Chemistry 5.7 pH of Strong and Weak Acids

Study Notes

pH Basics

pH scale: <7 indicates acidity and >7 indicates alkalinity.
Strong acids fully dissociate, while weak acids do not.

Strong Acids

HCl (0.01M): Fully dissociates into H+ and Cl-.
- [H+] = 0.01M results in pH 2.
Acetic acid (HC2H3O2): Weak acid (Ka not provided); will have a pH < 2 if 0.1M.
H2SO4 (0.01M): Diprotic acid; first H fully dissociates, resulting in pH ~2.
Second proton in H2SO4 is weakly acidic and contributes minimally to [H+].

Strong Bases

NaOH (0.01M): Fully dissociates, yielding [OH-] = 0.01M.
- pOH 2 results in pH 12.
Ba(OH)2 (0.01M): Gives [OH-] = 0.02M leading to pH 12.3.
CsOH (10M): Results in very high [OH-] of 10M, pH 15.

Weak Acids and Ka

Acetic acid (0.1M HC2H3O2): Ka = 1.75 x 10^-5.
Ka expression: Ka = [H+][A-]/[HA].
Simplifying: at equilibrium, [H+] = √(Ka × HA).
Calculation shows [H+] ~ 1.3 x 10^-3, resulting in pH ~2.9.

Weak Bases and Kb

Kb indicates the strength of weak bases, defined by the equilibrium: A- + H2O ⇌ OH- + HA.
Kb expression: Kb = [OH-][HA]/[A-].
Determining [OH-]: [OH-] = √(Kb × [A-]).

Water Equilibrium Constant (Kw)

Kw = [H+][OH-] = 1.0 x 10^-14 at 25°C.
Change in temperature affects Kw.
Relationship: Ka × Kb = Kw; pKa + pKb = 14.
Stronger acids have higher Ka and correspondingly lower Kb for conjugate bases.

Temperature Effects on pH

In pure water, [H+] = [OH-], leading to pH 7.
Acidic solutions: [H+] > [OH-].
Basic solutions: [OH-] > [H+].
Neutral pH shifts with temperature changes; at 37°C, pH 7 is considered basic if Kw increases.

Acid and Base Strength

Stronger acids have larger Ka, resulting in smaller pKa values.
Stronger bases have larger Kb, resulting in smaller pKb values.

Description

Explore the fundamental concepts of pH levels in strong and weak acids through interactive flashcards. This quiz covers the definitions and calculations necessary to understand how the pH of various acids is determined and the implications for acidity and basicity.