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Chapter 14 - Aqueous

Equilibria: Acids & Bases

CHM152 GCC

Why study acids & bases?

Acids and bases are important in

numerous reactions, from industrial

processes to biological ones.

Many household substances, including

cleaning solutions and food/beverages

that we consume, are acids or bases.

In the environment, the pH of rain,

water and soil can also have significant

effects.

Taste sour

Corrode most metals

Turn litmus paper red

React with bases to form

salt and water

examples: vinegar, lemon juice, gastric juice,

soft drinks

Acids Bases

Taste bitter, chalky

Feel soapy, slippery

Turn litmus paper blue

React with acids to form

salt and water

examples: milk of magnesia,

bleach, ammonia, detergents

Acid-Base Concepts

• CHM 151 taught Arrhenius theory

Acids produce hydrogen ions in water

HA (aq) H+ (aq) + A- (aq)

Bases produce hydroxide ions in water

MOH (aq) M+ (aq) + OH- (aq)

• 152 uses Brønsted-Lowry theory

Acid: Substance that can donate H+

Acid loses H+

Base: Substance that can accept H+

Base gains H+

Bronsted Lowry reactions involve the

transfer of 1 H+ ion

• Identify the acid, the base, the conjugate acid

and conjugate base.

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Acid-Base Pairs

• Identify the acid-base pairs above.

• Note: H2O is amphoteric – it can act as an acid or a base depending on what its reacting with.

Identify Conjugate Acid or Base

a) What is the conjugate acid of CO32-

b) What is the conjugate base of H2PO4-?

Strength of Acids/Bases Conjugate Acid-Base Pair Trends

1. A stronger acid loses its proton more readily

than a weaker acid and a stronger base gains

a proton more readily than a weaker base.

2. The stronger the acid, the weaker its

conjugate base. Likewise, the stronger the

base, the weaker its conjugate acid.

3. Proton transfer reactions proceed from the

stronger acid to the stronger base. Thus, the

equilibria lies on the side of the weaker acid-

base pair.

Strengths of Acids/Bases

• Competition for protons (H+)

e.g. HS-(aq) + HF(aq) H2S(aq) + F-(aq)

From Table 15.1

• Which acid is stronger?

• Which base is stronger?

• Which side does equilibria lie on?

Strong Acids

a substance that ionizes completely in water to

form H3O+ ions (H+). SA’s are strong electrolytes.

HNO3(aq) + H2O(l) → H3O+(aq) + NO3

- (aq)

A one way arrow is used since this reaction is

complete, all of the HNO3 molecules break

apart to form H3O+ and NO3

- ions.

These acids have very weak conjugate bases

For SA, eq lies very far to the right side!

KNOW 7 Strong acids! Memorize these!

HCl, HBr, HI, HClO4, HNO3, H2SO4 , HClO3

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Weak Acids

an acid that ionizes only to a small extent in H2O.

WA’s are weak electrolytes!

E.g. HCN(aq) + H2O(l) H3O+(aq) + CN-(aq)

• A two-way arrow is used since this reaction is not

complete but instead consists of an equilibrium

mixture of HCN molecules, H3O+ and CN- ions

• Equilibrium lies to the left since most of the acid

molecules have not ionized.

• Common WA’s: HF, HNO2, HCN, H3PO4, H2CO3,

carboxylic acids like CH3COOH

Strength of Acids

• HCl, H2SO4

• HNO2, HF

• H2O

Strong Bases

A substance that ionizes completely in water to

form OH- ions. SB’s are strong electrolytes!

SB dissociates in water; it doesn’t react with it:

NaOH(aq) Na+ (aq) + OH- (aq) (100 % ions)

Strong Bases: These Group 1A & 2A hydroxides

LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2,

Ba(OH)2, Sr(OH)2 Memorize the 8 SB’s!

•The reaction goes to completion – SB’s completely

dissociate into ions.

Weak Base

A base that ionizes only to a small extent in H2O.

WB’s are weak electrolytes!

NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)

Equilibrium lies to the left since most of the

ammonia molecules have not accepted protons

to form NH4+ and OH- ions.

Common examples of WB’s are ammonia and

amines (e.g. CH3NH2)

SA, WA, SB, WB solutions

Draw the following in water: HNO3, HF, Ca(OH)2, and NH3. Assume you have 3 formula units or molecules for each substance.

Hydronium Ions

• HA (aq) H+ (aq) + A- (aq)

• H+ is very reactive and will bond with O in

H2O to form H3O+

• H+ and H3O+ mean the same thing but H3O

+

is more accurate

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Dissociation of Water

• H2O (l) + H2O (l) H3O+ (aq) + OH- (aq)

• Identify the acid-base pairs

• This is the dissociation of water, an

amphoteric substance.

