Chapter 18 review - Lecture notes 18 PDF

Preview

Summary

Lecture review...

Description

Chapter 18. Equilibrium 18.1 Acids and bases in water 1. Arrhenius definitions: a. Acid = a substance with H in its formula that dissociates to produce H3O+ in water. b. Base = a substance with OH in its formula that dissociates to produce OH‒ in water. c. Neutralization = the reaction of H3O+ with OH‒ to produce water. 2. Strong acids completely dissociate in water, weak acids partially dissociate. 3. For the weak acid, HA, reaction with water results in an equilibrium state: HA(aq) + H2O(l) ⇄ H3O+(aq) + A‒(aq) 4. The equilibrium constant for a weak acid dissociation is known as Ka: +¿ ¿ H3O ¿ −¿ ¿ A ¿ ¿ K a=¿

18.2 Autoionization of water and the pH scale 1. Water can undergo an autoionization reaction: 2 H2O(l) = H3O+(aq) + OH‒(aq) 2. The equilibrium constant for autoionization is Kw: +¿ ¿ H3O ¿ −¿ ¿ OH ¿ K w=¿ 3. The concentrations of hydronium and hydroxide ions in pure water at 25.0oC can be determined from Kw: +¿ ¿ H3O ¿ −¿ ¿ OH ¿ ¿ 4. pH is defined using a logarithmic scale:

1

+¿ H 3 O¿ ¿ ¿ pH=−log¿ 5. pOH is defined similarly: −¿ OH ¿ ¿ ¿ pOH=−log ¿ 6. pKw is also defined: p K w =− log K w=14.00(at 25.0℃ ) +¿ +¿ ¿ ¿ H3O H3O ¿ ¿ 7. Acidic solutions have , and pH < 7.00; basic solutions have , and pH > −¿ −¿ ¿ ¿ OH OH ¿ ¿ 7.00.

18.3 Brønsted-Lowry acids and bases 1. Brønsted-Lowry definitions: a. Acid = a proton donor. b. Base = a proton acceptor. c. Neutralization = proton transfer reaction from an acid to a base forming a conjugate base and a conjugate acid. HA

+

Acid

B

=

Base

BH+

+

Conj. acid

A‒ Conj. base

2. HA and A‒ are a conjugate acid-base pair; BH+ and B are a conjugate acid-base pair.

18.4 Solving weak acid equilibria 1. ICE tables are very useful 2. For acids that dissociate very little, a simplifying assumption makes computation easier: C0 x < 0.05(5 %) , the > 400 , x ≪ C 0 and it can be neglected. Generally, if If C Ka 0 assumption is valid.

18.5 Acid strength 2

1. For nonmetal hydrides, acid strength increases with the electronegativity of the nonmetal element. 2. For oxo-acids, acid strength increases with the electronegativity of the central atom; and acid strength increases with the number of oxygen atoms surrounding the central atom. 3. Small, highly charged metal ions can hydrolyze in water to produce an acid solution: Al(H2O)63+(aq) + H2O(l) = Al(H2O)5(OH)2+(aq) + H3O+(aq)

18.6 Weak bases 1. Bases react with water to produce OH‒(aq) ions: B(aq) + H2O(l) = BH+(aq) + OH‒(aq) 2. The equilibrium constant for a weak base is given by: +¿ ¿ BH ¿ −¿ ¿ OH ¿ ¿ K b=¿ 3. Similarly, the conjugate base of a weak acid hydrolyzes as: A‒(aq) + H2O(l) = HA(aq) + OH‒(aq) 4. The equilibrium constant for the conjugate base is: −¿ ¿ OH ¿ −¿ A¿ ¿ ¿ [ HA ] ¿ K b =¿ 5. For any conjugate acid-base pair: K a × K b =K w ∧ p K a + p K b= p K w

3

18.7 Properties of salt solutions 1. Salts consisting of the cation of a strong base and the anion of a strong acid form neutral solutions. 2. Salts consisting of the cation of a weak base and the anion of a strong acid form acidic solutions. BH+ + H2O = B + H3O+ 3. Salts consisting of the cation of a strong base and the anion of a weak acid form basic solutions. A‒ + H2O = HA + OH‒ 4. Salts consisting of a small, highly charged cation and the anion of a strong acid form acidic solutions. Al(H2O)63+(aq) + H2O(l) = Al(H2O)5(OH)2+(aq) + H3O+(aq)

18.8 The leveling effect 1. In water, the strongest acid that can exist is H3O+ 2. In water, the strongest base that can exist is OH‒

18.9 Lewis acids and bases 1. A Lewis acid is an electron pair acceptor. 2. A Lewis base is an electron pair donor 3. Neutralization in the Lewis formalism is the formation of a coordinate-covalent bond.

4...