Problem 32 - Chemistry: The Central Science

Question

Explain the following observations: (a) \(\mathrm{NH}_{3}\) contains no \(\mathrm{OH}^{-}\) ions, and yet its aqueous solutions are basic; (b) HF is called a weak acid, and yet it is very reactive; (c) although sulfuric acid is a strong electrolyte, an aqueous solution of \(\mathrm{H}_{2} \mathrm{SO}_{4}\) contains more \(\mathrm{HSO}_{4}^{-}\) ions than \(\mathrm{SO}_{4}^{2-}\) ions.

Step-by-Step Solution

Answer

(a) NH3 does not contain OH- ions, but when dissolved in water, it reacts with water molecules to form NH4+ and OH- ions, making the solution basic. (b) HF is a weak acid due to its partial ionization in aqueous solutions; however, its reactivity stems from fluorine's high electronegativity, which makes it a strong nucleophile and an excellent ligand. (c) In aqueous H2SO4 solutions, there are more HSO4- ions than SO4^2- ions because H2SO4 ionizes almost completely into H+ and HSO4-, while the second ionization step (HSO4- to H+ and SO4^2-) is not as complete since HSO4- is a weaker acid.

Step 1: (a) Absence of OH- ions in NH3 and basicity of its aqueous solutions

Ammonia, NH3, is a neutral compound, as it neither contains OH- ions nor H+ ions. However, when dissolved in water, NH3 molecules react with water molecules to form NH4+ and OH- ions: \[ NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^- \] The presence of OH- ions in the solution indicates that the solution is basic. Therefore, although NH3 itself does not contain OH- ions, it forms a basic solution when dissolved in water.

Step 2: (b) HF as a weak acid and its reactivity

A weak acid is defined by its partial ionization in aqueous solutions. When dissolved in water, hydrofluoric acid, HF, does not completely ionize into H+ and F- ions, making it a weak acid: \[ HF ⇄ H^+ + F^- \] The equilibrium very much lies to the left, which means less H+ ions are released compared to strong acids. Nonetheless, HF is very reactive due to the high electronegativity of fluorine, making it a strong nucleophile and an excellent ligand. Fluorine can form strong bonds with other atoms, and this reactivity (not referring to basicity or acidity) is independent of HF's ionization.

Step 3: (c) Relative concentrations of HSO4- and SO4^2- ions in aqueous H2SO4 solutions

Sulfuric acid, H2SO4, is a strong acid, meaning that it ionizes almost completely in aqueous solutions into H+ ions and its conjugate base, HSO4-: \[H_2SO_4 \rightarrow H^+ + HSO_4^-\] However, the bisulfate ion (HSO4-) can also act as an acid, donating an additional H+ ion: \[HSO_4^- ⇄ H^+ + SO_4^{2-}\] This second ionization step is not as complete as the first one, as HSO4- is a weaker acid. Consequently, while sulfuric acid is a strong electrolyte, the equilibrium for the second ionization step lies more to the left, resulting in a higher concentration of HSO4- ions compared to SO4^2- ions in an aqueous solution of H2SO4.