11

Acids, Bases and PH

22

Some Properties of Acids

Produce H+ (as H3O+) ions in water (the hydronium ion is a

hydrogen ion attached to a water molecule)

Taste sour

Corrode metals

Electrolytes

React with bases to form a salt and water

pH is less than 7

Turns blue litmus paper to red “Blue to Red A-CID”

33

Some Properties of Bases

Produce OHProduce OH-- ions in water ions in water

Taste bitter, chalkyTaste bitter, chalky

Are electrolytesAre electrolytes

Feel soapy, slipperyFeel soapy, slippery

React with acids to form salts and waterReact with acids to form salts and water

pH greater than 7pH greater than 7

Turns red litmus paper to blue “Turns red litmus paper to blue “BBasic asic BBlue”lue”

44

Acid/Base definitions• Definition #1: Arrhenius

Acids:– produce H+ ions (or hydronium ions H3O+)

Bases:– produce OH- ions

(problem: some bases don’t have hydroxide ions!)

55Arrhenius acid is a substance that produces H+ (H3O+) in water

Arrhenius base is a substance that produces OH- in water

66

• Definition #2: Brønsted – Lowry

Acids:– proton donor

Bases:– proton acceptor

A “proton” is really just a hydrogen atom that has lost it’s electron!

77

A Brønsted-Lowry acid is a proton donorA Brønsted-Lowry base is a proton acceptor

acidconjugate

basebase conjugate

acid

88

The Brønsted definition means NHThe Brønsted definition means NH33 is a is a BASEBASE in in water — and water is itself anwater — and water is itself an ACIDACID

BaseAcidAcidBaseNH4

+ + OH-NH3 + H2OBaseAcidAcidBase

NH4+ + OH-NH3 + H2O

99

Conjugate PairsConjugate Pairs

1010

Learning Check!

Label the acid, base, conjugate acid, and Label the acid, base, conjugate acid, and conjugate base in each reaction:conjugate base in each reaction:

HCl + OHHCl + OH--   Cl   Cl-- + H + H22OO HCl + OHHCl + OH--   Cl   Cl-- + H + H22OO

HH22O + HO + H22SOSO44   HSO   HSO44-- + H + H33OO

++ HH22O + HO + H22SOSO44   HSO   HSO44-- + H + H33OO

++

1111

Lewis acid:- a substance that Lewis acid:- a substance that accepts an electron pair.accepts an electron pair.

Lewis base:- a substance that Lewis base:- a substance that donates an electron pair.donates an electron pair.

Definition #3 – Lewis

1212

Formation ofFormation of hydronium ion hydronium ion is also an excellent is also an excellent example.example.

Lewis Acids & BasesLewis Acids & Bases

•Electron pair of the new O-H bond Electron pair of the new O-H bond originates on the Lewis base.originates on the Lewis base.

HH

H

BASE

••••••

O—HO—H

H+

ACID

1313

HNO3, HCl, H2SO4 and HClO4 are strong acids.

Strong and Weak Acids/BasesStrong and Weak Acids/Bases

The strength of an acid (or base) is determined by the amount of IONIZATION.

1414

• Weak acidsWeak acids are much less than 100% ionized in water. are much less than 100% ionized in water.

One of the best known is acetic acid = CHOne of the best known is acetic acid = CH33COOHCOOH

Strong and Weak Acids/BasesStrong and Weak Acids/Bases

1515

• Strong Base:Strong Base: 100% dissociated in water.100% dissociated in water.

NaOH (aq) ---> NaNaOH (aq) ---> Na+ + (aq) + OH(aq) + OH- - (aq)(aq)

Strong and Weak Acids/BasesStrong and Weak Acids/Bases

• Weak base:Weak base: less than 100% ionized in waterless than 100% ionized in water

One of the best known weak bases is One of the best known weak bases is ammoniaammonia

NHNH3 3 (aq) + H(aq) + H22O (l) O (l) NH NH44+ + (aq) + OH(aq) + OH- - (aq)(aq)

1616

Equilibria Involving Equilibria Involving Weak Acids and BasesWeak Acids and Bases

Consider acetic acid, HCConsider acetic acid, HC22HH33OO22 (HOAc) (HOAc)

HCHC22HH33OO22 + H + H22O O H H33OO++ + C + C22HH33OO22 --

AcidAcid Conj. base Conj. base

Ka [H3O+][OAc- ]

[HOAc] 1.8 x 10-5Ka

[H3O+][OAc- ][HOAc]

1.8 x 10-5

(K is designated K(K is designated Kaa for ACID) for ACID)

K gives the ratio of ions (split up), to molecules K gives the ratio of ions (split up), to molecules

(don’t split up).(don’t split up).

