Acids and Bases Entry task: Feb 4 th Monday Sign off on Ch. 16 sec. 8-11.

Acidsand

Bases

Entry task: Feb 4th Monday

Sign off on Ch. 16 sec. 8-11

Acidsand

Bases

Agenda

Discuss Ch. 16 sec. 8-11 reading notes

HW: Ch. 16 Review ws

Acidsand

Bases

I can…• State how salt solutions can affect the

properties of an acid or base.• Explain the factors that make a strong acid is

“strong”.

Acidsand

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What is the relationship between Ka and Kb?

When two reactions are added to give a third reaction, the equilibrium constant for the third reaction is equal to the products of the equilibrium constants for the two added reactions

Acidsand

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Ka and Kb

Ka and Kb are related in this way:

Ka Kb = Kw (1.0 x10-14)

Therefore, if you know one of them, you can calculate the other.

Acidsand

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How does this relate to pKa and pKb?

Just like Ka x Kb = Kw

Just like pKa + pKb = pKw = 14

Acidsand

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Acid-Base Properties of Salt Solutions

• Hydrolysis is the reaction of dissolved ions with water to generate H+ ions or OH- ions

Example: C2H3O2−

(aq) + H2O(l) HC2H3O2(aq) + OH−(aq)

Acidsand

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Acid-Base Properties of Salt Solutions

Why do all cations (except alkali metals heavier alkaline earth metals) act as weak acids?

• Cu+(aq) + H2O(l) CuOH(aq) + H+

(aq)

They are ABLE to hydrolyze with water creating H+ ions thus a weak acid solution.

Acidsand

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Acid-Base Properties of Salt SolutionsThe pH of a solution may be qualitatively predicted using the following guidelines:

1. Salts derived from a strong acid and strong base are neutral. Examples: NaCl [ NaOH + HCl NaCl + H2O ]

Ca(NO3)2 [ Ca(OH)2 + HNO3 Ca(NO3)2 + 2H2O ]

2. Salts derived from a strong base and weak acid are basic. Examples: NaClO Ba(C2H3O2)2 strong base ion/weak acid ion strong base ion/weak acid

ion

[ NaOH + HClO NaClO + H2O ] [ BaOH + HC2H3O2 Ba(C2H3O2)2 + H2O ]

Acidsand

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Acid-Base Properties of Salt Solutions3. Salts derived from a weak base and strong acid are acidic.

Example: NH4Cl Mg(NO3)2 weak base ion/strong acid ion weak base ion/strong acid ion

[ NH3 + HCl NH4Cl + H2O ] [ Mg(OH)2 + HNO3 Mg(NO3)2 + 2H2O ]

4. Salts derived from a weak base and weak acid, both anions and cations are hydrolyzed. So it’s the extent of which each ion is hydrolyzed- compare Ka and Kb for both

Example: NH4CNweak base ion/weak acid ion

NH4+ has Ka of 5.6 x 10-10 and CN- has Kb of 2.0 x 10-5

The solution has a higher Kb so its basic.

Acidsand

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16.73 Predict whether aqueous solutions of the following compounds are acidic, basic or neutral;

a. NH4Br _____________

b. FeCl3 _____________

c. Na2CO3_____________

d. KClO4_____________

e. NaHC2O4 _________

Weak base – strong acidAcid

Weak base – strong acidAcid

Strong base – weak acidBase

Strong base – strong acidNeutral

Strong base – weak acidBase

Acidsand

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What makes a strong acid strong?WHY oh WHY are these STRONG ACIDS???

Strong acids:

HCl

HBr

HI

HNO3

HClO4

H2SO4

Acidsand

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Factors that Affect Acid StrengthConsider the acid with the formula, H-X. For this substance to be an acid we need the following:

POLARITY-

1. Keep in mind that acids are made of nonmetals.

2. The difference in electronegativity between the 2 covalently bonded determines if it will hydrolyze and pull the H+ off the water creating a “stronger” acid

3. NON-Polar bonds can not be hydrolyzed so they are neutral.

Acidsand

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Factors that Affect Acid StrengthConsider the acid with the formula, H-X. For this substance to be an acid we need the following:

BONDING STRENGTH-

1. HF is a weak acid because the H-F bond is sooo tight it has a hard time hydrolyzing.

2. HCl is a strong acid because the H-Cl bond is not as tight and will hydrolyze in water AND create more H+

Acidsand

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Factors that Affect Acid Strength• Consider the acid with the formula, H-X. For this substance

to be an acid we need the following:

CONJUGATE STABILITY

2. The H-X bond must be weak enough to be broken

3. The conjugate base, X−, must be stable. (The more stable the anion, the more acidic it will be.)

Acidsand

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Strength of Binary Acids• Acid strength increases across a period and down a group.

