http://www.ck12.org Chapter 21. Acids and Bases
21.3 Acid and Base Strength
Lesson Objectives
- Explain the difference between a strong acid and a weak acid.
- Explain the difference between a strong base and a weak base.
- Calculate the pH values of acidic or basic solutions based on the original concentration of the acid or base.
Lesson Vocabulary
- strong acid: An acid which when dissolved in water will completely transfer its proton to the solvent.
- weak acid: An acid whose dissociation favors the reactants at equilibrium; only a small number of the acidic
molecules dissociate to form H+. - strong base: A base whose dissociation favors the products at equilibrium; most moleculesdoremove a
proton from water. - weak base: A base whose dissociation favors the reactants at equilibrium; most molecules donot’ remove a
proton from water.
Check Your Understanding
- How are equilibrium constants calculated from data about the concentrations of each reacting species?
Strong vs. Weak Acids
So far, we have primarily been defining acids by their ability to donate an H+ion and bases by their ability to accept
an H+ion. However, acids and bases vary in their relative ability to undergo these processes.
In general, acids can be classified as strong or weak based on the extent to which they produce H 3 O+when dissolved
in water. For a generic acid, we can write the following equilibrium reaction:
HA(aq)+H 2 O(l) H 3 O+(aq)+A−(aq)Using the usual shorthand notation, this equation can also be written as follows:
HA(aq) H+(aq)+A−(aq)This type of equilibrium, in which a proton is being transferred to water, is often indicated by writing the equilibrium
constant as Ka. The relative position of this equilibrium for a given acid determines whether it will be considered
strong or weak. When dissolved in water, astrong acidwill completely transfer its proton to the solvent. In terms of