ACIDS&

BASES

Arrhenius Theory

1. in aqueous solution

2. Acid: produces H+

3. Base: produces OH-

HA H3O+ + A-

Acid

OH2

O

HHHA + O

HHH

+

+ A-

HCl(g) + H2O H3O+(aq) + Cl-(aq)

CH3COOH(l) + H2O = H3O+(aq) + CH3COO-(aq)

careless, but often seen

HCl HCl H H++ + Cl+ Cl--

CHCH33COOHCOOH HH++ + CH+ CH33COOCOO--

BaseNaOH(s) Na+(aq) + OH-(aq) OH2

Arrhenius acid/base reaction

acid + base H2O + a salt

HA + MOH HOH + MA

Monoprotic acid: HClHCl(aq) + NaOH(aq) H2O(l) + NaCl(aq)

H+ + Cl- + Na+ + OH- H2O + Na+ + Cl-

H+ + OH- H2O

HCl

Diprotic acid: H2SO4

H2SO4 (aq) + 2NaOH (aq) 2H2O(l) + Na2SO4 (aq)

H+ + OH- H2O

H2SO4

Triprotic acid: H3PO4

Polyprotic

H3PO4(aq) + 3NaOH(aq) 3H2O(l) + Na3PO4(aq)

H3PO4 + 3 OH- 3 H2O + PO43-

H3PO4

Bronsted-Lowry Theory1. aqueous & nonaqueous solutions

2. Acid: species donating a proton

HCl H+ + Cl-

H2SO4 H+ + HSO4-

CH3COOH H+ + CH3COO-

Bronsted-Lowry Theory3. Base: species accepting a proton

OH- + H+ HOH

H2O + H+ H3O+

NH3 + H+ NH4+

Conjugate acid-base pairs

acid1 + base1 acid2 + base2

conjugate pairs

HF + HOH

Conjugate acid-base pairs

conjugate pairs

HF + HOH H3O+ + F-

acid1 + base1 acid2 + base2

ALL Arrhenius reactions are Bronsted-Lowry

reactions

HCl + NaOH H2O + NaCl

NOT all Bronsted reactions are Arrhenius reactions

CH3COOH + NH3 NH4+ + CH3COO-

Amphiprotic = AmphotericCan act as either an acid or a base

HCl + HOH H3O+ + Cl-

NH3 + HOH NH4+ + OH-

NH3 + OH- NH2- + HOH

HOH + HOH H3O+ + OH-

ACID STRENGTHRelative ability of a

compound to donate a proton

Base strength is considered a result,

not a cause

Strong acid

100% dissociation

Weak acid

<100% dissociation

Notice this is NOT related to concentration

REVIEW

Electronegativity is the most significant factor influencing

the strength of acids & bases

HF > HCl > HBr > HI

as acids in non-aqueous solvents, or as pure gases

Look at difference inelectronegativities

2.1 H - F 4.02.1 H - Cl 3.02.1 H - Br 2.82.1 H - I 2.5

Most “ionic” is the most acidic

Nonpolar Polar Ionic

E 0 E 1.7 E 4.0

However,as acids

in aqueous solution

HF < HCl = HBr = HI

2.1 H - O 3.5competition!2.1 H - F 4.0

2.1 H - Cl 3.02.1 H - Br 2.82.1 H - I 2.5

Is methane acidic as a gas or in

aqueous solution?

2.1 H - C 2.5

The strength of oxy-acids are also

dependent on electronegativity.

Oxy-acids and bases have the

same fundamental structure

NaOH: Na - O - H 0.9 3.5 2.1

HClO: Cl - O - H 3.0 3.5 2.1

In water, the more “ionic” bond

dissociates, forming the acid or base

NaOH: Na - O - H 0.9 3.5 2.1

HClO: Cl - O - H 3.0 3.5 2.1

Are alcohols acids or bases?

C - O - H2.5 3.5 2.1

Acids in homologous series

are of different strength

Acid StrengthH2SO4 > H2SO3

HNO3 > HNO2

HClO4 > HClO3 > HClO2 > HClO

Structurally

H2SO4 = O2S(OH)2

H2SO3 = OS(OH)2

Need to examine formal charge of

central atom.

