Chapter 15:Aqueous Equilibria

Renee Y. Becker

Valencia Community College

Acid-Base Concepts: The Arrhenius Acid-Base Theory Generalized Arrhenius Acid HA(aq) H+

(aq) + A-(aq)

Acids are substances that dissociate in water to produce hydrogen ions

Generalized Arrhenius Base MOH(aq) M+(aq) + OH-

(aq)

Bases are substances that dissociate in water to yield hydroxide ions

 

Arrhenius theory has limitations

1.      Restricted to aqueous solutions

2.      Doesn’t account for the basicity of substances like ammonia (NH3) that don’t contain OH groups

Acid-Base Concepts: The Bronsted-Lowry Theory Bronsted-Lowry Theory

1.  Acid is any substance that can transfer a proton (H+ ion) to another substance

a) proton donors

2.   Base is any substance that can accept a proton

b)  proton acceptor

3.   Acid-base reactions are proton-transfer reactions

4.   The products of a Bronsted-Lowry acid-base reaction are themselves acids and bases

Acid-Base Concepts: The Bronsted-Lowry Theory

5.   The BH+ produced when base B accepts a proton from HA can itself donate a proton back to A-, BH+ is a

Bronsted-Lowry acid

6.   A- produced when HA loses a proton can itself accept a proton back from BH+, A- is a Bronsted-Lowry base

7. Chemical species whose formulas differ only by one proton are said to be conjugate acid-base pairs

Which of the following is a Bronsted-Lowry base but not an Arrhenius base?

1. NaOH

2. HCl

3. NH3

4. Mg(OH)2

Example 1

A) Write a balanced equation for the dissociation of each of the following Bronsted-Lowry acids in water

B) What is the conjugate base of the acid in each case?

1. H2SO4(aq)

2. HSO4-(aq)

3. H3O+

(aq)

4. NH4+

(aq)

Example 2

What is the conjugate acid of each of the following Bronsted-Lowry Bases?

1. HCO3-(aq)

2. CO32-

(aq)

3. OH-(aq)

4. H2PO4-(aq)

Acid Strength and Base Strength View of an acid-dissociation equilibrium

HA(aq) + H2O(l) H3O+

(aq) + A-(aq)

Acid Base Acid Base

1. Two bases, H2O and A- are competing for protons

2.  If H2O is a stronger base than A- the H2O molecules will get the protons and the solution will be mainly H3O

+ and A-

3. If A- is a stronger base than H2O, the A- ions will get the protons and the solution will be mainly HA and H2O

4. With equal concentrations of reactants and products, the proton is always transferred to the stronger base

HA(aq) + H2O(l) H3O+

(aq) + A-(aq)

Acid Base Acid Base

5. The direction of the reaction to reach equilibrium is proton transfer from the stronger acid to the stronger base to give the weaker acid and the weaker base

6. Strong acid is one that is almost completely dissociated in water

a) Acid-dissociation equilibrium of a strong acid lies nearly 100% to the right (products)

b)  Solution contains almost entirely H3O+ and A- ions

c)  HClO4, HCl, HBr, HI, HNO3, H2SO4

d) Strong acids have very weak conjugate bases (weaker than H2O)

HA(aq) + H2O(l) H3O+

(aq) + A-(aq)

Acid Base Acid Base

7. Weak acid is one that is only partially dissociated in water

a) Solution contains mainly HA

b) HNO2, HF, CH3CO2H

c) Acid-dissociation equilibrium lies essentially 100% to the left (reactants)

d) Very weak acids have strong conjugate bases

Example 3

If you mix equal concentrations of reactants and products, which of the following reactions proceed to the right and which proceed to the left?

1. HF(aq) + NO3-(aq) HNO3(aq) + F-

(aq)

2. NH4+

(aq) + CO32-

(aq) HCO3-(aq) + NH3(aq)

Dissociation of Water Most important property of water is its ability to act as an acid and a

base

1. In the presence of a base water acts as an acid

2. In the presence of an acid water acts like a base

Acid Base Acid Base

Dissociation of Water

3. Dissociation of water

a)      2 H2O(l) H3O+

(aq) +OH-(aq)

b)      Kw = [H3O+][OH-]

c)      Kw = ion-product for water

d)      Kw = 1 x10-14 @ 25C equilibrium constants are affected by temp

e)      [H3O+] > [OH-] acidic

f)       [H3O+] = [OH-] neutral

g)      [H3O+] < [OH-] basic

Example 4

• The concentration of OH- in a sample of seawater is 5 x 10-6 M. Calculate the concentration of H3O

+ ions, and classify the

solution as acidic, neutral, or basic.