• Write an equilibrium expression for this eqn.

• Kw is ion-product constant for water.

• Calculate Kw if [H3O+] = 1.0 x 10-7

(experimentally determined for pure H2O)

Dissociation of Water

• Kw = [H3O+][OH-] = 1x10-14

• We can compare [H3O+] & [OH-] for solutions.

• They’re equal in neutral solutions.

• Which one is greater in acidic solutions?

• Basic solutions?

• [H3O+] = 5.6 x 10-4 M. What is [OH-]?

• Is solution acidic or basic?

Acidic, Basic, Neutral Solns pH Scale

• Logarithmic scale (like Richter scale for

earthquakes).

• [H3O+] = 1x10-2 M has a pH of 2

• [H3O+] = 1x10-12 M has a pH of 12

• pH = -log [H3O+] [H3O

+] = 10-pH

• pOH = -log [OH-] [OH-] = 10-pOH

• Kw = [H3O+][OH-] = 1x10-14

• pH + pOH = 14

pH Sig Figs

[H3O+] = 5.6 x 10-4 M

• pH = -log (5.6 x 10-4) = 3.25 (3 related to

exponent; tells us acidity)

2 sig figs

Find [H3O+], pH, and pOH if [OH-] = 9.8x10-9 M

2 decimal places

Group Quiz #9

Calculate [H3O+], [OH-] and pH for a solution if

the pOH is 10.325.

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pH of Strong Acids/Bases

HNO3(aq) + H2O(l) H3O+(aq) + NO3

-(aq)

Calculate pH of 0.103 M HNO3 solution.

Sr(OH)2 Sr2+(aq) + 2OH-(aq)

Calculate pH of 0.020 M Sr(OH)2 soln.

Calculate [H3O+], [OH-], pH, and pOH of a solution

made by adding 2.250 g of LiOH to 250.0 mL of water.

Equilbria of Weak Acids

• HA(aq) + H2O(l) H3O+(aq) + A-(aq)

Write equilibrium expression.

• pKa = -log Ka: stronger acid = lower pKa

• % Ionization = [H3O+] / [HA]i x 100%

• As acid strength increases, indicate if the following will be higher or lower: [H3O

+]

pH

% ionization

Ka

pKa

pKa’s of Weak Acids Weak Acid Problems

• Set up ICE tables & Ka

• With small K values, assume x will be small to

simplify math.

WA problem types:

• Given [HA]i and Ka, find all equilibrium

concentrations and pH

• Given [HA]i and pH, find all equilibrium

concentrations and Ka

• Given [HA]i and %dissociation, find eq

concentrations, pH and Ka

Find pH of Weak Acid

Find the equilibrium concentrations for all

substances and the pH of a 0.50 M solution of

HOCl (hypochlorous acid). Ka = 3.5 x 10-8

ICE Table

HOCl + H2O H3O+ + OCl-

I

C

E

Example 14.10, Problems 14.14, 14.15

Find Ka for weak acid

• Find the Ka of a 1.25 M solution of nitrous

acid, HNO2. The pH of this solution is

measured to be 1.62.

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Percent Ionization

• % dissociation is the same calculation as

checking assumption of a small x.

% dissociation = ([H3O+]eq / [HA]i) x 100

• Will the percent dissociated increase or

decrease as an acid is diluted?

• Calculate the percent dissociation from the

previous problem.

Dilution and % Dissociation

• As we add

water, we can

dissociate

more. More

dilute = more

dissociation.

Find Ka for weak acid

• A 0.200 M solution of a weak acid is 9.4

percent dissociated. Using this information,

calculate [H3O+], [A-], [HA]eq, and Ka.

Group Quiz #10

• The acid-dissociation constant for benzoic

acid, HC7H5O2, is 6.3 x 10-5. Calculate the

equilibrium concentrations of H3O+,

C7H5O2-, and HC7H5O2 in the solution if the

initial concentration of the acid is 0.50 M.

Polyprotic Acids

• H2C2O4(aq) + H2O(l) HC2O4-(aq) + H3O

+(aq)

Ka1 = [HC2O4-][H3O

+] / [H2C2O4] = 5.9 x 10-2

• HC2O4-(aq) + H2O(l) C2O4

2-(aq) + H3O+(aq)

Ka2 = [C2O42-][H3O

+] / [HC2O4-] = 6.4 x 10-5

• In general, Ka1 > Ka2 > Ka3 … Why?

• pH of a polyprotic acid solution primarily arises from

[H+] formed in the 1st step.

Weak Bases

• Similar to WA problems but base hydrolysis reaction forms OH- (this is x!) and use Kb

• NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)

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

• Table 14.4: Kb Values for Weak Bases

• Why are amines (derivatives of NH3) basic?

