chapter 15: acids and bases

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Vanessa N. Prasad-PermaulCHM 1046

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 (HYRONIUM IONS, H3O+)

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 solutions2. Doesn’t account for the basicity of substances like ammonia (NH3) that don’t contain OH groups

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Acid-Base Concepts: The Bronsted-Lowry Theory

Bronsted-Lowry Theory1. Acid is any substance that can transfer a proton (H+ ion) to another substancea) proton donors

2. Base is any substance that can accept a protonb) 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

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Acid-Base Concepts: The Bronsted-Lowry Theory

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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

Example 1:

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

1. NaOH2. HCl3. NH3

4. Mg(OH)2

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EXAMPLE 15.1: IDENTIFYING ACIDS AND BASES

EXERCISE 15.1: IDENTIFYING ACIDS AND BASESLABEL EACH AS ACID OR BASE AND IT’S

CONJUGATE PAIR

H2CO3(aq) + CN-(aq) HCN(aq) + HCO3-(aq)

HSO4- + NH3 SO4

2- + NH4+

Example 2:

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)7

Example 3:

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)

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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)

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Lewis Acids and Bases

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Example 12:

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+

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EXAMPLE 15.2: IDENTIFYING LEWIS ACIDS AND BASES

EXERCISE 15.2: IDENTIFYING LEWIS ACIDS AND BASES

IDENTIFY THE LEWIS ACID AND BASE H

BF3 + CH3OH F3B:OCH3

O2- + CO2 CO32-

..

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

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(aq)

Acid Base Acid Base5. 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 watera) Acid-dissociation equilibrium of a strong acid lies nearly 100% to the right (products)b) Solution contains almost entirely H3O+ and A- ionsc) HClO4, HCl, HBr, HI, HNO3, H2SO4

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

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(aq)

Acid Base Acid Base7. Weak acid is one that is only partially

dissociated in watera) Solution contains mainly HAb) HNO2, HF, CH3CO2Hc) Acid-dissociation equilibrium lies essentially 100% to the left (reactants)d) Very weak acids have strong conjugate bases

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RELATIVE STRENGTHS OF ACIDS AND BASES

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EXAMPLE 15.3: REACTANTS OR PRODUCTS IN AN ACID-BASE REACTION

EXERCISE 15.3: REACTANTS OR PRODUCTS IN AN ACID-BASE REACTIONDETERMINE THE DIRECTION OF THE FOLLOWING REACTION FROM THE RELATIVE STRENGTHS OF ACIDS AND BASES

H2S(aq) + C2H302-(aq) HC2H3O2(aq) + HS-(aq)

Example 4:

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)

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Dissociation of Water

Most important property of water is its ability to act as an acid and a base1. In the presence of a base water acts as an acid2. In the presence of an acid water acts like a base

Acid Base Acid

Base 19

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 tempe) [H3O+] > [OH-] acidic

f) [H3O+] = [OH-] neutral

g) [H3O+] < [OH-] basic20

Example 5:

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.

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EXAMPLE 15.4: CALCULATING [H30+] AND [OH-]

EXERCISE 15.5: CALCULATING [H30+] AND [OH-]A SOLUTION OF BARIUM HYDROXIDE @ 25oC IS 0.125M. WHAT IS THE CONCENTRATION OF [H30+] AND [OH-]?

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 > 723

Example 6:

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

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THE pH SCALE

Example 7:

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

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Example 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

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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 abbreviatedc. Exhibit pH-dependent color change

2. The indicators are weak acids and have different colors in their acid (HIn) and conjugate base (In-) forms

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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

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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+]30

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 SO42-

ions

2. Majority of H3O+ comes from first dissociation

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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

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2. Example 2nd way Find the pH of a 0.01 M NaOH**

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EXERCISE 15.8: CALCULATING pH FROM [OH-]A SATURATED SOLUTION OF CALCIUM HYDROXIDE HAS AN [OH-] OF 0.025M. WHAT IS THE pH OF THE SOLUTION?

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EXAMPLE 15.5: CALCULATING pH FROM [H3O+]

EXERCISE 15.7: CALCULATING pH FROM [H3O+]WHAT IS THE pH OF A SAMPLE OF GASTRIC JUICE WHOSE HYDRONIUM ION CONCENTRATION IS 0.045M?

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EXAMPLE 15.6: CALCULATING [H3O+] FROM pH

EXERCISE 15.9: CALCULATING [H3O+] FROM pHA BRAND OF CARBONATED BEVERAGE HAS A pH OF 3.16. WHAT IS THE HYDRONIUM-ION CONCENTRATION OF THE BEVERAGE?

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

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Equilibria in Solutions of Weak Acids

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


(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

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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 9: 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-

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Example 10:

Calculate the pH and the concentrations of all species present (H3O+, F-, HF) in 0.050 M HF (Ka= 6.8 x10-4)

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ax2 + bx + C = 0

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

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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

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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 ExpressionKb = [NH4

+] [OH-] / [NH3]H2O omitted because it is a pure liquid!Kb = base-dissociation Constant:

Stronger base higher the Kb, lower the pKb43

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 11 Calculate the pH and the

concentrations of all species present in 0.40 M NH3

(Kb = 1.8 x 10-5)

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Relation Between Ka and Kb

NH4+

(aq) + H2O(l) H3O+(aq) + NH3(aq) Ka = [H3O+]

[ NH3] NH4

+

NH3(aq) + H2O(l) NH4

+(aq) + 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!!! 45

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)

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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 reactiona) Strong acid + strong base Neutral sol’nb) Strong acid + weak base Acidic sol’nc) Weak acid + strong base Basic sol’n

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a) Salts that yield neutral solutionsThe following ions do not react appreciably with water to produce either H3O+ or OH- ions:

Cations from strong bases1. 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

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b) Salts that yield Acidic solutionsThe 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+)

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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-

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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

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

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Factors that Affect Acid Strength

Hydrohalic acids

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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.


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.

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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.

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Stability of conjugate base

Acid strength increases as stability of conjugate base increases

The more resonance forms a conjugate base has, the more stable it is

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Let’s look atHClO4 HClO3 HClO2 HOCl

Draw LD structures for acid & conj base

List in order of increasing acid strength

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EXERCISE 15.4: DETERMINING ACID STRENGTHWHICH ONE OF THE FOLLOWING PAIRS OF ACIDS ARE STRONGER?NH3, PH3

HI, H2TeHSO3

-, H2SO3

H3AsO4, H3AsO3

HSO4-, HSeO4

-

chapter 15: acids and bases

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