Reactions of Acids & Bases

Reactions of Acids & Bases

Self-Ionization of WaterSelf-Ionization of Water

H2O + H2O H3O+ + OH-

Kw = [H3O+][OH-] = 1.0 10-14

In pure water at 25ºC, both H3O+ and OH- ions are found at concentrations of 1.0 x 10-7 M.

In all solutions at 25ºC, the product of the concentrations of H3O+ and OH- ions is equal to 1.0 x 10-14 M.

if [H3O+] > 1.0 10-7

It’s an acid …

if [OH-] > 1.0 10-7

It’s a base …

if [H3O+] = [OH-] = 1.0 10-7

It’s neutral …

pH = -log[H3O+]

pH ScalepH Scale

0

7INCREASING

ACIDITY NEUTRALINCREASING

BASICITY

14

pouvoir hydrogène (Fr.)“hydrogen power”

pH ScalepH Scale

pH of Common SubstancespH of Common SubstancespH of Common SubstancespH of Common Substances

pH ScalepH Scale

The pH scale runs from 0 to 14, but each unit represents a tenfold change in the concentration.The pH scale runs from 0 to 14, but each unit represents a tenfold change in the concentration.

The HThe H33OO++ concentration of 1 x 10 concentration of 1 x 1000 is not 14 times a concentration of 1 x 10 is not 14 times a concentration of 1 x 10-14-14, but it is a factor of 10, but it is a factor of 101414, which is 100 trillion times!, which is 100 trillion times!

The pH scale runs from 0 to 14, but each unit represents a tenfold change in the concentration.The pH scale runs from 0 to 14, but each unit represents a tenfold change in the concentration.

The HThe H33OO++ concentration of 1 x 10 concentration of 1 x 1000 is not 14 times a concentration of 1 x 10 is not 14 times a concentration of 1 x 10-14-14, but it is a factor of 10, but it is a factor of 101414, which is 100 trillion times!, which is 100 trillion times!

pH ScalepH Scale

What is the concentration of OH- ions in saturated lime if [H3O+] = 3.98 x 10-13 M? Is lime acidic, basic, or neutral?

BASIC

Practice Problem #1Practice Problem #1

[OH-] > 1.0 10-7

[H3O+][OH-] = 1.0 10-14

[3.98 10-13][OH-] = 1.0 10-14

[OH-] = 2.5 10-2 M or 0.025 M

Analysis of a sample of maple syrup reveals that the concentration of OH- ions is 5.0 x 10-8 M. What is the pH? Is it acidic, basic, or neutral?

[H3O+][OH-] = 1.0 10-14

Practice Problem #11Practice Problem #11

AcidicpH = -log[H3O+] pH = 6.7

[H3O+][5.0 x 10-8] = 1.0 10-14

[H3O+] = 2.0 10-7 M

More Sample Problems More Sample Problems

What is the pH of 0.080 M HNO3?What is the [H3O+] and the [OH-]?

pH = -log[H3O+]

pH = -log[0.080]

pH = 1.1Acidic

[H3O+] = 0.080 M[H3O+][OH-] = 1.0 x 10-14

[ OH-] = 1.0 x 10-14

[H3O+]

[ OH-] = 1.3 x 10-13

More Sample Problems More Sample Problems

What is the pH of 0.0123 M H2SO4?What is the [H3O+] and the [OH-]?

pH = -log[H3O+]

pH = -log[0.0246]

pH = 1.61Acidic

[H3O+] = 2(0.0123) M = 0.0246 M

[H3O+][OH-] = 1.0 x 10-14

[ OH-] = 1.0 x 10-14

[H3O+]

[ OH-] = 4.07 x 10-13

More Sample Problems More Sample Problems

The pH of a solution is 4.29What is the [H3O+] and the [OH-]?

pH = -log[H3O+]

Acidic

[H3O+][OH-] = 1.0 x 10-14

[ OH-] = 1.0 x 10-14

[H3O+]

[ OH-] = 1.95 x 10-10

10 = [H3O+]

[H3O+] = 5.13 x 10-5

-pH

BuffersBuffers

A buffer is a mixture that is able to release or absorb H+ ions, keeping a solution’s pH constant.

Most common buffers are mixtures of weak acids and their conjugate bases.

ExampleExample

H3O+ + C2H3O2 -

H2O + HC2H3O2

Acetic acid and acetate ion

When H3O+ ions are added to this solution, they react with the acetate ion.

pH changes only slightly

ExampleExample

OH- + HC2H3O2

H2O + C2H3O2 -

Acetic acid and acetate ion

When OH- ions are added to this solution, they react with the acetic acid.

pH changes only slightly

Buffer Capacity is the amount of acid or base that a buffer can neutralize.

All buffers have a limited capacity to neutralize added H3O+ or OH- ions.

If you add H3O+ or OH- ions beyond the buffer capacity, the ions will remain in solution, and the pH will change.

The greater the concentration of buffer in the solution, the greater the buffer capacity.

The human body must maintain the pH of blood between 7.35 and 7.45.

A pH outside this range can cause extreme illness or death.

Acid-Base TitrationAcid-Base Titration

Section 19-3Section 19-3

TitrationTitration

• An acid base titration is a carefully controlled neutralization reaction.

• Find concentration of an unknown solution by using a known “standard” solution

standard solution

unknown solution

Equivalence point Equivalence point • When enough standard

solution is added to neutralize all the acid or base in the unknown solution.• dramatic change in pH

• Determined by the Endpoint• indicator color change

TitrationTitration

Strong Acid with Strong Strong Acid with Strong BaseBase

Equivalence Point: pH = 7Equivalence Point: pH = 7 phenolphthaleinphenolphthalein

TitrationTitration

Weak Acid with Strong BaseWeak Acid with Strong Base Equivalence Point: pH > 7Equivalence Point: pH > 7 Phenolphthalein Phenolphthalein

TitrationTitration

Weak Weak Base with Strong Acid with Strong Acid Equivalence Point: pH < 7Equivalence Point: pH < 7 Methyl redMethyl red

TitrationTitration

TitrationTitration

moles H3O+ = moles OH-

MV #a = MV #b

M: MolarityV: volume#: # of H+ ions in the acid

or OH- ions in the base

TitrationTitration

42.5 mL of 1.3M KOH are required to neutralize 50.0 mL of H2SO4. Find the molarity of H2SO4.

H3O+

M = ?V = 50.0 mLn = 2

OH-

M = 1.3MV = 42.5 mLn = 1

MV#a= MV#b

M(50.0mL)(2)=(1.3M)(42.5mL)(1)

M = 0.55M H2SO4