1. Acids and Bases Chapter 16 Johannes N. Bronsted Thomas M. Lowry
Both independently developed Bronsted-Lowry theory of acids and bases. 1 1 1 1

2. Acids and Bases: A Brief Review
Classical Acids:
Taste sour
Donate H+ (called “H-plus” or “proton”)
Turn litmus red
Generally formed from H-Z, where Z = nonmetal
Classical Bases:
Taste bitter and feel soapy.
Donate OH- (called “O-H-minus” or “hydroxide”)
Turn litmus blue
Generally formed from MOH, where M = metal
Neutralization:
Acid + Base  Salt + water
H-Z + MOH  MZ + HOH
H+ in water is actually
in the form of H3O+,
“hydronium”

3. Brønsted-Lowry Acids and Bases
Proton Transfer Reactions
Brønsted-Lowry acid/base definition:
acid donates H+
base accepts H+.
Brønsted-Lowry base does not need to contain OH-.
Consider HCl(aq) + H2O(l)  H3O+(aq) + Cl-(aq):
HCl donates a proton to H2O. Therefore, HCl is an acid.
H2O accepts a proton from HCl. Therefore, H2O is a base.
Water can behave as either an acid or a base.
Amphoteric substances can behave as acids and bases.

4. Brønsted-Lowry Acids and Bases
Conjugate Acid-Base Pairs
Whatever is left of the acid after the proton is donated is called its conjugate base.
Similarly, whatever remains of the base after it accepts a proton is called a conjugate acid.
Consider
After H2O (base) gains a proton it is converted into H3O+ (acid). Therefore, H2O and H3O+ are conjugate acid-base pairs.
After HCl (acid) loses its proton it is converted into Cl- (base). Therefore HCl and Cl- are conjugate acid-base pairs.
Conjugate acid-base pairs differ by only one proton.

5. Brønsted-Lowry Acids and Bases
Conjugate Acid-Base Pairs
In each of following Bronsted-Lowry Acid/Base reactions,
Which is acid? base? conjugate acid? conjugate base?
HCl + H2O  H3O Cl-
acid
base
conj. acid
conj. base
NH3 + H2O  NH OH-
base
acid
conj. acid
conj. base
CH3NH2 + H2SO4  CH3NH HSO4-
conj. base
base
acid
conj. acid

6. Brønsted-Lowry Acids and Bases
Relative Strengths of Acids and Bases
The stronger the acid, the weaker the conjugate base.
The stronger the base, the weaker the conjugate acid.
H+ is the strongest acid that can exist in equilibrium in aqueous solution.
OH- is the strongest base that can exist in equilibrium in aqueous solution.

7. Strong and Weak Acids and Bases
Strong acids: completely ionized in water:
HCl, HNO3, H2SO4 (also HBr, HI)
Strong bases: completely ionized in water:
MOH, where M = alkali
M(OH)2, where M = alkaline earth
Weak acids: incompletely ionized in water:
any acid that is not strong - acetic acid, etc.
Ka is finite.
Weak bases: incompletely ionized in water:
any base that is not strong – NH3, etc.
Kb is finite.

8. The Autoionization of Water
The Ion Product of Water
In pure water the following equilibrium is established
but [H2O]2 = constant
(at 25oC)
This is called the autoionization of water

9. The Autoionization of Water
In pure water at 25oC, [H3O+][OH-] = 1 x 10-14
and also, [H3O+] = [OH-] = 1 x 10-7
(From now on, for simplification, let’s use the abbreviation:
[H+] = [H3O+]
which means [H]+ = [OH-] = 1 x 10-7
We define pH = -log [H+]
and pOH = -log [OH-]
In pure water at 25oC, pH = pOH = 7.00
pH + pOH = 14
pKw = 14
Acidic solutions have pH < 7.00
Basic solutions have pH > 7.00

10. The pH Scale
Conc. Drano
Battery acid

11. The pH Scale For [H+] = 3.4 x 10-5M, calculate pH, pOH and [OH-]
pH = 4.47, pOH = 9.53, [OH-] = 2.95 x 10-10
(b) For [OH-] = 4.4 x 10-3M, calculate pH, pOH, and [H+]
pOH = 2.36, pH = 11.64, [H+] = 2.27 x 10-12
(c) For pH= 8.9, calculate, pOH, [H+], [OH-]
pOH = 5.1, [H+] = 1.26 x 10-9, [OH-] = 7.94 x 10-6
(d) For pOH= 3.2, calculate pH, [H+], [OH-]
pH = 10.8, [H+] = 1.58 x 10-11, [OH-] = 6.3 x 10-4
The pH meter is the most accurate way to measure pH values of solutions.

