial molar concentration of solute necessarily have the same acidity. Use the ideal model and neglect the autoprotolysis of water where that approximation is valid.

The concept of neutralisation has complementary interpretations depending on the theoretical framework used.

1. a) Describe the neutralisation reaction in aqueous solution between solutions of sodium hydroxide, $\mathrm{NaOH}$, and nitric acid, $\mathrm{HNO}_3$, writing the net ionic equation and justifying the process according to Arrhenius's definition.
2. b) Now consider the reaction between the gases ammonia, $\mathrm{NH}_3(\mathrm{g})$, and hydrogen chloride, $\mathrm{HCl}(\mathrm{g})$, to form the white solid ammonium chloride, $\mathrm{NH}_4\mathrm{Cl}(\mathrm{s})$, in the complete absence of solvent. Explain whether this process is a neutralisation according to Arrhenius and according to Brønsted-Lowry, indicating the acid, the base and the proton transferred.