Conversely, base strength decreases across a period and down a group.

• HF is a weak acid because the bond is highly polar so the bond energy is high. Therefore, H-F doesn’t dissociate as much as HCl.

• The electronegativity difference between C and H is so small that the C-H bond is non-polar so CH4 is neither an acid nor a base.

Acidsand

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Factors Affecting Acid StrengthIn oxyacids, in which an –OH is bonded to another atom, Y, the more electronegative Y is, the more acidic the acid.

© 2012 Pearson Education, Inc.

Acidsand

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Strength of Oxyacids• As the number of O atoms attached to Y increases, the O–H

bond polarity increases.• Consequently, the electrons in the O-H bond are pulled away

from the H, and this makes it easier to ionize!• The strength of the acid will therefore increase.

Acidsand

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Strength of Carboxylic Acids• Why can CH3COO–H ionize, but CH3CH2O–H cannot?

1. The additional oxygen atom on the carboxyl group increases the polarity of the O–H bond and stabilizes the conjugate base.

2. The conjugate base of CH3COO− exhibits resonance further increasing the stability of the conjugate base.

The carboxylic acid strength also increases as the number of electronegative groups in the acid increases. Example: Acetic acid is much weaker than trichloroacetic acid.

CH3COOH vs. CCl3COOHweaker stronger

−

−

Acidsand

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Lewis Acids and Bases• There is one last way to define acids and bases…

(3) Lewis Definition:

- acids: electron pair acceptor

- bases: electron pair donor(Notice that it is the exact opposite of Brønsted-Lowry’s

definition…acids are proton donors; bases are proton acceptors.)

• Note: Lewis acids and Lewis bases do not need to contain protons, therefore, the Lewis definition is the most general definition of acids and bases we have.

• Lewis acids generally have an incomplete octet…(Example: BF3 )

• Lewis acids must have a vacant orbital (into which the electron pairs can be donated).

Acidsand

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• Here’s an example of a Lewis acid and Lewis base…

H – N: + B – F H – N – B – F

• Compounds with multiple bonds can act as Lewis acids.

- For example, consider the reaction:H2O(l) + CO2(g) H2CO3(aq)

• Water acts as the electron pair donor and carbon dioxide as the electron pair acceptor in this reaction.

Lewis Acids and Bases

H

H

H

H F

F

F

Fe- pair donor

(base)e- pair acceptor

(acid)

+

Acidsand

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Hydrolysis of Metal Ions• Metal ions are (+) charged and attract water molecules (via

the lone pairs on oxygen).• The higher the charge, the smaller the metal ion and the

stronger the M-OH2 interaction. Both factors increase Ka, so…

Summary: (1) Smaller ions are more acidic.

(2) Higher charged ions are more acidic.

Practice Problems: (1) Ca2+ vs. Zn2+ Which is more acidic in water?

As you go across the periodic table, size generally decreases, so Zn+2 is smaller than Ca+2 which makes Zn+2

more acidic. (2) Ca+2 vs. Al+3 Which is more acidic in water?

Al+3 is more acidic. Not only is its charge larger, but it is also a smaller cation.

Acidsand

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Reactions of Cations with Water• Greater charge and

smaller size make a cation more acidic.

© 2012 Pearson Education, Inc.

Acidsand

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Hydrolysis of Metal Ions

Weaker acidSmaller Ka

Higher pH

Stronger acidLarger Ka

Lower pH

Acidsand

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16.83 Based on all the notes in section 16.10, which is the stronger base among

the pair?

a. BrO- or ClO-: Why___________________________________

b BrO- or BrO2-:

Why___________________________________

c. HPO4-2 or H2PO4

-1 -: Why__________________________________

Think of it as who is the weakest acid.

ClO- because acid strength increase as we move down a group

BrO- - because it has less oxygens around the central atom

HPO4-2 - because it can accept more H+

Acidsand

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16.89. Identify the Lewis acid and Lewis base among the reactions in each of the following reactions.

A. Fe(ClO4)3 (s) + 6H2O (l) Fe(H2O)6+3 (aq) + 3ClO4

-(aq)

B. CN- (aq) + H2O (l) HCN (aq) + OH-

(aq)

C. (CH3)3N (g) + BF3 (g) (CH3)3NBF3 (s)

E-pair donor is base and e-pair acceptor is acid.

L-acid L-base

L-base L-acid

L-base L-acid

Acidsand

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