12. Use this “decision tree” to calculate pH
values of solutions of specific solutions.
Is it pure water? If yes, pH = 7.00.
Is it a strong acid? If yes, pH = -log[HZ]
Is it a strong base? If yes, pOH = -log[MOH]
or pOH = -log (2 x [M(OH)2])
Is it a weak acid? If yes, use the relationship
Ka = x2/(HZ – x), where x = [H+]
Is it a weak base? If yes, use the relationship
Kb = x2/(base – x), where x = [OH-]
Is it a salt (MZ)? If yes, then decide if it is neutral, acid,
or base; calculate its K value by the relationship
KaKb = Kw, where Ka and Kb are for a conjugate system;
then treat it as a weak acid or base.
Is it a mixture of a weak acid and its weak conjugate base?
It is a buffer; see next chapter.

13. 2. Strong Acids 3. Strong Bases Calculate the pH of 0.2 M HCl .
pH = -log[H+] = -log[0.2] = 0.70
3. Strong Bases
Calculate the pH of 0.2 M NaOH .
pOH = -log[OH-] = -log[0.2] = 0.70
pH = 14 – 0.70 = 13.30
Calculate the pH of 0.2 M Ba(OH)2.
pOH = -log[OH-] = -log[0.4] = 0.40
pH = 13.60

14. 4. Weak Acids
Weak acids are only partially ionized in solution, and are in equilibrium:
(shorthand):
OR:
(shorthand)
Ka is the acid dissociation constant.

15. 4. Weak Acids

16. 4. Weak Acids Calculate pH of 0.2 M solution of acetic acid, HC2H3O2
From previous slide, Ka= 1.8 x 10-5
From now on, we’ll use the shorthand notation for HC2H3O2 = HAc
Denote the acetate ion, C2H3O2- as “Ac-”
Equilibrium is then: HAc  H+ + Ac-
Initial conc (M):
-x x x
change:
at equilibrium:
0.2-x x x
Ka=
But, this is a quadratic equation!!
We can assume x is small if Ka < 10-4.
Then, x2/0.2= 1.8 x 10-5
x = 1.90 x 10-3 = [H+] = [Ac-]
pH = -log (1.90 x 10-3) = 2.72

17. 4. Weak Acids Polyprotic Acids
Polyprotic acids have more than one ionizable proton.
The protons are removed in steps not all at once:
It is always easier to remove the first proton in a polyprotic acid than the second.
Therefore, Ka1 > Ka2 > Ka3 etc.
Most H+(aq) at equilibrium usually comes from the first ionization (i.e. the Ka1 equilibrium).

18. 4. Weak Acids
Polyprotic Acids

19. Carbonic acid, H2CO3, Ka1=4.3 x 10-7
H2CO3  H+ + HCO Ka1 = 4.3 x 10-7
HCO3-  H CO Ka2 = 5.6 x 10-11
Since Ka1>>Ka2, nearly all H+ ions come from 1st equilibrium.
Therefore, the 1st equilibrium determines the pH.
Calculate pH of 0.4 M H2CO3 solution.
Ka = x2/HZ-x
4.3 x 10-7 = x2/0.4
x = [H+] = 4.15 x 10-4
pH = -log (4.1 x 10-4) = 3.38

20. 5. Weak Bases Weak bases remove protons from substances.
There is an equilibrium between the base and the resulting ions:
Example:
The base dissociation constant, Kb is defined as:

21. 5. Weak Bases
The larger Kb the stronger the base.

22. 5. Weak Bases x = 8.12 x 10 = [OH ] For convenience, denote it as “B”
What is pH of 0.15 M solution of methylamine, NH2CH3, a weak base?
For convenience, denote it as “B”
Then: B H2O  BH OH Kb =4.4 x 10-4
Initially:
change: x x x
At equil: x x x
Then: x =
8.12 x 10 - 3 =
[OH - ]

23. Relationship Between Ka and Kb
For a conjugate acid-base pair
Ka  Kb = Kw (constant)
Therefore, the larger the Ka, the smaller the Kb. That is, the stronger the acid, the weaker the conjugate base.
Taking negative logarithms:
-log Ka- log Kb= -log Kw
pKa + pKb = pKw
For HAc (acetic acid), the pKa = -log (1.8 x 10-5) = 4.74.
Thus, the pKb for Ac- (acetate 0 is 14 – 4.74 = 9.26

24. 6. Salts Salts may be acidic, basic or neutral.
Salts made by a strong acid and a strong base are neutral,
e.g., NaCl, KNO3.
Salts made by a weak acid and a strong base are weakly basic,
e.g., sodium acetate, NaAc, NaHCO3.
Salts made by a strong acid and a weak base are weakly acidic,
e.g., NH4Cl.
Calculate the pH of a 0.35 M solution of sodium acetate.
Since Ka Kb = Kw, where Ka = 1.8 x 10-5 for acetic acid,
Then Kb for NaAc (sodium acetate) is
Kb = Kw/Ka = 1.00 x 10-14/1.8 x 10-5 = 5.56 x
Now treat this as a weak base problem,
Kb = x2/base = 5.56 x = x2/0.35
x = [OH-] = 1.39 x 10-5
pOH = 4.85 and pH = 9.14