Cross-Topic Integration (23 of 26)
Two lewis structures of the canonical structures of the nitrite ion, NO2?.
Nitrite resonance forms. Nitrite is the conjugate base of the weak acid, nitrous acid.

Let us revisit a few favorite examples to discuss the underlying chemical thermodynamics governing the relative strength of acids. Why is ethanol a weaker acid than acetic acid? Why is acetic acid a weaker acid than trifluoracetic acid? When each of these acids disassociates, it forms a negatively charged conjugate base. Characteristics of the conjugate base which allow the negatively charged species to exist at a lower energy will reduce the enthalpy of the right side of the equation shifting the equilibrium towards disassociation. The internal energy differences among ethoxide, acetate, and trifluoroacetate underly the differences in the value of the respective pKa's. Acetic acid is a stronger acid than ethanol because resonant acetate holds its negative charge at lower energy than ethoxide. Trifluoroacetic acid is a stronger acid than acetic acid because the electronegative fluorines of trifluoroacetate stabilize the negative charge of the species by induction.

Similar reasoning, applies to the oxygen acids, the strength of which can be roughly estimated by the value of m in the variant empirical formula XOm(OH)n, as compared to the hydrogen number, n. The more oxygens, the more diffuse the negative charge on the anion. Sulfuric acid (H2SO4 rearranged to SO2(OH)2)is stronger than carbonic acid (H2CO3 rearranged to CO(OH)2) for example. It takes more resonance forms to describe sulfate than carbonate. The negative charge has more places to spread out.

Although HCl and HBr are both strong acids, HBr is a stronger acid than HCl. The anion radius of Cl- crowds the negative charge into a smaller space. A small metal sphere has a lower capacitance than a large metal sphere. The same amount of charge produces a higher voltage on the small sphere. (It is very good to abstract capacitance in this way. Capacitance describes how the geometry of a charge distribution determines its potential). We might say that chloride's geometry has lower capacitance than bromide's. Things aren't exactly so "physics simple" in the crazy, complicated world of chemistry, but this is a useful heuristic.