General Chemistry IIGeneral Chemistry IICHEM 152 Unit 2CHEM 152 Unit 2

Week 7

UA GenChem

Week 7 Reading Assignment

Chapter 15 – Sections 15.2 (acids/bases), 15.3 (acids/bases), 15.4 (Ka), 15.5 (pH)

UA GenChem

Equilibrium in Aqueous Equilibrium in Aqueous Solutions Solutions

Acids and BasesAcids and Bases

The ideas we have discussed about

chemical equilibrium are very useful to

understand the behavior of acids

and bases in water.

UA GenChem

Water’s Role as Acid or Base

Water acting as a Base

HA + H2O H3O+ + A-

base acid

Water acting as an Acid

B + H2O BH+ + OH-

acid base

Water behaves like a base when an acid is present. Water behaves

like an acid when a base is present.

UA GenChem

More About Water

HH22O can function as both an ACID and a BASE.O can function as both an ACID and a BASE.

In pure water there can be In pure water there can be AUTOIONIZATIONAUTOIONIZATION

Equilibrium constant for autoionization = KEquilibrium constant for autoionization = Kww

KKww = [H = [H33OO++] [OH] [OH--] = ] = 1 x 101 x 10-14-14 (exact) at 25 (exact) at 25 ooCC

OH-

H3O+

OH-

H3O+

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

UA GenChem

More About Water

If KIf Kww = [H = [H33OO++] [OH] [OH--] = 1.00 x 10] = 1.00 x 10-14-14 at 25 at 25 ooC, C,

what would be the concentration of Hwhat would be the concentration of H33OO++ and and

OHOH-- in pure water? in pure water?

OH-

H3O+

OH-

H3O+

Auto-ionizationAuto-ionization

In a In a neutral solutionneutral solution [H[H33OO++] = [OH] = [OH--]]

and so and so [H[H33OO++]] = = [OH[OH--]] = = 1.00 x 101.00 x 10-7-7 M M

UA GenChem

Calculating [HCalculating [H33OO++] and ] and [OH[OH--]]

Now, imagine that you add an acid or a base to pure Now, imagine that you add an acid or a base to pure water. water.

What are the new concentrations of HWhat are the new concentrations of H33OO++ and OH and OH-- in the in the system?system?

What happens if you add a strong What happens if you add a strong acid? acid?

What if you add a strong base?What if you add a strong base?

The answer depends on the type or acid or The answer depends on the type or acid or base that you add, but in all cases:base that you add, but in all cases:

KKww = = [H[H33OO++] [OH] [OH--]] = = 1 x 101 x 10-14-14 (exact) (exact) at 25 at 25 ooCC

UA GenChem

Calculating [HCalculating [H33OO++] and ] and [OH[OH--]]

Imagine that you add 0.0010 Imagine that you add 0.0010 moles of NaOH to 1.0 L of pure moles of NaOH to 1.0 L of pure

water. water. Calculate [HCalculate [H3OO++] and [OH] and [OH--]]

KKww = = [H[H33OO++] [OH] [OH--]] = = 1 x 101 x 10-14-14

[H[H33OO++] [0.0010 M]] [0.0010 M] = = 1 x 101 x 10-14-14

NaOH = strong electrolyte (and base!)NaOH = strong electrolyte (and base!)

0.0010 M NaOH 0.0010 M NaOH 0.0010 M OH 0.0010 M OH¯̄

[H[H33OO++] = ] = 1.0 x 101.0 x 10-11-11 M M

UA GenChem

Calculating [HCalculating [H33OO++] and [OH] and [OH--]]Now, do it yourself:

Calculate the concentration of OH- and H3O+ in a 6.0 M solution of

nitric acid (HNO3).

Analyze the results(How acidic is this solution?

