Acid-Base Equilibria

Acid-Base Equilibria

Reminder:

ACID A substance that forms H⁺ ions in water (Arrhenius Definition)

BASE A substance that forms OH^- ions in water (Arrhenius Definition)

HCl

H⁺ H⁺

H⁺ H⁺

H⁺

Cl^- Cl^- Cl^-

Cl^- Cl^-

ACID

NaOH

Na⁺

Na⁺

Na⁺ Na⁺

Na⁺

OH^-

OH^- OH^-

OH^- OH^-

BASE

PROPERTIES

ACID: Taste sour

Turn litmus paper red Proton (H⁺) donor

BASE: Taste bitter

Turn litmus paper blue Feel oily, slippery, soapy Proton (H⁺) acceptor

Common Acid and Bases

The 7 Strong Acids !!!!

1. HCl hydrochloric acid 2. HBr hydrobromic acid 3. HI hydroiodic acid

4. HNO₃ nitric acid

5. H₂SO₄ sulfuric acid 6. HClO₃ chloric acid

7. HClO₄ perchloric acid

Strong Acids: COMPLETELY DISSOCIATE! (Ionize)

HCl _(aq) H⁺_(aq) + Cl^-_(aq)

HNO_{3 (aq)} H⁺_(aq) + NO₃^- _(aq)

H₂ SO_{4 (aq)} 2 H⁺_(aq) + SO₄^2- _(aq)

HCl

H⁺ H⁺

H⁺ H⁺

H⁺

Cl^- Cl^- Cl^-

Cl^- Cl^-

ACID

Weak Acids: Partially Dissociate (Ionize) HF _(aq) H⁺_(aq) + F^-_(aq)

H₂SO_{3 (aq)} H⁺_(aq) + HSO₃^-_(aq)

HC₂H₃O_{2 (aq)} H⁺_(aq) + C₂H₃O₂^-_(aq)

HF

HF H⁺

HF HF

HF

HF

F^- HF

HF HF

Weak Acid

Strong Bases NaOH

KOH LiOH

Mg(OH)₂

Weak Bases

NH₃ ammonia

NH₃ + H₂O NH₄OH

Bronsted-Lowry Definitions

ACID : proton (H⁺) DONOR

BASE: proton (H⁺) ACCEPTOR

HCl + H₂O H₃O⁺ + Cl^-

NH₃ + H₂O NH₄⁺ + OH^-

Conjugate Acid/Base Pairs

ACID BASE

HCl Cl^-

NH₄⁺

F^- H₂CO₃

REMOVE H+

ADD H+

Strength of Conjugate Acid/Base Pairs

Strong Acid / Weak Conjugate Base

HCl H⁺ + Cl^-

H₂SO₄ H⁺ + HSO₄^-

Weak Acid / Stronger Conjugate Base

HF H⁺ + F^-

H₂CO₃ H⁺ + HCO₃^-

Warm-up

An acid is a proton __________ and a base is a proton ___________

What is the conjugate base for HF?

What is the conjugate acid for NH₃?

Which base is stronger: NO₃^- or C₂H₃O₂^-

A few properties of logarithms:

log[4x5] = log[⁴₅] =

log[10⁶] = -log[10^-9] = -log[3x10^-6]

Solve for x: log[x] = -2

Autoionization of Water

H₂O(l) H⁺_(aq) + OH^-_(aq)

H₂O_(l) + H₂O(l) H₃O⁺_(aq) + OH^-_(aq)

“HYDRONIUM”

Same thing!

[H⁺ ] = [H₃O⁺]

Or…

H₂O(l) H⁺_(aq) + OH^-_(aq) K_w =

Water Dissociation Constant

pH = -log[H⁺]

If [H⁺] = 0.01 M, calculate pH

If [H⁺] = 1x10^-8 M, calculate pH

K_w = [H⁺][OH^-] = 10^-14

-log[H⁺][OH^-] = -log[10^-14]

pH + pOH = 14

[H⁺] = 10^-pH M [OH^-] = 10^-pOH M

If the pH = 6

pOH = [H⁺] = [OH^-] =

pH = -log[H⁺] pOH = -log[OH^-]

The pH Scale

Basic Acidic

Lots of H⁺ Few OH^-

Strong Acid

Lots of OH^- Few H⁺

Strong Base

H^{+ OH}

- H⁺

OH^-

H^{+ OH-}

Measuring pH

pH Meter (Probe) pH Paper (Litmus )

pH Color Indicator

ACID BASE

Warm-up

pH = [H⁺] =

pOH = [OH^-] =

[H⁺][OH^-] =

pH + pOH =

STRONG ACIDS

What is [H⁺] of a 0.10 M HCl solution?

What is the pH?

What is the pOH?

What is [OH^-]?

What is the [H⁺] of a 0.00050 M HNO₃ solution?

What is the pH?

What is the pOH?

What is [OH^-]?

STRONG BASES

What is the [OH^-] of a 0.20 M KOH solution?

What is the pOH?

What is the pH?

What is [OH^-]?

WEAK ACIDS (K_a)

Acid Dissociation Constant

- High K_A : more acid dissociates - Low K_A : less acid dissociates

HF

HF H⁺

HF HF

HF

HF

F^- HF

HF HF

Weak Acid

Given pH find Ka:

The pH of a 0.10 M solution of formic acid is 2.38. What is the Ka?

What is the % ionization?

