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(8.2) Weak Acids & Bases: Ionization Constants
Percent Ionization for Weak Acids
Most weak acids ionize < 50% Percent ionization (p)
General Weak Acid: HA(aq) + H2O(l) H3O+ (aq) + A-
(aq)
• p varies depending on concentration: increase [HA ] decreases p
• This is caused by Le Chatelier’s Principle
• Remember, for strong acids we assume complete ionization (100%)
100xsoluteacidofionconcentrat
ionizedacidofionconcentratp
100x]HA[]H[
p)aq(
)aq(
Percent Ionization Calculation
Ex. 1
The pH of a 0.10mol/L methanoic acid (HCOOH) solution is 2.38. Calculate the percent ionization of methanoic acid.
Ans: 4.2%
Ionization Constants
Same as K, but represents acids (Ka) and bases (Kb)
Weak Acids
A weak acid is a weak electrolyte that doesn’t completely ionize when mixed with water to form H+
General formula: HA (aq) + H2O (l) H3O+ (aq) + A-
(aq)
Ka is the acid ionization constant for weak acids (don’t include pure states or units, just as with K)
The stronger the acid the larger the Ka
Ka values are found in Appendix C9 p 803
][HA
][A]O[HK
(aq)
(aq)(aq)3a
Calculating Ka
Ex. 2
A 0.24 mol/L solution of H2CO3 has a pH of 3.49.
Determine the Ka for H2CO3 when one H+ is donated.
Step 1: Write out balance chemical equation, determine we know/what we
want to know
Step 2: Determine formula for calculating Ka
Step 3: Complete an ICE table
Step 4: Determine [H3O+]
Step 5: Substitute [H3O+] into Ka equation
Ans: 4.4x10-7 (let’s compare to Appendix C Table p803 – Ka1)
Recall: B-L acids can also be polyprotic-capable of losing
(donating) more than one proton. Polyprotic acids lose their protons in separate steps or
reactions.
Ex:
H2CO3(aq) + H2O(l) HCO3-(aq) + H3O+
(aq) (Ka1)
HCO3-(aq) + H2O(l) CO3
2-(aq) + H3O+
(aq) (Ka2)
Polyprotic Acids (Ka values)
1st
H+
lost
2nd
H+
lost
Calculating Ka from Percent Ionization
Ex.3
Calculate the acid ionization constant (Ka )of acetic acid if a 0.1000mol/L solution at equilibrium at SATP has a percent ionization of 1.3% (Hint: ICE table)
Ans: 1.7x10-5
Weak Bases
Do not react completely in an aqueous solution Are molecules or ions that remove a proton from H2O and therefore
produce OH-
General Formula: B:(aq) + H2O (l) OH-(aq) + HB+
(aq)
To act as a weak B-L base a compound must have an atom with lone pairs of electrons since this is where the H+ is accepted from water
A substance can undergo two chemical changes, one can represent a strong base while another weak
For example: Na3PO4(aq) 3Na+(aq) + PO4
3-(aq) (strong)
PO43-
(aq) + H2O(l) OH- aq) + HPO4
2-(aq) (weak)
Weak Bases
Base ionization constant (Kb)
B:(aq) + H2O (l) OH-(aq) + HB+
(aq)
In the above equation H2O acts as a B-L acid, and B as a B-L base. H20 and OH- are conjugate base pairs, as are B: and HB+
Kb values are also found in Appendix C9 pg. 803
][B
][HB][OHK
(aq)
(aq)(aq)b
Relationship Between Ka and Kb for Conjugate Base Pairs
• Recall: Conjugate Pairs – an acid and base that differ by one hydrogen
• Lets consider the hypothetical weak acid, HX, and its conjugate base, X -
HX (aq) + H2O (l) H3O+ (aq) + X- (aq)
][HX
][X]O[HK
(aq)
(aq)(aq)3a
Relationship Between Ka and Kb for Conjugate Base Pairs
• Now consider the hypothetical weak base, X -
X- (aq) + H2O (l) HX (aq) + OH- (aq)
• Now let’s put that together
HX (aq) + H2O (l) H3O+ (aq) + X- (aq) Ka
X- (aq) + H2O (l) HX (aq) + OH- (aq) Kb
H2O (l) H+ (aq) + OH-
(aq) Kw
][X
][OH][HXK
(aq)-
(aq)-
(aq)b
Relationship Between Ka and Kb for Conjugate Base Pairs
][HX
][X]O[HK
(aq)
(aq)(aq)3a
][X
][OH][HXK
(aq)-
(aq)-
(aq)b
KxK ba
][X][OH][HX
x][HX
][X]O[H(aq)
-
(aq)-
(aq)
(aq)
(aq)-
(aq)3
][OH]O[H (aq)(aq)3 Kw
Recall: Autoionization of water H2O(l) + H2O(l) H3O+
(aq) + OH-(aq)
Kw=1.00x10-14 **must remember this value**
Relationship Between Ionization Constants for Conjugate Base Pairs
For acids and bases whose chemical formulas differ by only one hydrogen (conjugate pairs) the following apply:
Kw = KaKb Kb =Kw/Ka Ka = Kw/Kb
• Therefore if only the Ka value is available in the table, we can determine the conjugate pairs Kb by using the above equations
Note: these equations show the larger the Ka the smaller the Kb
Important Concepts Between Conjugate Pairs
Summary : Text p562
Important Concepts Between Conjugate Pairs
Summary : Text p562
Using Kw to Calculate Ka & Kb
Ex.4
What is the value of the base ionization constant (Kb) for the acetate ion, C2H3O2
- (aq)
at SATP?
Ans: 5.6x10-10
Example Calculations
Ex.5
Calculate the percent ionization of propanoic acid, HC3H5O2(aq), if a 0.050 mol/L solution has a pH of 2.78.
Ans. 3.3%