How do you know)

UA GenChem

Acidity and BasicityAcidity and Basicity

A solution is considered acidic when[H3O+] > [OH-]

A solution is considered basic when[H3O+] < [OH-]

A solution is neutral when

[H3O+] = [OH-]

UA GenChem

A common way to express acidity and A common way to express acidity and basicity is with pHbasicity is with pH

pH = - log [HpH = - log [H33OO++] or [H] or [H33OO++]=10]=10-pH-pH

Acidity and BasicityAcidity and Basicity

The # of sig figs in the concentrationThe # of sig figs in the concentration

== the number of decimal places in pH the number of decimal places in pH

In an acidic solution,In an acidic solution,

[H[H3OO++]= 1.25 x 10]= 1.25 x 10-4-4 M at 25 M at 25 ooCC

pH pH =-log (1.25 x 10=-log (1.25 x 10-4-4)= -(-3.903) )= -(-3.903) = = 3.9033.903

UA GenChem

What is the pH of Black Coffee, [H3O+]= 1.0 x 10-5 M?

UA GenChem

What is the H3O+ concentration in sea water if pH=8.30?

UA GenChem

What is the OH¯ concentration in acid rain if pH=5.25?

UA GenChem

[H[H33OO++], [OH], [OH--] and ] and pHpH

General conclusion @25ºC General conclusion @25ºC

Basic solution Basic solution pH > 7 pH > 7

Neutral Neutral pH = 7pH = 7 Acidic solutionAcidic solution pH < 7pH < 7

UA GenChem

Other pX ScalesOther pX Scales

In generalIn general pX pX = = -log X-log X

and so and so pOHpOH = = - log [OH- log [OH--]]

Which solution is more basic, one that has a pH of 5.5 or one with a pOH of 8.5?

If KIf Kww = [H = [H3OO++] [OH] [OH--] = 1 x 10] = 1 x 10-14-14

Taking the log of both sidesTaking the log of both sides

-log (10-log (10-14-14) = - log [H) = - log [H3OO++] + (-log ] + (-log

[OH[OH--])])

pKpKww = 14 = pH + pOH = 14 = pH + pOH

UA GenChem

Methods for Measuring the pH of an Aqueous

Solution

pH meterpH (indicator)

paper

UA GenChem

According with this theory:According with this theory:

ACIDS DONATE HACIDS DONATE H++ IONS IONS

(hydrogen ion donors)(hydrogen ion donors)

HA(aq) + HHA(aq) + H22O(l) O(l) A A--(aq) + H(aq) + H33OO++(aq)(aq)

BASES ACCEPT HBASES ACCEPT H++ IONS IONS

(hydrogen ion acceptors)(hydrogen ion acceptors)

AA--(aq) + H(aq) + H22O(l) O(l) HA(aq) + OH HA(aq) + OH--(aq)(aq)

HH++

HH++

BrBrØØnstednsted––Lowry TheoryLowry Theory

UA GenChem

Strong or WeakStrong or WeakGenerally, we divide acids and bases into Generally, we divide acids and bases into

STRONG STRONG or or WEAKWEAK ones ones

STRONG ACID:STRONG ACID: HNOHNO33(aq) + H(aq) + H22O(l) O(l) HH33OO++(aq) + NO(aq) + NO33

--(aq)(aq)

HNOHNO3 3 (nitric acid) is about 100% dissociated (nitric acid) is about 100% dissociated

in water.in water.

HH++

The equilibrium constant

is much larger than 1.][

]][[

3

33

HNO

NOOHKa

UA GenChem

Strong Acid?Strong Acid? HA(aq) + HHA(aq) + H22O(l) O(l) HH33OO++(aq) + A(aq) + A--(aq)(aq)

Write Ka for this acid and comment

on its value?

1][

]][[ 3

HA

AOHKa

UA GenChem

HNO3, HCl, HBr, HI, H2SO4 and HClO4 are strong acids

Strong AcidsStrong Acids

The H3O+ ion forms strong

hydrogen bonds with surrounding water molecules

UA GenChem

Weak Acids Weak acids donate a small

fraction of their H+’s– most of the weak acid

molecules do not donate H+ to water

– Roughly 1% ionized in water

[H3O+] << [HF]

HF H+ + F-

HF + H2O H3O+ + F-

1][

]][[ 3

HF

FOHKa

UA GenChem

Weak acids are much less than 100% ionized in water.Weak acids are much less than 100% ionized in water.

Weak AcidsWeak Acids

HA + H2O H3O+ + A-

HH++

1][

]][[ 3

HA

AOHKa

UA GenChem

Strong Base:Strong Base: 100% dissociated in water 100% dissociated in water

NaOH(aq) NaOH(aq) Na Na++(aq) + (aq) + OHOH--(aq)(aq)

Strong BasesStrong Bases

Solubility rules – all soluble hydroxides.Solubility rules – all soluble hydroxides.