Acid Strength

CH3COOH> CH3CH2OH

CF3COOH > CH3COOH

Need to examine inductive effect of neighboring

atoms.

pH

pK

Ka , Kb , Kw

2H2O H3O+ + OH-

KH O OH

H Oeq [ ][ ]

[ ]3

22

Keq [H2O]2 = [H3O+ ][OH-]

Kw = [H3O+ ][OH-]

whereKw (25oC ) = 1 x 10-14

in a neutral solution[H3O+ ] = [OH-]

1 x 10-14 = [H3O+ ]2 = [OH-]2

[H3O+ ] = [OH-] = 1 x 10-7

pX = -log X

pK = -log K pH = -log [H3O+]pOH = -log [OH-]

leveling effect of H2O limits [H3O+ ] & [OH-] to that

controlled by H2O

upper limit [H3O+ ] = 1

lower limit [H3O+ ] =

1 x 10-14

pH scale

0 7 14

acid neutral base

highest [H3O+ ] on left

lowest [H3O+ ] on right

[H3O+ ] and [OH-]

must be considered together

Kw = [H3O+ ][OH-]

-log Kw = -log {[H3O+ ][OH-]}

-log Kw = {-log [H3O+ ]} + {-log[OH-]}

pKw = pH + pOH

but Kw = 1 x 10-14

14 = pH + pOH

Relationship betweenconjugate

acids & bases

HA + H2O H3O+ + A-

A- + H2O HA + OH-

KH O A

HA

KHA OH

A

a

b

[ ][ ][ ]

[ ][ ][ ]

3

]A[

]OH][HA[

]HA[

]A][OH[KK 3

ba

K KH O A

HAHA OH

Aa b

[ ][ ]

[ ][ ][ ]

[ ]3

Ka x Kb = [H3O+ ][OH-] = Kw

Ka. Kb = [H3O+ ][OH-] = Kw

Ka. Kb = Kw

SUMMARYpH = -log [H3O+ ]pOH = -log [OH-][H3O+ ][OH-] = 1 x 10-14

pH + pOH = 14Ka

. Kb = Kw

Applications ofAcid-BaseConcepts

for weak acids & bases, refer to

Appendix H for Ka & Appendix I for Kb

values in Kotz & Treichel

1. What is the pH of a solution that is 0.025 M KOH?

2. What is the pH of a 0.20 M acetic acid solution?

3. 100 mL of 0.10 M CH3COOH are mixed with 20.0 mL of 0.10 M NaOH. What is the pH of the solution?

4. Calculate the percent ionization of 0.10 M methylamine (CH3NH2).

pH of Salts & Oxides

What effect does the addition of a

salt to water have upon the pH of the

water?

H2O equilibrium is the prime factor in

the behavior of solutions.

pH of a salt solution is dependent upon the strength of the

salt as an electrolyte.

Example 1

NaCl(s) + HOH NaOH(aq) + HCl(aq)

Example 1

NaCl(s) + HOH NaOH (aq) + HCl(aq) strong base strong acid

Na+ + OH- + H+ + Cl- Na+ + HOH + Cl-

thus, NaCl in water has NO effect on pH

Example 2

NaCN(s) + HOH NaOH(aq) + HCN(aq)

Example 2

NaCN(s) + HOH NaOH (aq) + HCN(aq) strong base weak acid

Na+ + OH- + HCN

CN- is the anion of the weak acid HCN

CN- + HOH HCN + OH-

NaCN(s) + HOH Na+ + OH- + HCN strong base weak acid

thus, NaCN in water produces a/n ??

solution

thus, NaCN in water produces a BASIC

solution

5. What is the pH of a 0.010 M sodium cyanide solution?

Example 3

NH4Cl(s) + HOH NH4OH (aq) + HCl(aq)