1717Ionization Constants for Acids/Bases Ionization Constants for Acids/Bases

AcidsAcids ConjugateConjugateBasesBases

Increase strength

Increase strength

1818

The The pH scalepH scale is a way of is a way of expressing the strength of expressing the strength of acids and bases. acids and bases.

Under 7 = acidUnder 7 = acid 7 = neutral 7 = neutral

Over 7 = base Over 7 = base

The pH scale The pH scale

1919

pH of Common SubstancespH of Common Substances

2020Calculating the pH

pH = - log [H+](Remember that the [ ] mean Molarity)

Example:

If [H+] = 1 X 10-10

pH = - log 1 X 10-10

pH = - (- 10)

pH = 10

Example:

If [H+] = 1.8 X 10-5

pH = - log 1.8 X 10-5

pH = - (- 4.74)

pH = 4.74

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More Examples:More Examples:

Find the pH of:Find the pH of:

1) A 0.15 M solution of Hydrochloric acid .1) A 0.15 M solution of Hydrochloric acid .

2) A 3.00 X 102) A 3.00 X 10-7-7 M solution of Nitric acid . M solution of Nitric acid .

2222

More About WaterMore About WaterHH22O can function as both an ACID and a BASE.O can function as both an ACID and a BASE.

In pure water there can beIn pure water there can be AUTOIONIZATIONAUTOIONIZATION

Equilibrium constant for water = KEquilibrium constant for water = Kww

KKww = [H = [H33OO++] [OH] [OH--] =] = 1.00 x 101.00 x 10-14-14 at 25 at 25 ooCC

OH-

H3O+

OH-

H3O+

In a In a neutral neutral solution [Hsolution [H33OO++] = [OH] = [OH--] = 1.00 x 10] = 1.00 x 10-7-7 M M

2323

2424pOH

• pOH does not really exist, but it is useful for pOH does not really exist, but it is useful for changing bases to pH.changing bases to pH.

pOH = - log [OHpOH = - log [OH--]]Since pH and pOH are on opposite ends,Since pH and pOH are on opposite ends,

pH + pOH = 14pH + pOH = 14

2525

pHpH [H+][H+] [OH-][OH-] pOHpOH

2626Examples:Examples:

1- What is the pH of :1- What is the pH of :

0.0010 M NaOH solution?0.0010 M NaOH solution?

soln:soln:

[OH-] = 0.0010 (or 1.0 X 10[OH-] = 0.0010 (or 1.0 X 10-3-3 M) M)

pOH = - log 0.0010pOH = - log 0.0010

pOH = 3pOH = 3

pH = 14 – 3 = 11pH = 14 – 3 = 11

OR:OR:

KKww = [H = [H33OO++] [OH] [OH--]]

[H[H3OO++] = 1.0 x 10] = 1.0 x 10-11-11 M M

pH = - log (1.0 x 10pH = - log (1.0 x 10-11-11) = 11.00) = 11.00

2727

[OH[OH--]]

[H[H++]] pOHpOH

pHpH

1010 -pOH

-pOH

1010 -pH-pH-Log[H

-Log[H++]]

-Log[OH

Log[OH

--]]

14 -

pOH

14 -

pOH

14 -

pH

14 -

pH

1.0

x 10

1.0

x 10-1

4-14

[OH[O

H-- ]]

1.0

x 10

1.0

x 10-1

4-14

[H[H

++ ]]

2828

The OH- ion concentration of a blood sample is 2.5 x 10-7 M. What is the pH of the blood?

More Examples:

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