Other common strong bases include KOH and Other common strong bases include KOH and Ca(OH)Ca(OH)22..

Ionic compounds that that add so much OHIonic compounds that that add so much OH¯̄ as as to overwhelm the balance of Hto overwhelm the balance of H33OO++ and OH and OH¯̄

UA GenChem

Weak BasesWeak Bases

Weak base:Weak base: only a small percentage only a small percentage ionized in waterionized in water

One of the best known weak bases is One of the best known weak bases is ammoniaammonia

NHNH33(aq) + H(aq) + H22O(liq) O(liq) NH NH44++(aq) + (aq) + OHOH--

(aq)(aq)

HH++

How would you express Kb for this

reaction?

][

]][[

3

4

NH

NHOHKb

Weak bases are molecules that have an affinity Weak bases are molecules that have an affinity to ACCEPT an Hto ACCEPT an H++

UA GenChem

Conjugate Acid-Base Pairs

NHNH44++ / NH / NH3 3 is a is a conjugate acid-base pairconjugate acid-base pair

— related by the gain or loss of H— related by the gain or loss of H++

Every acid has a conjugate base Every acid has a conjugate base and vice-versa. and vice-versa.

Consider the acid-base reaction:

HH++ HH++

Acid

UA GenChem

The Conjugate Pairs in Some Acid-Base Reactions

Base Acid+Acid Base+

Conjugate Pair

Conjugate Pair

Conjugate Acid-Base Pairs

HPO42- SO3

2-+ PO43- HSO3

-+

Identify the acid, the base, and the conjugate acid/base pairs:

UA GenChem

Identify the acid and its conjugate base in: HF + H2O F¯ + H3O+

1. HF, F¯

2. H2O, H3O+

3. HF, H3O+

4. H2O, F¯

5. HF, H2O

UA GenChem

Identify the base and its conjugate acid in: CN¯ + HCOOH HCOO¯ + HCN

1. CN¯, HCOO¯

2. HCOOH, HCN3. CN¯, HCN4. HCOOH, CN¯

5. HCOOH, HCOO¯

UA GenChem

Identify the conjugate acid-base pairs:

H2PO4¯ + OH¯ H2O + HPO4

2-

1. OH¯, H2O

2. OH¯, HPO42-

3. H2PO4¯, OH¯

4. H2PO4¯, H2O

5. H2PO4¯, HPO4

2-

UA GenChem

Identify the conjugate acid-base pairs:

N2H5+ + H2SO4 HSO4

¯ + N2H62+

1. N2H5+, H2SO4

2. H2SO4, HSO4¯

3. H2SO4, N2H62+

4. N2H5+, HSO4

¯

5. N2H5+, N2H6

2+

UA GenChem

Relative Strengths of

Acid and Bases

As acid strength decreases, base

strength increases; the weaker the acid,

the stronger its conjugate base.

As base strength decreases, acid

strength increases; the weaker the base,

the stronger its conjugate acid.

UA GenChem

Relative Strengths of Acid and Bases

The stronger acid and the stronger base will always react to form a

weaker conjugate base and a weaker

conjugate acid. Would you expect these acids and bases to react?

What would be the outcome?

CN-(aq) + HCl(aq)

HF(aq) + Cl-(aq)

H2O(l) + S2-(aq)

UA GenChem

For the reaction: CN¯ + HCl Cl¯ + HCN

1. K is large2. K is ~13. K is small

UA GenChem

For the reaction: Cl¯ + HF F¯ + HCl

1. K is large2. K is ~13. K is small

UA GenChem

For the reaction: S2- + H2O HS¯ + OH¯

1. K is large2. K is ~13. K is small

UA GenChem

Summary Activity

1. Label the following as strong acids, weak acids, strong bases, weak bases:

HCl HNO2 (CH3)2NH

HCOOH Ca(OH)2

UA GenChem

2. What is the [H3O+] of a 0.0050 M Ca(OH)2 solution?

UA GenChem

3. What is the pOH of a 0.0028 M HNO3

solution?