1. Just When It’s Safe to GoJust When It’s Safe to Go
Back Into the WaterBack Into the Water
The Acidic or Basic Quality of SaltsThe Acidic or Basic Quality of Salts

2. Acid Equilibrium ReviewAcid Equilibrium Review
 WeakWeak acidsacids form equilibria in aqueous solutionsform equilibria in aqueous solutions
according to the following general patternaccording to the following general pattern
 HX (aq) HHX (aq) H+1+1
(aq) + X(aq) + X-1-1
(aq)(aq)
 The equilibrium constant follows the formThe equilibrium constant follows the form
 KKaa = [H= [H+1+1
][X][X-1-1
]/[HX]]/[HX]
 The reaction can be shifted according toThe reaction can be shifted according to
LeChatelier’s PrincipleLeChatelier’s Principle

3. Base Equilibrium ReviewBase Equilibrium Review
 Weak bases form equilibria in water similar toWeak bases form equilibria in water similar to
the example of ammoniathe example of ammonia
 NHNH33 + HOH NH+ HOH NH44
+1+1
+ OH+ OH-1-1
 The equilibrium constant would beThe equilibrium constant would be
 KKbb = [NH= [NH44
+1+1
][OH][OH-1-1
]/[NH]/[NH33]]
 Again, LeChatelier’s Principle can be used toAgain, LeChatelier’s Principle can be used to
explain or predict shiftsexplain or predict shifts

4. And, Water Has Its Equilibrium TooAnd, Water Has Its Equilibrium Too
 Water forms an equilibrium that can be affectedWater forms an equilibrium that can be affected
by any substance dissolved into solutionby any substance dissolved into solution
 The equilibrium involves the water molecule,The equilibrium involves the water molecule,
hydrogen ion, and hydroxide ionhydrogen ion, and hydroxide ion
 HH22O HO H+1+1
+ OH+ OH-1-1
 The equilibrium constant, KThe equilibrium constant, Kww, takes the form, takes the form
 KKww = [H= [H+1+1
][OH][OH-1-1
]]
 KKww = 1 x 10= 1 x 10-14-14

5. Conjugate Acids and BasesConjugate Acids and Bases
 An acid-base reaction produces new materialsAn acid-base reaction produces new materials
which may be classified as conjugate acids (orwhich may be classified as conjugate acids (or
bases)bases)
 Weak acid (or base) equilibria often produceWeak acid (or base) equilibria often produce
anions (or cations) with significant base (or acid)anions (or cations) with significant base (or acid)
propertiesproperties
 The weaker the original acid (or base) theThe weaker the original acid (or base) the
stronger its conjugate base (or acid)stronger its conjugate base (or acid)

6. Acetic Acid, for exampleAcetic Acid, for example
 When acetic acid dissolves in water it formsWhen acetic acid dissolves in water it forms
hydrogen ion and acetate ionhydrogen ion and acetate ion
 CHCH33COOH (aq) CHCOOH (aq) CH33COOCOO-1-1
+ H+ H+1+1
 The acetate ion has some basic properties (It isThe acetate ion has some basic properties (It is
the conjugate base for acetic acid in thethe conjugate base for acetic acid in the
reaction.)reaction.)
 Therefore, it can be expected that solutionsTherefore, it can be expected that solutions
(which might carelessly be considered neutral)(which might carelessly be considered neutral)
containing acetate ion might be basiccontaining acetate ion might be basic

7. An Acetate Solution ExampleAn Acetate Solution Example
 Suppose a solution of sodium acetate, CHSuppose a solution of sodium acetate, CH33COOCOO--
NaNa++
, is made, is made
 The acetate and sodium ions separateThe acetate and sodium ions separate
 CHCH33COOCOO--
NaNa++
CHCH33COOCOO--
+ Na+ Na++
 Each ion may have acidic or basic propertiesEach ion may have acidic or basic properties
 To analyze this we see what would be producedTo analyze this we see what would be produced
if each ion reacted with waterif each ion reacted with water

