Chapter 19 - Reactions of Acids and Bases

Water is amphoteric - can act as either an acid or base H 2 O H + + OH - (acting as acid) H 2 O H + + OH - (acting as acid) H 2 O + H + H 3 O + (acting as base) H 2 O + H + H 3 O + (acting as base) Water forms an equilibrium with itself Water forms an equilibrium with itself H 2 O + H 2 O H 3 O + + OH - H 2 O + H 2 O H 3 O + + OH - [H 3 O + ] [OH - ] Keq = [H 2 O] 2 [H 2 O] 2

K eq [H 2 O] 2 = [H 3 O + ] [OH - ] K eq [H 2 O] 2 = [H 3 O + ] [OH - ] H 2 O is a constant (the amount does not change much on percent basis) H 2 O is a constant (the amount does not change much on percent basis) K w = K eq [H 2 O] 2 = [H 3 O + ] [OH - ] K w = K eq [H 2 O] 2 = [H 3 O + ] [OH - ] K w = [H 3 O + ] [OH - ] = 1.0 x K w = [H 3 O + ] [OH - ] = 1.0 x K w is the ion product constant for water K w is the ion product constant for water K w = x 2 = 1.0 x K w = x 2 = 1.0 x x = 1.0 x x = 1.0 x 10 -7

If [H 3 O + ] > 1.0 x the solution is acidic If [H 3 O + ] > 1.0 x the solution is acidic If [H 3 O + ] < 1.0 x the solution is basic If [H 3 O + ] < 1.0 x the solution is basic See sample problem #1 See sample problem #1 Assignment – practice problems 1 & 2 on page 628 Assignment – practice problems 1 & 2 on page 628

pH scale – a more compact way to express [H 3 O + ] pH = -log [H 3 O + ] pH = -log [H 3 O + ] If pH < 7 the solution is acidic If pH < 7 the solution is acidic If pH > 7 the solution is basic If pH > 7 the solution is basic Example problem: What is the pH if Example problem: What is the pH if [H 3 O + ] = 7.3 x M? pH = -log (7.3 x ) = 4.14 Do practice problems 3 & 4 on page 630

More on pH Measuring pH – use acid-base indicators (will change color with pH) Measuring pH – use acid-base indicators (will change color with pH) Strong acids dissociate completely. What is the pH of a.1 M solution of HCl? What is the pH of a 1 M solution of HCl? What is the pH of a 6 M solution of HCl? One is not the lowest pH we can have.

Buffers – resist change in pH by absorbing or releasing H + ions. The most common buffers are mixtures of weak acids and their conjugate bases. The most common buffers are mixtures of weak acids and their conjugate bases. Example buffers: HC 2 H 3 O 2 + NaC 2 H 3 O 2 Example buffers: HC 2 H 3 O 2 + NaC 2 H 3 O 2 Acetic acid and its conjugate base the C 2 H 3 O 2 - ion NH 4 Cl and NH 3 (acid and conj. Base)

How a buffer works show diagram

The buffering capacity is limited by the amount of HC 2 H 3 O 2 and NaC 2 H 3 O2. If we run out of either the buffering capacity is exceeded. The buffering capacity is limited by the amount of HC 2 H 3 O 2 and NaC 2 H 3 O2. If we run out of either the buffering capacity is exceeded. Equal amount of acetic acid and acetate ion stablize the pH near 4.7 Equal amount of acetic acid and acetate ion stablize the pH near 4.7 We can derive this from the Ka of acetic acid, which is 1.8 x We can derive this from the Ka of acetic acid, which is 1.8 x 10 -5

[H 3 O + ] [C 2 H 3 O 2 - ] Ka = = 1.8 x [HC 2 H 3 O 2 ] [HC 2 H 3 O 2 ] If [C 2 H 3 O 2 - ] and [HC 2 H 3 O 2 ] are equal they cancel out of the equation Ka = [H 3 O + ] = 1.8 x M pH = -log 1.8 x = 4.74

Experimental Determination of acid concentration The concentration of a very strong acid can be determined by pH. The concentration of a very strong acid can be determined by pH. The concentration of a weak acid is more accurately determined by an acid/base titration. The concentration of a weak acid is more accurately determined by an acid/base titration. An acid-base titration is a carefully controlled neutralization reaction. (see page 637) An acid-base titration is a carefully controlled neutralization reaction. (see page 637)

Acid – Base Titration Equivalence point – when we have added just enough base to neutralize all of the acid. Equivalence point – when we have added just enough base to neutralize all of the acid. End point – when we are at the color change point of a titration End point – when we are at the color change point of a titration Phenolphthalein indicator – changes color over a pH range of 8 – 10 (clear to red) Phenolphthalein indicator – changes color over a pH range of 8 – 10 (clear to red)

Do example problem on page 638 together in class Do example problem on page 638 together in class Assignment – problems 5 & 6 on page 638 Assignment – problems 5 & 6 on page 638 Questions page (1-19, 32, 33) Questions page (1-19, 32, 33) Problem bank 37, 39, 41, 43, 45, 47, 49, 51, 53, 54, 56, 58, 60 (answers on page 945) Problem bank 37, 39, 41, 43, 45, 47, 49, 51, 53, 54, 56, 58, 60 (answers on page 945)