Strong and Weak Acids and Bases

Green & Damjii – Chapter 8 – Section 3

Chang - Chapter 15

Copyright © The McGraw-Hill Companies, Inc.  Permission required for reproduction or display.

An Arrhenius acid is a substance that produces H+ (H3O+) in water

4.3

There are multiple definitions for the terms ‘acid’ and ‘base’.

You will need to be familiar with three (3) of them:• Arrhenius acids and bases• Bronsted-Lowry acids and bases• Lewis acids and bases

Today’s notes will focus on the Arrhenius definition!

Strong Electrolyte – 100% dissociation[like a soluble salt…or strong acids… or strong bases…]

NaCl (s) Na+ (aq) + Cl- (aq)H2O

Weak Electrolyte – not completely dissociated[like an insoluble salt… or weak acids… or weak bases]

AgCl (s) Ag+ (aq) + Cl- (aq)

15.4

Electrolyte – substance whose aqueous solution can conduct electricity –

because of the presence of mobile ions

H2O

Strong Acids are strong electrolytes

HCl (aq) + H2O (l) H3O+ (aq) + Cl- (aq)

HNO3 (aq) + H2O (l) H3O+ (aq) + NO3- (aq)

HClO4 (aq) + H2O (l) H3O+ (aq) + ClO4- (aq)

H2SO4 (aq) + H2O (l) H3O+ (aq) + HSO4- (aq)

15.4

HCl (aq) H+ (aq) + Cl- (aq)H2O

H3O+ is the hydronium ion

General Equation for dissociation of a strong acid:

HA (aq) + H2O (l) H3O+ (aq) + A- (aq)

or HA (aq) H+ (aq) + A- (aq)

H2O

15.4

HF (aq) + H2O (l) H3O+ (aq) + F- (aq)

Weak Acids are weak electrolytes

HNO2 (aq) + H2O (l) H3O+ (aq) + NO2- (aq)

HSO4- (aq) + H2O (l) H3O+ (aq) + SO4

2- (aq)

H2O (l) + H2O (l) H3O+ (aq) + OH- (aq)

General equation for the dissociation of a weak acid:

HA (aq) + H2O (l) H3O+ (aq) + A- (aq) or

H2OHA (aq) H+ (aq) + A- (aq)

Strong Acid Weak Acid

15.4

percent ionization = Ionized acid concentration at equilibrium

Initial concentration of acidx 100%

For a monoprotic acid HA

Percent ionization = [H+]

[HA]0

x 100% [HA]0 = initial concentration

15.5

Molecular Structure and Acid Strength

H X H+ + X-

The stronger the bond

The weaker the acid

HF << HCl < HBr < HI

15.9

Molecular Structure and Acid Strength

15.9

NOTE: HF is a weak acid…

• H-F bond is very polar… which might make you think it should be easy to break… but

• H-F bond is VERY strong

• HF molecule can strongly hydrogen bond to water… (unlike the other hydrogen halides) stabilizing the undissociated molecule.

Ka – A Measure of Acid Strength

Ka is the equilibrium constant for the dissociation of an acid – aka the acid dissociation constant

• a large Ka (> 1) means the products (dissociated ions) are favored – indicating a strong acid

• a small Ka (< 1) means the reactants are favored – indicating a weak acid

Ka – A Measure of Acid StrengthNAME of ACID FORMULA Ka

boric acid H3BO3 5.9 x 10−10

hydrogen cyanide HCN 5.9 x 10−10

carbonic H2CO3 4.3 x 10−7

hydrogen sulfide H2S 1.3 x 10−7

acetic acid CH3COOH 1.8 x 10−5

formic acid HCOOH 1.8 x 10−4

hydrofluoric acid HF 6.9 x 10−4

citric acid H3C6H5O7 7.4 x 10−4

phosphoric acid H3PO4 7.1 x 10−3

trichloroacetic acid CCl3COOH 3.0 x 10−1

http://www.csudh.edu/oliver/chemdata/data-ka.htm

Molecular Structure and Acid Strength

Z O H Z O- + H+d- d+

In general, the acid will be stronger if the O-H bond is more polar and easier to break. This happens if:

(1) Z (the non-metal) is more (very) electronegative

The electronegativity of some common elements =

N(3.0) Cl(3.0) S(2.5) C(2.5) P (2.1)

So…

…H2SO4 is a stronger acid than H3PO4

…HNO3 is a stronger acid than H2CO3

15.9

Molecular Structure and Acid StrengthHere is another example, using the same rule…

Oxoacids having different central atoms (Z) that are from the same group and that have the same oxidation number.

