Bases

Sodium hydroxide

Preferred IUPAC nameSodium hydroxide

Systematic nameSodium oxidanide

Other namesCaustic soda

Lye

Graphics Source: Wikipedia

Acid/Base Definitions Arrhenius Model

Acids produce hydrogen ions in aqueous solutions

Bases produce hydroxide ions in aqueous solutions

Bronsted-Lowry Model Acids are proton donors Bases are proton acceptors

Lewis Acid Model Acids are electron pair acceptors Bases are electron pair donors

Dissociation of Strong Bases

Strong bases are metallic hydroxidesGroup I hydroxides (NaOH, KOH) are

very solubleGroup II hydroxides (Ca, Ba, Mg, Sr)

are less solublepH of strong bases is calculated

directly from the concentration of the base in solution

MOH(s) M+(aq) + OH-(aq)

Reaction of Weak Bases with Water

The base reacts with water, producing its conjugate acid and hydroxide ion:

CH3NH2 + H2O CH3NH3+ + OH- Kb = 4.38 x 10-4

4 3 3

3 2

[ ][ ]4.38 10

[ ]b

CH NH OHK x

CH NH

Kb for Some Common Weak Bases

Base FormulaConjugat

e AcidKb

Ammonia   NH3  NH4+  1.8 x 10-5 

 Methylamine  CH3NH2  CH3NH3+  4.38 x 10-4 

 Ethylamine  C2H5NH2  C2H5NH3+  5.6 x 10-4 

 Diethylamine  (C2H5)2NH  (C2H5)2NH2+  1.3 x 10-3 

 Triethylamine   (C2H5)3N   (C2H5)3NH+  4.0 x 10-4 

 Hydroxylamine  HONH2   HONH3+

   1.1 x 10-8 

 Hydrazine H2NNH2  H2NNH3+

   3.0 x 10-6 

 Aniline  C6H5NH2   C6H5NH3+

   3.8 x 10-10 

 Pyridine  C5H5N   C5H5NH+    1.7 x 10-9 

Reaction of Weak Bases with Water

The generic reaction for a base reacting with water, producing its conjugate acid and hydroxide ion:

B + H2O BH+ + OH-

[ ][ ]

[ ]b

BH OHK

B

(Yes, all weak bases do this – DO NOTendeavor to make this complicated!)

A Weak Base Equilibrium Problem

What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ?

Step #1: Write the equation for the reaction

NH3 + H2O NH4+ + OH-

A Weak Base Equilibrium Problem

What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ?

Step #2: ICE it!

I

C

E

0.50 0 0

- x +x +x

0.50 - x xx

NH3 + H2O NH4+ + OH-

A Weak Base Equilibrium Problem

Step #3: Set up the law of mass action

0.50 - x xxE

)50.0()50.0(

))((108.1

25 x

x

xxx

What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ?

NH3 + H2O NH4+ + OH-

A Weak Base Equilibrium Problem

Step #4: Solve for x, which is also [OH-]

0.50 - x xxE

)50.0(108.1

25 x

x [OH-] = 3.0 x 10-3 M

NH3 + H2O NH4+ + OH-

What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ?

A Weak Base Equilibrium Problem

52.4)100.3log( 5 xpOH

Step #5: Convert [OH-] to pH

0.50 - x xxE

What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ?

NH3 + H2O NH4+ + OH-

48.900.14 pOHpH

• Soft cover book Pg. 291-294

• Text Pg. 713 do question 16.75• Then you can practice for hw 16.76

• For conjugate acid base pairs only• Ka x Kb = Kw

• pKa = -logKa

• pKb = -logKb

• pKa + pKb = 14

Now do question 16.78 on Pg. 713

Hydrolysis of Salts

• Salt of strong acid and strong base – neutral

• Salt of weak acid and strong base – basic• Salt of strong base and weak acid – acidic• In general Ka>Kb acidic• Kb>Ka basic• Soft cover book Pg. 387

• Text Pg. 697• Text pg. 698 sample 16.18 and 16.19

Reactions of Anions with Water

• Anions are bases.• As such, they can react with water in a

hydrolysis reaction to form OH− and the conjugate acid:

X−(aq) + H2O(l) HX(aq) + OH−(aq)

Reactions of Cations with Water

• Cations with acidic protons (like NH4

+) will lower the pH of a solution.

• Most metal cations that are hydrated in solution also lower the pH of the solution.

Reactions of Cations with Water

• Attraction between nonbonding electrons on oxygen and the metal causes a shift of the electron density in water.

• This makes the O-H bond more polar and the water more acidic.

• Greater charge and smaller size make a cation more acidic.

Effect of Cations and Anions

1. An anion that is the conjugate base of a strong acid will not affect the pH.

2. An anion that is the conjugate base of a weak acid will increase the pH.

3. A cation that is the conjugate acid of a weak base will decrease the pH.

Effect of Cations and Anions

4. Cations of the strong Arrhenius bases will not affect the pH.

5. Other metal ions will cause a decrease in pH.

6. When a solution contains both the conjugate base of a weak acid and the conjugate acid of a weak base, the affect on pH depends on the Ka and Kb values.

Factors Affecting Acid Strength

• The more polar the H-X bond and/or the weaker the H-X bond, the more acidic the compound.

• Acidity increases from left to right across a row and from top to bottom down a group.

Factors Affecting Acid Strength

In oxyacids, in which an OH is bonded to another atom, Y, the more electronegative Y is, the more acidic the acid.

Factors Affecting Acid Strength

For a series of oxyacids, acidity increases with the number of oxygens.

Factors Affecting Acid Strength

Resonance in the conjugate bases of carboxylic acids stabilizes the base and makes the conjugate acid more acidic.

Lewis Acids

• Lewis acids are defined as electron-pair acceptors.

• Atoms with an empty valence orbital can be Lewis acids.

Lewis Bases

• Lewis bases are defined as electron-pair donors.• Anything that could be a Brønsted–Lowry base is

a Lewis base.• Lewis bases can interact with things other than

protons, however.

• Soft covered book Pg. 288• Example and practice