The pH Scale IntroductionThe term pH is derived from the French puissance d’hydrogene

(power of hydrogen) and refers to the power of 10 (the exponent) used to express the molar H3O

+ concentration

Only works for Strong Acids and Strong Bases

pH = -log [H3O+] [H3O

+] = 10-pH pOH = -log [OH-] 

pH + pOH = 14 kw = [H3O+] [ OH-] = 1.0 x10-14

Acidic solution pH < 7 Neutral solution pH = 7

Basic solution pH > 7

Example 5

Calculate the pH of the following solutions

a) A sample of seawater that has OH- of

1.58 x 10-6 M

b) A sample of seawater that has H3O+

6.0 x 10 –5 M

Example 6

Calculate the concentrations of H3O+ and OH-

in each of the following

a) Human blood pH = 7.40

b) A cola beverage pH = 2.8

If you have an acidic solution you would expect?

1. [H3O+] < 1 x 10-7

2. [OH-] > 1 x 10-7

3. [H3O+] > 1 x 10-7

4. [OH-] = 1 x 10-7

Measuring pH Acid-base indicator

1.    The approximate pH of a solution can be determined by using an acid-base indicator

a. A substance that changes color in a specific pH range

b. HIn abbreviated

c.  Exhibit pH-dependent color change

2. The indicators are weak acids and have different colors

in their acid (HIn) and conjugate base (In-) forms

Measuring pH

 

2.   More accurate pH measurements can be made by using a pH meter

a) A device that measures the pH-dependent electrical potential of the test solution

The pH in Solutions of Strong Acids and Strong Bases Strong acids are nearly 100% dissociated in aqueous

solution

a) H3O+ and A- concentrations are equal to the initial

concentration of the acid

b) Concentration of undissociated HA molecules is essentially zero

c) The pH of a solution of a strong monoprotic acid is calculated from the H3O

+ concentration

pH = -log[H3O+]

The pH in Solutions of Strong Acids and Strong Bases

d) Calculation of the pH of a diprotic (H2SO4)

solution is more complicated because 100% of the H2SO4 molecules dissociate to give H3O

+ and

HSO4- ions

1. Less than 100% of the resulting HSO4-

ions dissociate to give H3O+ and SO4

2- ions

2. Majority of H3O+ comes from first dissociation

The pH in Solutions of Strong Acids and Strong Bases

The most familiar Strong bases are alkali metal hydroxides, MOH, (NaOH, KOH)

a) Strong bases are nearly 100% dissociated in aqueous solution

b) The pH of a strong base solution can be found 2 ways

1.       Example first way

Find the pH of a 0.01 M NaOH

The pH in Solutions of Strong Acids and Strong Bases

2. Example 2nd way

Find the pH of a 0.01 M NaOH**

Equilibria in Solutions of Weak Acids

Introduction

1. A weak acid is not the same as a dilute solution of a strong acid

2. A strong acid is 100% dissociated in aqueous solution

3. A weak acid is only partially dissociated

Equilibria in Solutions of Weak Acids

The dissociation of a weak acid in water is characterized by an equilibrium equation

HA(aq) + H2O(l) H3O+

(aq) + A-(aq)

Ka = [H3O+] [A-] / [HA]

Ka is the acid-dissociation constant

Equilibrium equation, pure water is left out!!

pKa = -log Ka

Larger the Ka the stronger the acid

Smaller the pKa the stronger the acid

Calculating Equilibrium Concentrations in Solutions of Weak Acids Once the Ka value for a weak acid has been measured it

can be used to calculate equilibrium concentrations and the pH in a solution of the acid

Example 8: Calculate the pH and the concentrations of all species present in a 0.10 M HCN (Ka = 4.9 x 10-10)

General equation: HCN + H2O H3O+ + CN-

Example 9

Calculate the pH and the concentrations of all species present (H3O

+, F-, HF, and OH-) in

0.050 M HF (Ka= 6.8 x10-4)

Percent Dissociation in Solutions of Weak Acids

% dissociation = ([HA] dissociated / [HA] initial) x 100%

1. The higher the % dissociation the stronger the acid

2. % dissociation increases as Ka increases

3. % dissociation in weak acids increases as dilution increases

Polyprotic Acids 1. Acids that contain more than one dissociable proton

1. Dissociate in a stepwise manner

1. Each dissociation has it’s own Ka labeled Ka1 Ka2 etc.

4. Ka1 > Ka2 > Ka3 it is harder to take a hydrogen from a

compound that is already electron rich, proton deficient

5. The principal reaction is the first dissociation

6. Essentially all the H3O+ in the solution comes from the

first dissociation

Equilibria in Solutions of Weak Bases Weak bases accept a proton from water to give the

conjugate acid of the base and OH- ions

NH3(aq) + H2O(l) NH4+

(l) + OH-(aq)

Equilibrium Expression

Kb = [NH4+] [OH-] / [NH3]

H2O omitted because it is a pure liquid!

