ChemistryChapter 19: Acids, Bases, and Salts

Section 1: Acid-Base Theories

Upon completion of this lesson and homework, student will be able to:

• List properties of acids and bases;• Relate the characteristics of acids and bases to

[H+] and [OH-];• Identify conjugate acid-base pairs;• Explain how pH relates to [H+]; and• Describe how the pH scale relates to acidity or

alkalinity.

Properties of acids and bases

• Acids (Acidic Solutions)– Taste sour– Can be strong or weak electrolytes in

aqueous solution– Change the color of an acid-base

indicator

I’m acidic!

• Bases (Alkaline Solutions)– Taste bitter– Feel slippery– can be strong or weak electrolytes

in aqueous solution– Change the color of an acid-base indicator

I’m basic!

What makes an acid an acid?• Acids are substances that add H+ to solution

– Examples: HCl, HNO3, HF

• H+ is a proton (H that lost electron)• H+ can attach to H2O, making H3O+

What makes a base a base?• Bases add OH- to solution• Two ways that happens:Ionic compounds with OH– anion

– Ex.: NaOH (s) → Na+(aq) + OH–(aq)

Compounds that pull H+ off of H2O, leaving OH–

– Example: NH3

Summarizing…

H+

OH-

Which is which?

Conjugate Acids and BasesConjugate acid-base pair: two substances that

differ only by the presence of H+

The conjugate is the “other half”

How good are acids and bases at this proton trading?

• Strong acids completely dissociate in water– They lose all their protons– Their conjugate bases don’t

attract protons, so they are quite weak (not considered a base)

• Weak acids only dissociate partially in water– Their conjugate bases can

attract protons– Conjugates are weak bases

How do we measure acidity?• pH scale: used to measure relative acidity or

basicity of solutionpH = –log[H+]

pH < 7 is acidic pH > 7 is basicpH = 7 is neutral

Objective check: Can you...

• List properties of acids and bases?• Relate the characteristics of acids and bases to

[H+] and [OH–]?• Identify conjugate acid-base pairs?• Explain how pH relates to [H+]?• Describe how the pH scale relates to acidity or

alkalinity?

ChemistryChapter 19: Acids, Bases, and Salts

Section 2: Hydrogen Ions and Acidity

Upon completion of this lesson and homework, student will be able to:

• Explain the autoionization of water;• Explain how pH relates to [H+];• Calculate pH when given [H+];• Calculate and interpret values of [H+], [OH–] and

pH to determine if a substance is an acid or a base; and

• Use acid-base indicators to estimate pH of all kinds of stuff.

Water, H+ and OH–

• “Pure” water isn’t all H2O moleculesAutoionization: the splitting of water molecules

into ions

• Only happens with a very small fraction of water molecules

[H+][OH-] = 1.0 x 10-14

brackets mean concentration in mol/L

Come on, lucky sevens!

• In a neutral solution, [H+] and [OH–]are balanced (equal)

• Since [H+][OH-] = 1.0 x 10-14,• [H+] = [OH–] =• Acids tip the scales toward more [H+], bases

toward [OH–]• When one goes up, the other must go down

Those exponents are a pain• How do we scale exponents to nicer numbers?• log(10x) = x• pH = –log [H+]• In pure water, pH = −log (1.0 10−7) = 7.00• An acid has a higher [H+] than pure water, so

pH is <7• A base has a lower [H+] than pure water, so

pH is >7

The “other” p scale

• pOH = -log[OH-]• Since [H+][OH-]=1 x 10−14

• pH + pOH = 14• These relationships among pH, pOH, [H+],

and [OH-] can lead to hours of chemistry problem solving fun...

Summing up…pH = –log[H+]

Acids:pH < 7[H+] > 1 x 10-7

[OH–] < 1 x 10-7

Bases:pH > 7[OH–] > 1 x 10-7

[H+] < 1 x 10-7

NeutralpH = 7

[H+]= [OH–]=10-7

pH pOH

[H+] [OH–][H+][OH–] = 1 x 10–14

–log[H+] = pH10–pH = [H+]

–log[OH–] = pOH

10–pOH = [OH–]

pH + pOH = 14

Acid-Base Indicators• Indicators change color at a given pH range

Objective check: Can you...

