acids & bases
DESCRIPTION
ACIDS & BASES. Arrhenius Theory. 1. in aqueous solution 2. Acid: produces H + 3. Base: produces OH -. Acid. HA H 3 O + + A -. +. O. O. -. HA +. + A. H. H. H. H. H. HCl(g) + H 2 O H 3 O + (aq) + Cl - (aq). - PowerPoint PPT PresentationTRANSCRIPT
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