Intersection 11

11/12/05

Reading: 16.8 (p794-800) 17.2 (p 828-836)

Equilibrium Representations

Water Projects…Now What?

Outline

• Strong vs. Weak Acids and Bases• Which H’s are acidic? Which groups are basic?• Periodic Trends and Acid Strength• What affects the pH of a solution?• Polyprotic acids• Concept Questions• Acid/Base Titration: A Closer Look

Strong vs. Weak

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Vocabulary

Strong (16 definitions):

Weak (10 definitions):

Favorable reaction..

ionizing freely in solution

ionizing only slightly in solution

Strong Reaction

exothermic

spontaneous

product favored

goes to completion

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Strong Acids An acid that dissociates completely (the equilibrium is shifted all of the way to

its conjugate base and hydronium ion) is said to be a strong acid.

HCl(aq) +  H2O(l) →   H3O+

(aq) + Cl-(aq)

acid                                           conj. base

An acid that does not dissociate completely (an equilibrium is established in solution between the acid, its conjugate base, and hydronium ion) is said to be a weak acid.

HClO2(aq) + H2O(l)  ↔  H3O+

(aq) + ClO2- (aq)     

acid                                          conj base

Ka = ([H3O+][ClO2

-]) / [HClO2]

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Strong BasesA base that dissociates completely (the equilibrium is shifted all of

the way to its conjugate acid and hydroxide) is said to be a strong base.

NaOH(aq) +  H2O(l) →   OH-(aq) + Na+

(aq) + H2O(l)

base                                             conj. acid

A base that does not dissociate completely (an equilibrium is established in solution between the base, its conjugate acid, and hydroxide) is said to be a weak base.

(CH3)3N(aq)  +  H2O(l)   ↔ (CH3)3NH+(aq)  + OH-

(aq)    base                            conj. acid

Kb = ([(CH3)3NH+][OH-]) / [(CH3)3N]

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Strong Acid

Weak Acid

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There are six strongly dissociating acids:HCl HNO3

HBr HClO4

HI H2SO4 

There are also five bases that dissociate completely in solution (strong):LiOH Ca(OH)2

NaOH Ba(OH)2

KOH You should commit the strong acids and bases to memory.Appendix F in your text book lists Ka and Kb values for many weakly

dissociating acids and bases.

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Problem 1

Trimethylamine (CH3)3N has a Kb of 6.5 x10-5.  Write out its chemical reaction with water:  What is the [OH-] of a 0.010 M solution of triethylamine?  What is the pOH?  What is the pH?

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Which H’s are acidic?What groups are basic?

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What kinds of hydrogen atoms (protons) are acidic?

H-halogen (HF, HCl, HBr, HI)

H2O

H2S (Ka1 = 8.9x10-8)

Oxoacids (H-polyatomic ions) (H2CO3, HNO3, etc.) HCN

O

C

HO

C

CC

O

OH

OH

H

H

OH

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Organic Acids

CR

O

OH

+ H2O

CR

O

O-

+ H3O+

RCOOH

CR

O-

O

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Bases

OH

R3N

H2N CH C

CH3

OH

O

H2N CH C

CH2

OH

O

CH2

C

OH

O

HN

C OH

O

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Periodic Trends and Acid Strength

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Groups 7 and Period 2

Acid strength: HI > HBr > HCl> HF

HF>H2O >> H3N >>> CH4

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Oxoacids I

HOY + H2O H3O+ + OY-

Since a negatively charged ion must be formed, it will be most stable when Y is an atom that is most effective at attracting electron density to itself, thereby stabilizing the negative charge. So the more electronegative atom as Y, yields a stronger acid

Acid strength: HOI < HOBr < HOCl

Y = I, Br, Cl

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Oxoacids II

What trend do you see and why?

Name Hypochlorous Chlorous Chloric Perchloric

Formula HOCl HOClO HOClO2 HOClO3

Experimental Ka

3.2 x10-8 1.3 x10-2 1x102 2 x107

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What affects the pH?

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Question 1

• How could you change the pH of a solution of acetic acid (CH3COOH)?

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Question 2

Apatite, Ca5(PO4)3OH is the mineral in teeth.Ca5(PO4)3OH(s)  5 Ca+2(aq)  + 3PO4

-4(aq)  +  OH-(aq)

Sour milk contains lactic acid.  Not removing sour milk from the teeth of young children can lead to tooth decay.  Use chemical principles to explain why.

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Can salts affect the pH of a solution?

Ca(OH)2 Na(CH3COO)

NH4Cl

NaNO3

Ksp = 7.9x10-6

Kb NH3 = 1.8 x10-5

Ka CH3COOH = 1.8 x10-5

NH4+ + H2O NH3 + H3O+

NaH2PO4

Na2HPO4

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Name ions found in salts that would not affect the pH:

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Question 3

A carbonated beverage is left open to the atmosphere. Will the pH change?

CO2(aq) + H2O(l) ↔ H2CO3(aq)

H2CO3 + H2O(l) ↔ H3O + + HCO3¯ Ka1 = 4.2 × 10-7

HCO3¯ + H2O(l) ↔ H+ + CO32¯ Ka2 = 4.8 × 10 -11

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