• Lone e- pair (d-) on N attracts H (d+) from water. N wins the competition and pulls H+ off of water.

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Weak Bases

• Calculate the pH of a 0.50 M dimethylamine ((CH3)2NH) solution. Kb = 5.4 x 10-4

Example 14.12, Problems 14.19, 14.20

Group Quiz #11

A 0.065 M solution of methylamine, CH3NH2,

has a pH of 11.70. What is Kb for CH3NH2?

Relation Between Ka and Kb

• Weak acid: Ka = [H+][A-] / [HA]

• Weak base: Kb = [HA][OH-] / [A-]

• Multiply: Ka * Kb

• Ka·Kb = [H+] [OH-] = Kw Ka·Kb = 10-14

• True only if the acid and base are conjugate pairs!

• What is the Kb for hydrazine (N2H4) if Ka for N2H5+ is

1.1 x 10-8?

• Example 14.13, Problem 14.21

Properties of Salts

• Acid + Base Salt + Water

• Salts are ionic products of acid-base reaction.

• Cation in salt: comes from the base

• Anion in salt: comes from the acid

• Salts can be neutral, acidic, or basic depending

on the strength of the acid or base from which

they’re made

Hydrolysis Reactions

• Many salts cause pH shifts when dissolved in

water. These salts contain the conjugate acid (or

base) of a weak base (or acid).

NH4Cl: NH4+ + H2O NH3 + H3O

+

• pH < 7

NaCH3CO2: CH3COO- + H2O CH3COOH + OH-

• pH > 7

• These reactions are called hydrolysis.

General Ion Categories

1) Acidic ions: Most cations (except 1A & 2A)

E.g. NH4+, Al3+, Cu2+

2) Neutral Ions: cations from strong bases and

anions from strong acids

Cations: Group 1A & 2A (except Be)

Anions: NO3-, Cl-, Br -, I-, ClO4

-, ClO3-(except HSO4

-)

3) Basic Ions: Most anions derived from weak

acids.

E.g. F-, CN-, CH3COO- , NO2-

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Types of Salts

• Neutral Salts

NaOH + HCl NaCl + H2O

Acid and base are strong, but resulting salt is neutral.

Neutral ions don’t hydrolyze!

• Acidic Salts

HCl + NH3 NH4Cl

strong acid + weak base

Ammonium salts are acidic (cation from weak base, anion from strong acid)

Types of Salts

Basic Salts

• Salts made from weak acids and strong bases give basic salts

• Anion from weak acid; cation from strong base

• Give an example.

Acidic cations and basic anions

(NH4)2CO3: NH4+ acts as weak acid; CO3

2- acts as weak base. Have to compare Ka and Kb values in order to determine if its acidic or basic.

Example Salt Problems

• Is KF an acidic, basic or neutral salt?

Write the hydrolysis reaction and calculate the pH of a 0.10 M KF solution. Ka (HF) = 3.5x10-4

• Is NH4Cl acidic, basic, or neutral?

Write the hydrolysis reaction and calculate the pH of a 0.10 M NH4Cl solution.

Kb (NH3) = 1.8 x 10-5

Classify Salts

• Classify the following salts as acidic, basic,

or neutral. For acidic or basic salts, write

the reaction of hydrolysis.

KBr

NaF

LiCN

NH4Cl

Group Quiz #12

• Classify the following salts as acidic, basic, or neutral. For acidic or basic salts, write the equation of hydrolysis.

NH4ClO3

Ba(NO2)2

CaI2

Lewis Acids and Bases

• Broadest acid base definition since Lewis acids don’t have to contain H+.

• Looks at electron transfer instead of H+ transfer

• Acids are electron-pair acceptors, bases are electron-pair donors

H+ + :NH3 NH4+

Ammonia donates the electron pair to H+ to make the bond; H+ accepts the electron pair

This definition is used in CHM235/236 where you learn to push e- pairs!

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Lewis Acids and Bases

Metal cations= Lewis acids

Small, highly charged metals also make

acidic salts; Al(H2O)63+ (Ka = 1.4 x 10-5)

High charge on metal electrons on O are

attracted to +metal cation.

Metal acids

• Al3+ forms acid solutions! Easier to break

O-H bonds and form hydronium!

Acid Strength Factors

• Why is HF weak but HCl, HBr, and HI strong?

• Determined by strength and polarity of H-X bond.

• H-F is strongest in series (H and F are small and

close together); other bonds are weaker and break

easily in water

Acid Strength Factors

• In the same row, bond strengths are relatively

similar, but electronegativity changes are greater

Acid Strength Factors