Acid–Base Theories
Questions 1–4The Brønsted–Lowry definition describes acids as proton donors and bases as proton acceptors. The Lewis definition describes acids as electron-pair acceptors and bases as electron-pair donors. Which statement best compares the two?
Every Brønsted acid acts as an electron-pair acceptor (the H+ accepts a lone pair from the base), so all Brønsted acids are also Lewis acids. However, the Lewis definition additionally covers species like BF3, AlCl3, and metal cations that accept electron pairs without any proton transfer involved. This makes the Lewis framework strictly broader than Brønsted–Lowry.
BF3 is a classic Lewis acid. Which orbital argument best explains why BF3 behaves as an acid under the Lewis definition?
BF3 has a trigonal planar geometry with boron in an sp2 hybridized state. The unhybridized 2p orbital on boron is empty and oriented perpendicular to the molecular plane. This vacant orbital can accept a lone pair from a Lewis base (such as NH3 or F−), forming a dative bond and producing a tetrahedral adduct like BF3·NH3. The electron deficiency at boron is the defining feature of its Lewis acidity.
The Lux–Flood definition is particularly useful in molten salt and high-temperature oxide chemistry. Under this framework, which of the following correctly identifies a Lux–Flood acid and base?
In the Lux–Flood definition, a base is an oxide ion (O2−) donor and an acid is an oxide ion acceptor. For example, CaO (base) donates O2− to SiO2 (acid) in the reaction: CaO + SiO2 → CaSiO3. This framework is especially useful in metallurgy, ceramics, and molten salt chemistry where protons are absent and oxide transfer is the dominant acid–base process.
Using Pearson's HSAB (Hard–Soft Acid–Base) theory, classify the following species: Ag+, Fe3+, F−, I−. Which combination would form the most stable complex?
Ag+ is a classic soft acid (large, polarizable, low charge density), while Fe3+ is a hard acid (small, high charge, low polarizability). F− is a hard base (small, electronegative, low polarizability), and I− is a soft base (large, polarizable). HSAB predicts that hard acids prefer hard bases and soft acids prefer soft bases: thus AgI (soft–soft) and FeF3 (hard–hard) are the thermodynamically favoured combinations.
Aqueous Acid–Base Chemistry
Questions 5–8
HF is a weak acid in water (pKa ≈ 3.45) despite fluorine being the most electronegative element. What is the primary reason for this unexpected weakness?
The H–F bond dissociation energy (~570 kJ/mol) is much higher than H–Cl (~432 kJ/mol), H–Br (~366 kJ/mol), or H–I (~298 kJ/mol). Although F− has a large hydration enthalpy due to its small size and high charge density, the thermodynamic cycle shows that the very large bond energy cost dominates, making HF a weak acid compared to HCl, HBr, and HI. High electronegativity alone does not guarantee strong acidity — bond strength is the decisive factor.
Rank the following oxoacids in order of increasing acid strength: HClO, H2SO3, H2SO4, HClO4. Which structural rule governs oxoacid strength?
Pauling's rules for oxoacid strength state that for an oxoacid of general formula (HO)mE(=O)n, the acid strength increases with n, the number of terminal (non-protonated) oxygen atoms. Each terminal O withdraws electron density from the E–O–H bond, weakening it and facilitating proton loss. HClO has n = 0 (very weak), H2SO3 has n = 1, H2SO4 has n = 2, and HClO4 has n = 3, making it the strongest.
Al(OH)3 and Zn(OH)2 are amphoteric hydroxides — they dissolve in both strong acids and strong bases. What primarily determines whether a metal hydroxide is amphoteric?
Amphoteric hydroxides occur for metals near the metal–nonmetal diagonal of the periodic table (Al, Zn, Sn, Pb, Cr in certain oxidation states). These metals have intermediate electronegativity: enough to form covalent M–O bonds (allowing dissolution in base to form aluminate/zincate ions) but not so much that the hydroxide is purely acidic. In acid, Al(OH)3 dissolves as [Al(H2O)6]3+; in base, it dissolves as [Al(OH)4]−.
The diagonal relationship between Li and Mg means their chemistry is remarkably similar despite being in different groups. How does HSAB help explain this?
Moving one step right and one step down in the periodic table (Li → Mg), the increase in ionic charge (+1 → +2) is offset by the increase in ionic radius, resulting in similar charge density (charge/radius ratio). Both Li+ (r = 76 pm) and Mg2+ (r = 72 pm) are small, hard cations that form strong bonds with hard bases like F− and O2−. This explains shared properties such as forming insoluble carbonates and fluorides, unlike Na+.
Non-Aqueous Solvents & Advanced Concepts
Questions 9–12
In water, HCl, HBr, and HI are all equally strong acids, yet their intrinsic (gas-phase) acidities differ substantially. What is this phenomenon called, and why does it occur?
The leveling effect means that any acid stronger than the solvent's conjugate acid (H3O+ in water) will be fully deprotonated, making H3O+ the strongest acid that can exist in aqueous solution. Since HCl, HBr, and HI are all stronger than H3O+, they all appear equally strong in water. To differentiate their true acidities, a less basic solvent such as acetic acid or liquid HF must be used, where the discrimination window is wider.
Liquid ammonia (bp −33 °C) is an important non-aqueous solvent for acid–base chemistry. How does it differ from water as a solvent system?
Liquid ammonia autoionizes as 2 NH3 ⇌ NH4+ + NH2−, with a much larger pKs (~33) than water (pKw = 14). Because NH3 is a stronger base than H2O, acids that are weak in water (such as acetic acid) become effectively strong in liquid ammonia. Conversely, species like NaNH2 (sodium amide) serve as the strong base in this solvent, analogous to NaOH in water.
The Drago–Wayland equation: −ΔH = EAEB + CACB provides a quantitative approach to acid–base interactions. How does it improve upon HSAB theory?
HSAB is qualitative — it tells you that hard–hard and soft–soft combinations are preferred but cannot predict how strong an interaction will be. The Drago–Wayland equation assigns each acid and base two parameters: E (electrostatic, analogous to "hardness") and C (covalent, analogous to "softness"). The product EAEB + CACB gives a quantitative prediction of the interaction enthalpy, making it far more useful for comparing competing reactions.
Superacids are acids stronger than 100% sulfuric acid. Which of the following correctly describes a superacid system?
A superacid is defined (by Gillespie) as any acid system with a Hammett acidity function H0 lower (more negative) than that of pure H2SO4 (H0 = −12). The system HSO3F/SbF5 ("magic acid") achieves H0 values as low as −23. SbF5 acts as a Lewis acid that abstracts F− from HSO3F, dramatically increasing the proton-donating ability. These systems can protonate extremely weak bases, including saturated hydrocarbons (C–H bonds).