ACIDS AND BASES

CHAPTER 15

I. Arrhenius Acids and Bases

(What we have been using to this point)

Arrhenius Acid is a substance that, when dissolved in water, increases the concentration of hydrogen ion, H+ or (hydronium ion H3O

+).

Arrhenius Base is a substance that, when dissolved in water, increases the concentration of hydroxide ions, OH-.

II. Bronsted-Lowry Acids & Bases

A. Definitions

1. Bronsted Acid: H+ (proton) donor

Ionizable hydrogen in the acid structure is usually bonded to an electronegative atom.

2. Bronsted Base: H+ (proton) acceptor

Base structure must have an unshared pair of electrons.

B. Broader Definition For Acid-Base Reactions

Consider: HCl + NH4OH NH4Cl + H2O

Consider: HF + HCO3- H2CO3 + F-

C. Amphiprotic Compounds

Some compounds can act as a Bronsted acid in one case and a Bronsted base in another case. It depends on the molecules the amphiprotic compound is reacting with.

Example: Water

HCl + H2O H3O+ + Cl-

NH3 + H2O NH4+ + OH-

D. Conjugate Acid-Base Pairs A conjugate acid-base pair consists of two species in an acid-base reaction, one acid and one base, that differ by the loss or gain of a proton.

Conjugate acid- species formed when a base has accepted a proton.

Conjugate base- species formed when an acid has donated or removed a proton.

Example: Acetic Acid

CH3COOH ⇌ CH3COO- + H+

CH3COOH + H2O ⇌ CH3COO- + H3O+

Example Problems

1. What is the conjugate base of HNO3 ?

2. What is the conjugate acid of NH2- ?

3. Label all species in the following reaction.

H2PO4- + HCO3

- ⇌ H2CO3 + HPO42-

E. Strong vs. Weak Acids and Bases

1. Strong Acids and Bases

Essentially go to 100% ionization

Rxn normally shown with single headed arrow.

Strong Acids Strong BasesHCl LiOH

HBr NaOH

HI KOH

HNO3 Ca(OH)2

H2SO4 Ba(OH)2

HClO4 Sr(OH)2

HNO3 → H+ + NO3-

2. Weak Acids and Bases

Partially dissociate or ionize, reaching some dynamic equilibrium state.

Rxn normally shown with double headed arrow.

HCN H⇌ + + CN-

Weak acids and bases significant in biological systems.

F. Examples of Organic Weak Acids and Bases

Carboxylic Acids: (weak acids)

RCO2H or RCOOH

Anion formed by ionization is resonance stabilized

Examples: pyruvic acid, lactic acid, acetic acid

Amines: (weak bases)

Based on ammonia, with differing # of “R” groups

Primary, secondary, tertiary

G. Polyprotic Acids and Bases

Polyprotic acids can donate more than one proton sequentially.

H2SO4

H3PO4

H2S

Polyprotic bases can accept more than one proton sequentially.

SO42-

PO43-

S2-

III. Self-Ionization of Water

DescriptionPure water slightly ionizes on its own (autoionizes)

with equilibrium reactant favored.

H2O(l) + H2O(l) ⇌ H3O+(aq) + OH-(aq)

Determine the Kc value.

Kc for water ionization = Kw

Kw = ionization constant for water or

= ion product constant for water

Kw = [H+] [OH-] = 1 x 10-14 at 25oC

What are [H+] and [OH-] for pure water at 25oC?

True for Water and Dilute Aqueous Solutions

(Including Acid and Base Solutions of Interest)

[H+] [OH-] = 1 x 10-14 or

[H3O+] [OH-] = 1 x 10-14

Neutral Solution [H+] = [OH-]

Acidic Solution [H+] > [OH-]

Basic Solution [H+] < [OH-]

or [H3O+]

IV. pH Scale

In most aqueous weak acid and base solutions, the H+ concentrations are very small. Easier to express concentrations using a logarithmic relationship. p = - log

Thus: pH = -log [H+]

pOH = -log [OH-]

What is the relationship between pH and pOH?

[H+] [OH-] = 1 x 10-14

(-log [H+]) + (-log [OH-]) = -log (1 x 10-14)

pH + pOH = 14

pH + pOH = 14

The pH Scale

Since pH is a logarithmic scale, cola drinks (pH about 2.5) are about ____ times as acidic as

tomatoes (pH about 4.5)

Recall for problem solving:

****[H+] is the same as [H3O+]

1) pH = -log [H+]

2) [H+] = 10-pH or antilog (-pH) = [H+]

3) pOH = -log [OH-]

4) [OH-] = 10-pOH or

antilog (-pOH-) = [OH-]

5) [H+] [OH-] = 1 x 10-14

6) pH + pOH = 14

Problem Solving:

1) A blood sample (considered a dilute aqueous solution) contains 7.2 x 10-8 mol H+ per liter.

a. What is the pH of the blood sample?b. What is the [OH-] of the blood sample?c. Is the blood sample acidic, basic, or neutral?

2) The pH of an aqueous solution is 8.7a. What is the [H+]?b. What is the pOH?

V. Strong Acids and Bases Strong acids and bases

undergo essentially complete

dissociation (ionization).

Equilibrium expressions not

needed for problem solving.

Example: What is the pH and [H+] for a

0.05 M HCl solution?

Problem Solving for Weak Acids and Bases

1. They only dissociate or ionize partially in solution.

2. This means we have an equilibrium situation, and can solve problems using the techniques we learned earlier for dealing with equilibrium problems. (ICE tables…)

3. Equilibrium Constants (Kc)

Ka for weak acids

Kb for weak bases

VI. Weak Acid Equilibrium Rxns

A. Acid Ionization Constant 1. Definition

Give the acid ionization constant expression for

the acid: HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)

or HA(aq) ⇌ H+(aq) + A-(aq)

Ka = ?

