Key Concepts and Summary
The relative strengths of acids and bases are reflected in the magnitudes of their ionization constants; the stronger the acid or base, the larger its ionization constant. A reciprocal relation exists between the strengths of a conjugate acid-base pair: the stronger the acid, the weaker its conjugate base. Water exerts a leveling effect on dissolved acids or bases, reacting completely to generate its characteristic hydronium and hydroxide ions (the strongest acid and base that may exist in water). The strengths of the binary acids increase from left to right across a period of the periodic table (CH4 < NH3 < H2O < HF), and they increase down a group (HF < HCl < HBr < HI). The strengths of oxyacids that contain the same central element increase as the oxidation number of the element increases (H2SO3 < H2SO4). The strengths of oxyacids also increase as the electronegativity of the central element increases [H2SeO4 < H2SO4].
Key Equations
| $K_a=\frac{[H_3O^+][A^-]}{[HA]}$ |
| $K_b=\frac{[HB^+][OH^-]}{[B]}$ |
| $K_a\times K_b=1.0×10^{-14}=K_w$ |
| Percent ionization $=\frac{[H_3O^+]_{eq}}{[HA]_0}\times 100$ |
Practice Questions
Explain why the neutralization reaction of a strong acid and a weak base gives a weakly acidic solution.
Explain why the neutralization reaction of a weak acid and a strong base gives a weakly basic solution.
Solution
The salt ionizes in solution, but the anion slightly reacts with water to form the weak acid. This reaction also forms OH−, which causes the solution to be basic.
Use this list of important industrial compounds (and this table) to answer the following questions regarding: CaO, Ca(OH)2, CH3CO2H, CO2, HCl, H2CO3, HF, HNO2, HNO3, H3PO4, H2SO4, NH3, NaOH, Na2CO3.
(a) Identify the strong Brønsted-Lowry acids and strong Brønsted-Lowry bases.
(b) List those compounds in (a) that can behave as Brønsted-Lowry acids with strengths lying between those of H3O+ and H2O.
(c) List those compounds in (a) that can behave as Brønsted-Lowry bases with strengths lying between those of H2O and OH−.
The odor of vinegar is due to the presence of acetic acid, CH3CO2H, a weak acid. List, in order of descending concentration, all of the ionic and molecular species present in a 1-M aqueous solution of this acid.
Solution
$[H_2O]>[CH_3CO_2H]>[H_3O^+]≈[CH_3CO_2^-]>[OH^-]$
Household ammonia is a solution of the weak base NH3 in water. List, in order of descending concentration, all of the ionic and molecular species present in a 1-M aqueous solution of this base.