Given K_a find pH:

What is the pH of a 0.25 M solution of acetic acid?

(Ka =1.8x10^-5)

What is the % ionization?

0.25 HC₂H₃O₂

HC₂H₃O₂

H⁺ HC₂H₃O₂

HC₂H₃O₂ HC₂H₃O₂

HC₂H₃O₂

C₂H₃O₂^- HC₂H₃O₂ HC₂H₃O₂

HC₂H₃O₂

pH = 7 pH = 0

pH = 14 1 L

1 M H⁺ 10^-14 M OH^- 1 g H⁺ 1.7x10^-14 g OH^-

999 g H₂O

10^-7 M H⁺ 10^-7 M OH^- 10^-7 g H⁺ 1.7x10^-6 g OH^-

1000 g H₂O

10^-14 M H⁺ 1 M OH^- 10^-14 g H⁺ 17 g OH^-

983 g H₂O

Given Ka find pH (Think x!)

Write the equation for the dissociation of lactic acid.

(HC₃H₅O₃)

What is the pH of a 0.50 M solution of Lactic acid?

(Ka = 1.4x10^-4)

If the pH of a 0.4 M solution of acetic is 2.1, determine the value of Ka.

Base Dissociation Constant (Kb)

NH₃ (aq) + H₂O (l) NH₄⁺ (aq) + OH^- (aq) Kb =

NH₄⁺ (aq) + H₂O (l) H₃O⁺ (aq) + NH₃ (aq) Ka =

Kw = Ka x Kb

What is the pH of 0.25 M NH₃? (Kb = 1.8x10^-5)

What is Ka for NH₄⁺?

Measuring pH Activity

- Use a VERY SMALL piece of pH paper - Rinse the pH meter after each test - DO NOT mix up samples!!!!

Warm-up:

What is the pH of 0.040 M HNO₃?

What is the pH of 6x10^-4 M KOH?

Calculate the pH of 0.50 M HF. (Ka = 6.8x10^-4)

Common Ion

A solution containing a weak acid and it’s conjugate base.

0.25 HC₂H₃O₂

H⁺ C₂H₃O₂^-

0.25 NaC₂H₃O₂

C₂H₃O₂^-

C₂H₃O₂^- C₂H₃O₂^-

C₂H₃O₂^-

H⁺

H⁺

H⁺ H⁺

HC₂H₃O2 H⁺ + C₂H₃O₂^-

What is the pH of a solution containing 0.25 M Acetic acid with 0.25 M sodium acetate?

Diprotic Acids

H₂CO3 H⁺ + HCO₃^1- Ka = 4.5x10^-7

HCO₃- H⁺ + CO₃^2- Ka = 5.6x10-11

Buffers

Solution the helps maintain a stable pH

BUFFER = Weak Acid + Conjugate Base Ex) HC₂H₃O₂ / NaC₂H₃O₂

NH₄⁺ / NH₃ Work Best at a pH ~ pKa

H⁺ C₂H₃O₂^- C₂H₃O₂^-

C₂H₃O₂^-

C₂H₃O₂^- C₂H₃O₂^-

H⁺

H⁺ H⁺

H⁺

Add Acid: H⁺ Add Base: OH^-

HC₂H₃O₂ / C₂H₃O₂^-

0.25 HC₂H₃O₂

H⁺ C₂H₃O₂^-

0.25 NaC₂H₃O₂

C₂H₃O₂^-

C₂H₃O₂^- C₂H₃O₂^-

C₂H₃O₂^-

H⁺

H⁺

H⁺ H⁺

HC₂H₃O2 H⁺ + C₂H₃O₂^-

Henderson-Hasselbach Equation: Buffers

MOLES OF NaOH Added

pH

14 13 12 10 9 8 7 6 5 4 3 2 1

H⁺

H⁺ H⁺

H⁺ H⁺ H⁺ H⁺ H⁺

H⁺ H⁺

H⁺ H⁺

NaOH

OH^-

OH^- OHOH^--

OH^- OH^- OH^- OH^-

OH^-

OH^-

Strong Acid/Strong Base

HCl

MOLES OF NaOH Added

pH

14 13 12 10 9 8 7 6 5 4 3 2 1

H⁺

H⁺ H⁺

H⁺ H⁺ H⁺ H⁺ H⁺

H⁺ H⁺

H⁺ H⁺

NaOH

OH^-

OH^- OHOH^--

OH^- OH^- OH^- OH^-

OH^-

OH^-

Weak Acid/Strong Base

HC₂H₃O₂

Use funnel to

fill SLOWLY to zero mark with NaOH

Add 20 mL of HCl with grad cylinder

Add 2 drops of indicator Slowly add NaOH

until color stays PINK!!!

Buret

Measure pH

M_AV_A = M_BV_B

Unknown 20 mL 0.20 M mL from Buret at endpoint

Solve for M_A !!!

ACID BASE

MOLES OF NaOH Added

pH

14 13 12 10 9 8 7 6 5 4 3 2 1

NaOH

Diprotic Acid (H₂CO₃)

H₂CO₃

HCO₃^-

CO₃^2-

H₂CO₃ H₂CO₃ H₂CO₃ H₂CO₃ HCO₃^- HCO₃^-

HCO₃^- HCO₃^-

CO₃^2-

CO₃^2- CO₃^2- CO₃^2-

HO^-

Structure and Acid Strength