Just When It’s Safe Just When It’s Safe to Go Back Into the to Go Back Into the

WaterWaterThe Acidic or Basic Quality of The Acidic or Basic Quality of

SaltsSalts

Acid Equilibrium ReviewAcid Equilibrium Review

Weak Weak acidsacids form equilibria in aqueous form equilibria in aqueous solutions according to the following solutions according to the following general patterngeneral pattern HX (aq) HHX (aq) H+1+1 (aq) + X (aq) + X-1-1 (aq) (aq)

The equilibrium constant follows the The equilibrium constant follows the formform KKaa = [H = [H+1+1][X][X-1-1]/[HX] ]/[HX]

The reaction can be shifted according The reaction can be shifted according to LeChatelier’s Principleto LeChatelier’s Principle

Base Equilibrium ReviewBase Equilibrium Review

Weak bases form equilibria in water Weak bases form equilibria in water similar to the example of ammoniasimilar to the example of ammonia NHNH33 + HOH NH + HOH NH44

+1+1 + OH + OH-1-1

The equilibrium constant would be The equilibrium constant would be KKbb = [NH = [NH44

+1+1][OH][OH-1-1]/[NH]/[NH33]]

Again, LeChatelier’s Principle can be Again, LeChatelier’s Principle can be used to explain or predict shiftsused to explain or predict shifts

And, Water Has Its And, Water Has Its Equilibrium TooEquilibrium Too

Water forms an equilibrium that can be Water forms an equilibrium that can be affected by any substance dissolved into affected by any substance dissolved into solutionsolution

The equilibrium involves the water The equilibrium involves the water molecule, hydrogen ion, and hydroxide ionmolecule, hydrogen 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

Conjugate Acids and Conjugate Acids and BasesBases

An acid-base reaction produces new An acid-base reaction produces new materials which may be classified as materials which may be classified as conjugate acids (or bases)conjugate acids (or bases)

Weak acid (or base) equilibria often Weak acid (or base) equilibria often produce anions (or cations) with produce anions (or cations) with significant base (or acid) propertiessignificant base (or acid) properties

The weaker the original acid (or The weaker the original acid (or base) the stronger its conjugate base base) the stronger its conjugate base (or acid)(or acid)

Acetic Acid, for exampleAcetic Acid, for example

When acetic acid dissolves in water it When acetic acid dissolves in water it forms hydrogen ion and acetate ionforms hydrogen ion and acetate ion CHCH33COOH (aq) CHCOOH (aq) CH33COOCOO-1-1 + H + H+1+1

The acetate ion has some basic The acetate ion has some basic properties (It is the conjugate base for properties (It is the conjugate base for acetic acid in the reaction.)acetic acid in the reaction.)

Therefore, it can be expected that Therefore, it can be expected that solutions (which might carelessly be solutions (which might carelessly be considered neutral) containing acetate considered neutral) containing acetate ion might be basicion might be basic

An Acetate Solution An Acetate Solution ExampleExample

Suppose a solution of sodium acetate, Suppose a solution of sodium acetate, CHCH33COOCOO--NaNa++, is made, is made

The acetate and sodium ions separate The acetate and sodium ions separate CHCH33COOCOO--NaNa++ CH CH33COOCOO-- + Na + Na++

Each ion may have acidic or basic Each ion may have acidic or basic propertiesproperties

To analyze this we see what would be To analyze this we see what would be produced if each ion reacted with produced if each ion reacted with water water

These Are Called Hydrolysis These 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 . The NaOH is strong and dissociates 100%. Thus, no is strong and dissociates 100%. Thus, no difference between [Hdifference between [H+1+1] and [OH] and [OH-1-1] is ] is createdcreated

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 . Acetic acid is weak and doesn’t dissociate 100%. acid is weak and doesn’t dissociate 100%. So, more [OHSo, more [OH-1-1]>[H]>[H+1+1]. It’s a basic solution.]. It’s a basic solution.

Generalizing for Generalizing for HydrolysisHydrolysis

Check the reaction impact of the ions Check the reaction impact of the ions from a salt (MX) with waterfrom a salt (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 If either the acid or base produced is weak the salt has acidic/basic weak the salt has acidic/basic propertiesproperties

These reactions are equilibrium These reactions are equilibrium reactions and will have K valuesreactions and will have K values

K Values for Hydrolysis K Values for Hydrolysis ReactionsReactions

Using a specific example of Using a specific example of sodium acetate the sodium acetate the hydrolysis reaction is hydrolysis reaction is

The KThe Kaa value for acetic acid value for acetic acid is is

The K formula for the The K formula for the hydroysis reaction would behydroysis reaction would be

Inverting the KInverting the Kaa gives gives

Mulitplying this by KMulitplying this by Kww gives gives

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--]]

A Specific Example A Specific Example Predict a pH for 0.25 M Predict a pH for 0.25 M

sodium acetate sodium acetate solution.solution.

The reaction for acetate The reaction for acetate ion is:ion is:

CHCH33COOCOO-- CH CH33COOH + OHCOOH + OH--

KKaa for acetic acid is for acetic acid is 1.8x101.8x10-5-5

KKhh = 1x10 = 1x10-14-14/1.8x10/1.8x10-5-5

= 5.56x10= 5.56x10-10-10

Equilibrium “chart”Equilibrium “chart”CHCH33COOCOO-- CH CH33COOH + COOH +

OHOH--

S 0.25S 0.25 0 0 0 0

-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

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