(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) + H 2 O (l)  H 3 O + (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%)

Percent Ionization Calculation Ex. 1 The pH of a 0.10mol/L methanoic acid (HCOOH) solution is Calculate the percent ionization of methanoic acid. Ans: 4.2%

Ionization Constants Same as K, but represents acids (K a ) and bases (K b )

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) + H 2 O (l)  H 3 O + (aq) + A - (aq) K a 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 K a K a values are found in Appendix C9 p 803

Calculating K a Ex. 2 A 0.24 mol/L solution of H 2 CO 3 has a pH of Determine the K a for H 2 CO 3 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 K a Step 3: Complete an ICE table Step 4: Determine [H 3 O + ] Step 5: Substitute [H 3 O + ] into K a equation Ans: 4.4x10 -7 (let’s compare to Appendix C Table p803 – K a1 )

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: H 2 CO 3(aq) + H 2 O (l)  HCO 3 - (aq) + H 3 O + (aq) (K a 1 ) HCO 3 - (aq) + H 2 O (l)  CO 3 2- (aq) + H 3 O + (aq) (K a 2 ) Polyprotic Acids (K a values) 1 st H + lost 2 nd H + lost

Calculating K a from Percent Ionization Ex.3 Calculate the acid ionization constant (K a )of acetic acid if a mol/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 H 2 O and therefore produce OH - General Formula : B: (aq) + H 2 O (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: Na 3 PO 4(aq)  3Na + (aq) + PO 4 3- (aq) (strong) PO 4 3- (aq) + H 2 O (l)  OH - aq) + HPO 4 2- (aq) (weak)

Weak Bases Base ionization constant (K b ) B: (aq) + H 2 O (l)  OH - (aq) + HB + (aq) In the above equation H 2 O acts as a B-L acid, and B as a B-L base. H 2 0 and OH - are conjugate base pairs, as are B: and HB + K b values are also found in Appendix C9 pg. 803

Relationship Between K a and K b 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) + H 2 O (l)  H 3 O + (aq) + X - (aq)

Relationship Between K a and K b for Conjugate Base Pairs Now consider the hypothetical weak base, X - X - (aq) + H 2 O (l)  HX (aq) + OH - (aq) Now let’s put that together HX (aq) + H 2 O (l)  H 3 O + (aq) + X - (aq) K a X - (aq) + H 2 O (l)  HX (aq) + OH - (aq) K b H 2 O (l)  H + (aq) + OH - (aq) K w

Relationship Between K a and K b for Conjugate Base Pairs Recall: Autoionization of water H 2 O (l) + H 2 O (l)  H 3 O + (aq) + OH - (aq) K w =1.00x **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: K w = K a K b K b =K w /K a K a = K w /K b Therefore if only the K a value is available in the table, we can determine the conjugate pairs K b by using the above equations Note: these equations show the larger the K a the smaller the K b

Important Concepts Between Conjugate Pairs Summary : Text p562

Important Concepts Between Conjugate Pairs Summary : Text p562

Using K w to Calculate K a & K b Ex.4 What is the value of the base ionization constant (K b ) for the acetate ion, C 2 H 3 O 2 - (aq) at SATP? Ans: 5.6x10 -10

Example Calculations Ex.5 Calculate the percent ionization of propanoic acid, HC 3 H 5 O 2 (aq), if a mol/L solution has a pH of Ans. 3.3%