Kb = base-dissociation Constant:

Stronger base higher the Kb, lower the pKb

Equilibria in Solutions of Weak Bases

• Equilibria in solutions of weak bases are treated the same as weak acids. We will use the same procedure

Example 10

• Calculate the pH and the concentrations of all species present in 0.40 M NH3

(Kb = 1.8 x 10-5)

Relation Between Ka and Kb NH4

+(aq) + H2O(l) H3O

+(aq) + NH3(aq) Ka = [H3O

+][ NH3]

NH4+

 

NH3(aq) + H2O(l) NH4+

(l) + OH-(aq) Kb = [NH4

+][OH-]

NH3

 

Net: 2 H2O(l) H3O+

(aq) + OH-(aq)

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

14

Ka x Kb = Kw for conjugate acid-base pairs only!!!

Relation Between Ka and Kb

• The equilibrium constant for the net reaction equals the product of the equilibrium constants for the reactions added

• For any conjugate acid-base pair, the product of the acid-dissociation constant for the acid and the base-dissociation constant for the base always equals the ion-product constant for water Kw = Ka x Kb

• As the strength of an acid increases (larger Ka) the

strength of its conjugate base decreases (smaller Kb)

Acid-base Properties of Salts Introduction:

1. When an acid neutralizes a base, an ionic compound called a salt is formed.

2. Salt solutions can be neutral, acidic, or basic

3. The pH depends on the acid-base properties of the cations and anions that result from the reaction

a) Strong acid + strong base Neutral sol’n

b) Strong acid + weak base Acidic sol’n

c) Weak acid + strong base Basic sol’n

Acid-base Properties of Salts

a) Salts that yield neutral solutionsThe following ions do not react appreciably with water to produce either H3O

+ or OH- ions:

Cations from strong bases

1. Alkali metal cations of group 1A (Li+, Na+, K+)2. Alkaline earth cations of group 2A (Ca2+, Sr2+,

Ba2+) except for Be2+

Anions from strong monoprotic acids:

1. Cl-, Br-, I-, NO3-, CLO4

-

Salts that contain only these ions give neutral solutions in pure water

Acid-base Properties of Salts

b) Salts that yield Acidic solutions

The anion is neither an acid nor a base but the cation is a weak acid

1. The acidity of hydrated main-group cations increases from left to right in the periodic table, as the metal ion charge increases and the metal ion size decreases

(Li+ < Be2+;Na+ < Mg2+ < Al3+)

2. Transition metal cations , (Zn2+, Cr3+, Fe3+)

Acid-base Properties of Salts

c) salts that yield basic solutions

The cation is neither an acid nor a base, but the anion is a weak base

1. NO2-, F-, CH3CO2

-, CO2-

Acid-base Properties of Salts

d) Salts that contain acidic cations and basic anions

1. If Ka > Kb pH < 7

2. If Ka < Kb pH > 7

3. Ka = Kb pH = 7

Factors that Affect Acid Strength Introduction:Why is one acid stronger than another? It is mainly determined by the strength and polarity H-A bond.

1. The strength of the H-A bond is the enthalpy required to dissociate HA into H atom and A atom.

2. The polarity of the H-A bond increases with an increase in the electronegativity of A

a)      This is related to the ease of electron transfer from H atom to an A atom to give H+ and A-

b)      The weaker the H-A bond, the stronger the acid

c)      The more polar the H-A bond the stronger the acid

Factors that Affect Acid Strength

Hydrohalic acids

The variation in polarity in this series is much less important than the variation in bond strength, HF – 567 kJ/mol to HI – 299 kJ/mol

In general for binary acids in the same group of the periodic table, the H-A bond strength is the most important determinant of acidity.

Factors that Affect Acid Strength

The H-A bond strength generally decreases with increasing size of element A down a group, so acidity increases. Size of halogens F-small get bigger down the group, bond strength decreases and acidity increases from HF to HI.

Factors that Affect Acid Strength

For binary acids of elements in the same row of the periodic table, changes in the H-A bond strength are smaller, and the polarity of the H-A bond is the most important determinant of acid strength.

Lewis Acids and Bases Lewis acid – An electron pair acceptor

Lewis base- An electron pair donor

1. All lewis bases are Bronsted-Lowry bases

2. All Bronsted-Lowry bases are lewis bases

3. Lewis acid is more general than the Bronsted-Lowry (all lewis acids are not also

Bronsted- Lowry acids)

Lewis Acids and Bases

Example 11

For each of the following reactions, identify the lewis acid and the lewis base

a) AlCl3 + Cl- AlCl4-

b) 2 NH3 + Ag+ Ag(NH)2+

c) SO2 + OH- HSO3-

d) 6 H2O + Cr3+ Cr(OH2)63+