Example 3

NH4Cl(s) + HOH NH4OH (aq) + HCl(aq) weak base strong acid

NH4OH + H+ + Cl-

NH4+ is the cation of the

weak base NH4OH

NH4+ + HOH NH3 + H3O+

NH4Cl(s) + HOH -> NH4OH + H+ + Cl-

weak base strong acid

thus, NH4Cl in water produces a/n ??

solution

thus, NH4Cl in water produces an ACID

solution

6. What is pH of a 0.10 M ammonium chloride solution?

Example 4

NH4CN(s) + HOH NH4OH (aq) + HCN(aq) weak base weak acid

NH4+ is the cation of

the weak base NH4OH

NH4++ HOH NH3 + H3O+

CN- is the anion of the weak acid HCN

CN- + HOH HCN + OH-

thus, NH4CN in water produces a/n

?? solution

The pH of a solution formed from the cation of a weak base and the anion

of a weak acid is dependent on the relative strength of the weak acid

and weak base.

Ka(HCN) = 6.2 x 10-10

[Text: Table 5.1] Appendix H A-23

Kb(NH4OH) = 1.8 x 10-5

[Text: Table 5.3] Appendix I A-25

thus, NH4CN in water produces a/n

?? solution

thus, NH4CN in water produces a BASIC

solution, because the weak base is stronger (ionizes more) than the

weak acid

Acidity of Oxides

SO2 + HOH ??

SO2 + HOH H2SO3

[O2]

H2SO4

SO2 + HOH H2SO3

[O2]

H2SO4

Covalent oxides are acidic & are referred to as acid anhydrides

Na2O + HOH ??

Na2O + HOH 2NaOH(aq)

Na2O + HOH 2NaOH(aq)

Ionic oxides are basic& are referred to as

basic anhydrides

Lewis Acid-Base

Theory

Acid

substance capable of accepting an e- pair

Lewis acidmust have an empty

valence level orbital

i.e. H+ has an empty 1s orbital which can accept an e- pair

Thus, H+ is an acid under all three theories

Arrhenius

Bronsted-Lowry

Lewis

Lewis Acid-Base TheoryAcid: substance capable

of accepting an e- pair

Base

substance capable of donating an e- pair

Examples of Lewis bases

OH- , NH3 , F-

all have unbonded pairs of e- available for

donation

Elements of Group 13 (3A) form compounds

that make excellent Lewis acids

another “typical”Lewis acid-base

reaction:

Reaction of a Lewis Acid and Lewis Base

New bond formed using electron pair from the Lewis base.

Coordinate covalent bond

Notice geometry change on reaction.

Formation of hydronium ion is also an excellent example.

Lewis Acids & Bases

•Electron pair of the new O-H bond originates on the Lewis base.

HH

H

BASE

••••••

O—HO—H

H+

ACID

Lewis Acid/Base Reaction

H3BO3 + H2O H2BO3- + H3O+

?

NO!

H3BO3 + 2H2O B(OH)4- + H3O+

is Al(OH)3 an

acid or base?

Amphoterism of Al(OH)3

This explains AMPHOTERIC nature of some metal hydroxides.

Al(OH)3(s) + 3 H+ Al3+ + 3 H2O

Here Al(OH)3 is a Brønsted base.

Al(OH)3(s) + OH- Al(OH)4-

Here Al(OH)3 is a Lewis acid.

Al3+ O—H-••••

••Al3+ O—H-

••••

••

Lewis Acids & Bases

Transition metal ions also very good

Lewis Acids

Other good examples involve metal ions.

Lewis Acids & Bases

HH

••

BASE

••••••

Co2+

ACID

O—HO—H

Co2+

Zn2+ + HOH ?..

Zn2+ => [Ar] 4s0 3d10 4p0

Zn(H2O)42+

..

Reaction of NH3 with Cu2+

(aq)

Formation of complexions is a Lewis

acid-base reaction

Lewis Acid-Base Interactions in Biology

The heme group in hemoglobin can interact with O2 and CO.

The Fe ion in hemoglobin is a Lewis acid

O2 and CO can act as Lewis bases

Heme group

LewisBronsteadArrhenius

Inclusiveness of the Acid/Base Definitions