2. What does the Ka tell you?

Larger Ka stronger acid

(more of the acid ionizes in water to form H+)

See Table 15.5, for examples

Which is the strongest acid?

Acetic acid Ka = 1.8 x 10-5

Boric Acid Ka = 7.3 x 10-10

B. Ka From Equilibrium Concentrations

A 0.10 M solution of phenol (weak acid) has a pH of 5.43. Calculate the Ka for phenol.

C. pH Calculations from Ka of Weak Acid

The weak monoprotic acid, parahydroxybenzoic acid, has a Ka of 2.6 x 10-5. Answer the following questions for a 0.200 M solution of parahydroxybenzoic acid.

1. What is the pH of the solution?

2. What percent of the parahydroxybenzoic acid has ionized in this solution?

D. Comment on Polyprotic Acids

Example: H3PO4

Ka’s = 7.5 x 10-3, 6.2 x 10-8, 3.6 x 10-13

1. Show the three acid ionization equations

with their accompanying Ka values.

2. Which is the strongest acid?

3. How would you go about calculating the pH of a given H3PO4 solution?

VII. Weak Base Equilibrium Rxns

A. Base Ionization Constant

(base will accept proton from H2O, forming OH-)

1. Definition

Give the base ionization constant expression for

the base: B(aq) + H2O(l) ⇌ BH+(aq) + OH-(aq)

Kb = ?

2. Relative Strength of Weak Bases

a. The Kb for ammonia is 1.8 x 10-5.

The Kb for phosphate ion is 2.8 x 10-2.

Which is the strongest base?

b. Which will have a higher pH:

a 1.0 M ammonia solution, or

a 1.0 M phosphate ion solution

B. Kb From Equilibrium Concentrations

The pain killer, morphine, is a weak base. A 0.01 M morphine solution has a pH of

10.1. Calculate the Kb for morphine.

C. pH Calculations from Kb of Weak Base

The weak base methylamine (CH3NH2) has a Kb of 5.0 x 10-4. Answer the following

questions for a 0.080 M aqueous solution of methylamine.

a. Write the chemical equation.

b. Write the base ionization equilibrium expression.

c. Calculate the [OH-], pOH, and pH for solution.

VIII. Relationship of Ka and Kb

A. Conjugate Acid and Base Reaction May Be Written in Both Directions. For Example:

HA + H2O ⇌ H3O+ + A- Ka =

A- + H2O ⇌ HA + OH- Kb =

** Reaction will go in direction of stronger acid and base to the weaker acid and base.

** Compare Ka and Kb to decide direction.

B. Mathematical Relationship of Ka and Kb

(for previous equations)

Thus:

Ka x Kb = [H+] [OH-] = Kw

Ka x Kb = 1 x 10-14

and pKa + pKb = 14

3

3a b

H O A HA OHK K H O OH

HA A

C. Example Problems

Consider the dihydrogen phosphate ion, H2PO41-.

Ka = 6.2 x 10-8.

1. What is its conjugate base?

2. Show the chemical reaction to which Ka applies for H2PO4

1-

4. Determine the Kb value for HPO42-.

5. Which direction is the reaction favored?

IX. Acid-Base Reactions of Salts(Ions as Acids and Bases)

A. Salts

1. Ionic compounds (salts) formed from acid-base reaction.

HA + MOH M+A- + H2O

acid base salt

2. The salt (MA) could leave the resulting aqueous solution to be neutral, acidic, or basic.

B. Hydrolysis of Salts

Salts may react with water (hydrolysis reaction involving the splitting of a water molecule) to produce acidic or basic solutions.

For hypothetical ionic compound MA, consisting of

Mx+ cation and Ax- anion, consider possible

hydrolysis reactions and possible consequences:

Mx+ + H2O M(OH)(x-1)+ + H+ acidic solution

Ax- + H2O HA(x+1)- + OH- basic solution

If both cation and anion hydrolyze, compare Ka and Kb

C. Predicting If Hydrolysis Occurs

1. Write the hypothetical hydrolysis rxns for salt.

(one for cation and one for anion)

2. Look at the hypothetical hydrolysis products.

3. If a strong acid or strong base was hypothetically

produced, these hydrolysis reactions DO NOT

occur. No acid or base properties upon

hydrolysis. Recall Table of Strong Acids and Bases

4. If a weak acid or base was produced, these hydrolysis reactions DO occur and produce acid or base properties. For hydrolysis products:

H+ : acidic OH- : Basic

D. Example Problems

Will an aqueous solution of the salt be acidic,

basic, or neutral?

1) NaCl

2) NH4Cl

3) NH4CN

Cumulative Example Problem

The cyanate ion, OCN-, is a weak base. The Ka for HOCN (cyanic acid) is 3.4 x 10-8. Calculate the pH of a 0.200 M solution of NaOCN.

X. Lewis Acids and Bases

A. Some Acid-Base Reactions Do Not Fit

Either Arrehenius or Bronsted –Lowry

Definitions.

Consider:

HCl + NaOH NaCl + H2O

Al3+ + 6 H2O Al (H2O)63+

B. Lewis Acids and Bases

1. Lewis Acid: electron pair acceptor

(Accepts a pair of electrons to form a new bond)

** Usually has an incomplete octet of electrons

2. Lewis Base: electron pair donor

(Donates a pair of electrons to form a new bond)

** Must have at least one lone pair of electrons

C. Identify as Lewis acid or Lewis base?

H2O Cr3+ BF3

XI. Acid Rain

Read Section 15.12

What is acid rain and what are its environmental effects?