8. These Are Called HydrolysisThese Are Called Hydrolysis
ReactionsReactions
 Sodium ion reacting with waterSodium ion reacting with water
 NaNa+1+1
+ HOH NaOH + H+ HOH NaOH + H+1+1
 The products are NaOH and HThe products are NaOH and H+1+1
. The NaOH is strong. The NaOH is strong
and dissociates 100%. Thus, no difference betweenand dissociates 100%. Thus, no difference between
[H[H+1+1
] and [OH] and [OH-1-1
] is created] is created
 Acetate ion reacting with waterAcetate ion reacting with water
 CHCH33COOCOO-1-1
+ HOH CH+ HOH CH33COOH + OHCOOH + OH-1-1
 The products are CHThe products are CH33COOH and OHCOOH and OH-1-1
. Acetic acid is. Acetic acid is
weak and doesn’t dissociate 100%. So, more [OHweak and doesn’t dissociate 100%. So, more [OH--
11
]>[H]>[H+1+1
]. It’s a basic solution.]. It’s a basic solution.

9. Generalizing for HydrolysisGeneralizing for Hydrolysis
 Check the reaction impact of the ions from a saltCheck the reaction impact of the ions from a salt
(MX) with water(MX) with water
 MM+1+1
+ HOH MOH + H+ HOH MOH + H+1+1
 XX-1-1
+ HOH HX + OH+ HOH HX + OH-1-1
 If either the acid or base produced is weak theIf either the acid or base produced is weak the
salt has acidic/basic propertiessalt has acidic/basic properties
 These reactions are equilibrium reactions andThese reactions are equilibrium reactions and
will have K valueswill have K values

10. K Values for Hydrolysis ReactionsK Values for Hydrolysis Reactions
 Using a specific example of sodiumUsing a specific example of sodium
acetate the hydrolysis reaction isacetate the hydrolysis reaction is
 The KThe Kaa value for acetic acid isvalue for acetic acid is
 The K formula for the hydroysisThe K formula for the hydroysis
reaction would bereaction would be
 Inverting the KInverting the Kaa givesgives
 Mulitplying this by KMulitplying this by Kww givesgives
 So, KSo, Khh = K= Kww/K/Kaa
 CHCH33COOCOO--
CHCH33COOH + OHCOOH + OH-1-1
 KKaa = [H= [H+1+1
][CH][CH33COOCOO--
]/[CH]/[CH33COOH]COOH]
 KKhh = [CH= [CH33COOH][OHCOOH][OH--
]/[CH]/[CH33COOCOO--
]]
 1/K1/Kaa = [CH= [CH33COOH]/[CHCOOH]/[CH33COOCOO--
][H][H++
]]
 KKww/K/Kaa = [CH= [CH33COOH][HCOOH][H++
][OH][OH--
]]
[CH[CH33COOCOO--
][H][H++
]]
KKhh = [CH= [CH33COOH][OHCOOH][OH--
]/[CH]/[CH33COOCOO--
]]

11. A Specific ExampleA Specific Example
 Predict a pH for 0.25 MPredict a pH for 0.25 M
sodium acetate solution.sodium acetate solution.
 The reaction for acetate ionThe reaction for acetate ion
is:is:
CHCH33COOCOO--
CHCH33COOH + OHCOOH + OH--
 KKaa for acetic acid is 1.8x10for acetic acid is 1.8x10-5-5
 KKhh = 1x10= 1x10-14-14
/1.8x10/1.8x10-5-5
= 5.56x10= 5.56x10-10-10
 Equilibrium “chart”Equilibrium “chart”
CHCH33COOCOO--
CHCH33COOH + OHCOOH + OH--
S 0.25S 0.25 00 00
∆∆ -x x x-x x x
Eq 0.25-x x xEq 0.25-x x x
 5.56x105.56x10-10-10
= x= x22
/0.25 */0.25 *
x = 1.18 x 10x = 1.18 x 10-5-5
So, pOH = 4.93 andSo, pOH = 4.93 and
pH = 9.07pH = 9.07
* The “x” term was dropped as x << 0.25

12. I’m Not Sayin’ This Will Help But…I’m Not Sayin’ This Will Help But…