![Measuring strength
• Ionization is reversible.
• HA ↔ H+
+ A-
• This makes an equilibrium
• Acid dissociation constant = Ka
• Ka = [H+
][A-
] (water is constant)
[HA]
• Stronger acid = more products (ions),
thus a larger Ka (Table 20.8, page 602)](https://image.slidesharecdn.com/20sec4-101028113525-phpapp02/85/20sec4-6-320.jpg)
![What about bases?
• Strong bases dissociate completely.
• MOH ↔ M+
+ OH-
(M = a metal)
• Base dissociation constant = Kb
• Kb = [M+
][OH-
]
[MOH]
• Stronger base = more dissociated
ions are produced, thus a larger Kb.](https://image.slidesharecdn.com/20sec4-101028113525-phpapp02/85/20sec4-7-320.jpg)
20sec4
- 1. 20.4 Strengths of Acids and Bases
- 2. Strength • Acids and Bases are classified according to the degree to which they ionize in water: –Strong are completely ionized in aqueous solution; this means they ionize 100 % –Weak ionize only slightly in aqueous solution • Strength is different from concentration
- 3. Strength • Strong – means it forms many ions when dissolved (100 % ionization) • Mg(OH)2 is a strong base- it falls completely apart when dissolved. –But, not much dissolves- so it is not concentrated
- 6. Measuring strength • Ionization is reversible. • HA ↔ H+ + A- • This makes an equilibrium • Acid dissociation constant = Ka • Ka = [H+ ][A- ] (water is constant) [HA] • Stronger acid = more products (ions), thus a larger Ka (Table 20.8, page 602)
- 7. What about bases? • Strong bases dissociate completely. • MOH ↔ M+ + OH- (M = a metal) • Base dissociation constant = Kb • Kb = [M+ ][OH- ] [MOH] • Stronger base = more dissociated ions are produced, thus a larger Kb.
- 8. Strength vs. Concentration • The words concentrated and dilute tell how much of an acid or base is dissolved in solution - refers to the number of moles of acid or base in a given volume • The words strong and weak refer to the extent of ionization of an acid or base • Is a concentrated weak acid possible?
- 9. Practice • Write the expression for HNO2 • Write the Kb for NH3 (as NH4OH)
- 10. - Page 610