Acid strength increases with increasing electronegativity of Z

H O Cl O

O••

••••••

••

••••

••••

H O Br O

O••

••••••

••

••••

••••

Cl is more electronegative than Br, therefore HClO3 is

stronger than HBrO3

HClO3 > HBrO3

15.9

Molecular Structure and Acid Strength

Z O H Z O- + H+d- d+

In general, the O-H bond will be easier to break if:

(2) Z (the non-metal) is in a high oxidation state – as indicated by an increase in the number of attached groups.

15.9

HClO4 > HClO3 > HClO2 > HClO

perchloric acid is stronger than

chloric acid which is stronger than

chlorous acid which is stronger than

hypochlorous acid.

Molecular Structure and Acid Strength

Which one of the acids in each of the following groups would be strongest?

(a) H2SO3 H2SO4

sulfurous acid sulfuric acid  (b) HNO2 HNO3

nitrous acid nitric acid

 (c) H3PO3 H3PO4

phosphorous acid phosphoric acid

15.9

What pattern do you notice between the formula and the name of these oxyacids?

Using this pattern, what would be the formula for sulfurous acid?

Strong Acids

hydrochloric acid HCl

sulfuric acid H2SO4

nitric acid HNO3

hydrobromic acid HBr

hydroiodic acid HI

chloric acid HClO3

perchloric acid HClO4

Weak Acids

ethanoic (or acetic) acid CH3COOH

water H2Ohydrofluoric acid HFphosphoric acid H3PO4

formic acid HCOOHhydrocyanic acidhydrogen cyanide

HCN

hydrogen sulfide H2Strichloroacetic acid CCl3COOH

How does acid strength relate to properties of acids?

• pH ?• strong acids have lower pH than weak acids

• electrical conductivity?• strong acids have greater electrical conductivity

than weak acids

• reaction with active metals? carbonates? bases?• strong acids react more vigorously

than weak acids

An Arrhenius base is a substance that produces OH- in water

4.3

Strong Bases are strong electrolytes

NaOH (s) Na+ (aq) + OH- (aq)H2O

KOH (s) K+ (aq) + OH- (aq)H2O

Ba(OH)2 (s) Ba2+ (aq) + 2 OH- (aq)H2O

15.4

15.4

F –(aq) + H2O (l) OH– (aq) + HF (aq)

Weak Bases are weak electrolytes

NO2- (aq) + H2O (l) OH– (aq) + HNO2 (aq)

NH3 (aq) + H2O (l) OH– (aq) + NH4+ (aq)

CH3 CH2NH2 (aq) + H2O (l) OH– (aq) + CH3 CH2NH3+ (aq)

Strong Baseslithium hydroxide LiOHsodium hydroxide NaOH

potassium hydroxide KOHrubidium hydroxide RbOHcesium hydroxide CsOHcalcium hydroxide Ca(OH)2

strontium hydroxide Sr(OH)2

barium hydroxide Ba(OH)2

Weak Basesammonia NH3

water H2Oethyl amine CH3 CH2NH2

ammonium hydroxide NH4OHtrimethylammonia (CH3)3N

pyridine C5H5N

How does base strength relate to properties of bases?

• pH ?• strong bases have ________ pH than weak bases

• electrical conductivity?• strong bases have ________ electrical conductivity

than weak bases

• reaction with acids?• strong bases react ________ with acids

than weak bases

NOTE: Do NOT confuse the strength of an acid or base

with its concentration !

Circle the ‘strongest’ species in each pair below:

(1) 6.0 M HCl vs 3.0 M HCl

(2) 2.0 M HCl vs 2.0 M CH3COOH

(3) 0.3 M HCl vs 3.0 M CH3COOH

(4) 0.3 M CH3COOH vs 3.0 M CH3COOH

Underline the ‘least concentrated’ solution in each pair above.

Read Section 8.3 - PP 213-214Strong and Weak Acids & Bases Do Ex 8.3 on p. 214 # 1-5 (all)

HOMEWORK: Due_______________