• Explain the autoionization of water?• Explain how pH relates to [H+]?• Calculate pH when given [H+]?• Calculate and interpret values of [H+], [OH–]

and pH to determine if a substance is an acidor a base?

• Use acid-base indicators to estimate pH of all kinds of stuff?

ChemistryChapter 19: Acids, Bases, and Salts

Section 3: Strengths of Acids and Bases

Upon completion of this lesson and homework, student will be able to:

• Describe what defines a strong acid or base;• Calculate pH of solutions of strong acids and bases;• Describe what defines weak acids and bases;• Differentiate between acid strength and acid

concentration.

Strong Acids• Strong acids: dissociate

completely and irreversibly inaqueous solution into H+ and anion

• Seven strong acids:HCl, HBr, HI, HNO3, H2SO4, HClO3,

and HClO4.• To calculate pH of a strong

acid, use the acid’s concentration as [H+]

H+ Cl-

pH of a Strong Acid

Example: What is the pH of a 0.10 M solution of HCl?

So if you have 0.10 M HCl, have [H+] = 0.10 M

pH = -log(0.10) pH = 1.00

Strong Bases• Strong bases: dissociate completely

and irreversibly in aqueous solutioninto metal cations and OH-

• Examples: soluble hydroxides of alkali metal and heavier alkaline earth metal hydroxides (Ca2+, Sr2+, and Ba2+)

• To calculate pH of a strong base, use the base’s concentration to determine [OH-]

OH-

Na+

pH of a Strong Base

Example: What is the pH of a 0.15 M solution of NaOH?

• If you have 0.15 M NaOH, [OH-] = 0.15M pOH = -log(0.15)pOH = 0.82pH = 14.00 – 0.82 = 13.18

pH of a Strong Base

Example 2: What is the pH of a 0.050 Msolution of Ca(OH)2?

• Ca(OH)2 dissociates completely:• Ca(OH)2 → Ca2+ +2OH-

• So [OH-] = 2(0.050 M) = 0.10 MpOH = -log(0.10) pOH = 1.00pH = 14.00 –1.00 = 13.00

Weak acids are more complicated...Weak acids: don’t dissociate completely

That’s soweak!

• Most of the acid holds onto H+

• Reach equilibrium:HA(aq) A−(aq) + H+(aq)

• Equilibrium concentration of undissociated acid is much higher than concentration of H+

Weak Bases

Weak bases: react with water to form the hydroxide ion and the conjugate acid of the base

• Don’t bring OH−, have to make it• Like weak acids, exist in equilibrium

Weak Bases

Most of the weak base doesn’t make OH−

I’m weak...

DeLo

ng

Difference between strength and concentration

• Concentration: how much acid or base in a solution (usually mol/L)

• Strength: how much of substancewill dissociate and make solutionacidic or basic

• A strong acid solution of concentration equal to a weak acid will always have a lower pH

• Same for strong and weak bases

Objective check: Can you...

• Describe what defines a strong acid or base?• Calculate pH of solutions of strong acids and

bases?• Describe what defines weak acids and bases?• Differentiate between acid strength and acid

concentration?

ChemistryChapter 19: Acids, Bases, and Salts

Section 4: Neutralization Reactions

Upon completion of this lesson and homework, student will be able to:• Define the term neutralization;

• Describe the products of an acid-base reaction; and• Define the terms titration and equivalence point.

Acid-Base Reactions

Neutralization: reaction of acid with base to produce water + salt

Is that salt?

Salt: compound with cation of base and anion of acid

+ = +

TitrationTitration: reaction of a solution of unknown concentration with a solution of known (standard) concentrationEquivalence point: when stoichiometrically equivalent amounts of known and unknown are present For strong acid & strong base, solution is neutral at

equivalence point

[?]+ =[0.1 M]

pH 7

How strong is your acid?• If you have acid of unknown

concentration...• Add base of known concentration

until neutralized• Vol. base x [base] = moles base• At equivalence point:

moles base = moles acid

How do I know it’s

neutralized?

Titration of a Strong Acid with a Strong Base

From the start of the titration to near the equivalence point, pH goes up slowly.

Objective check: Can you...

• Define the term neutralization?• Describe the products of an acid-base

reaction?• Define the terms titration and equivalence

point?