The Chemistry of Acids and Bases

Acids

Acids

Bases

Some Properties of Acids• Produce H+

• 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”• React with certain metals to produce hydrogen gas.• React with carbonates and bicarbonates to produce

carbon dioxide gas

Some Common AcidsMonoprotic acids: one acidic (ionizable)hydrogenHNO3 - nitric acid HCl - hydrochloric acid

Diprotic acids: two acidic (ionizable)hydrogensH2SO4 - sulfuric acid

Triprotic acids: three acidic (ionizable)hydrogensH3(PO4)phosphoric acidH3(C6H5O7) citric acid

Some Properties of Bases

• Produce OH- ions in water• Taste bitter, chalky• Are electrolytes• Feel soapy, slippery• React with acids to form salts and water• pH greater than 7• Turns red litmus paper to blue “Basic Blue”

Some Common BasesNaOH sodium hydroxide lye

KOH potassium hydroxide liquid soap

Ba(OH)2 barium hydroxide stabilizer for plastics

Mg(OH)2 magnesium hydroxide Milk of magnesia

Al(OH)3 aluminum hydroxide Maalox (antacid)

Acid/Base definitionsDefinition #1: Arrhenius (traditional)

Acids – produce H+ ions (H3O+)HX H+

(aq) + X-(aq)

Bases – produce OH- ionsXOH -----> X+

(aq) + OH-(aq)

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

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

Arrhenius base is a substance that produces OH- in water

But: some bases don’t have hydroxide ions! Now what?????

Acid/Base DefinitionsDefinition #2: Brønsted – Lowry

Acids – proton donorBases – proton acceptor

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

acid

conjugate base

base

conjugate acid

H+ + Base = Conjugate acid of Base+

Acid = H+ + Conjugate base of Acid-

Conjugate Pairs

Water can be an acid or a baseAmphoteric

Learning Check!

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

HCl + OH-   Cl- + H2O

H2O + H2SO4   HSO4- +

H3O+

Acids & Base DefinitionsDefinition #3 – Lewis Acids and Bases

Lewis base – a substance that donates an electron pair

Lewis acid – a substance that accepts an electron pair

Lewis Acids & BasesFormation of hydronium ion is also an excellent

example.

H H

H

BASE

••••••

O—HO—HH+

ACID

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

Lewis Acid Base Reaction

Water

Water molecules collide with one another: self-ionization reaction 2H2O H3O+ + OH- The reaction does not form very much H3O+ or OH-.

1 Liter water = ~55 moles of water molecules, but only forms 1.0 x 10-7 moles of H3O+ and OH-

[H3O+] and [OH-] in pure water = 1.0 x 10-7 M. It is the 7 in the exponent or power of this number

that gives neutral water a pH of 7.

Ionization constant of water

Why isn’t water in the equilibrium constant Kw ?

Neutral solution: [H3O+] = [OH-] = [10-7]pH = - log [H+] pOH = - log [OH-]

Neutral solution: pH = pOH = 7Acidic Solutions: [H+] > [OH-]Basic Solutions (Alkaline): [OH-] > [H+] pH + pOH must always add up to 14

pH Scale: indicates strength of acid or base

Example: If [H+] = 1 X 10-10

pH = - log 1 X 10-10

pH = - (- 10)pH = 10 basicpOH = 4

Example: If [H+] = 1.8 X 10-5

pH = - log 1.8 X 10-5

pH = - (- 4.74)pH = 4.74 acidicpOH = 9.26

Try These!Find the pH of these:1) A 0.15 M solution of Hydrochloric acid2) A 3.00 X 10-7 M solution of Nitric acid3) A 0.15 M solution of sodium

hydroxide

pH testing• There are several ways to test pH

– Blue litmus paper (red = acid)– Red litmus paper (blue = basic)– pH paper (multi-colored)– pH meter (7 is neutral, <7 acid, >7 base)– Universal indicator (multi-colored)– Indicators like phenolphthalein– Natural indicators like red cabbage,

radishes

pH indicators• Indicators are dyes that can

be added that will change color in the presence of an acid or base.

• Some indicators only work in a specific range of pH

TitrationBuret with standardized solution

Unknown solution

End point

Past End point

End point – Visual Equivalence Point: StoichiometricThe point at which neutralization is

achieved (acid = base)

Titration Curves

Strong Acids vs Weak Acids• An acid that

nearly completely dissociates

• All molecules of the acid break up to form the ions soluble in water

• An acid that only slightly dissociates in a water solution

• Only a small percent of acid molecules donate their hydrogen, and most remain the same.

Weak acids have much lower percent ionization.CH3COOH(l) + H2O(l) H3O+

(aq) + CH3COO-(aq)

0.10 mol << 0.10 mol << 0.10 molmany molecules few ionsOnly about 5% ionization at 25C

A strong acid essentially ionizes 100%. HCl(g) + H2O(l) H3O+

(aq) + Cl-(aq)

0.10 mol 0.10 mol 0.10 molAfter ionization: a few molecules; but many ions

Strong Acids and BasesStrong Acids Strong Bases

HClO4 perchloric acid LiOH lithium hydroxide

HCl hydrochloric acid NaOH sodium hydroxide

HNO3 nitric acid KOH potassium hydroxide

H2SO4 sulfuric acid RbOH rubidium hydroxide

HBr hydrobromic acid CsOH cesium hydroxide

HI hydriodic acidCa(OH)2 calcium hydroxideHClO3Chloric acid* Sr(OH)2strontium hydroxide Ba(OH)2 barium

hydroxideNot included in every list

H+ A - H+ A - HA

A - H+ A - H+ A –

H+ A - H+ A - H+

A - HA H+ A -

H+ A - H+ A - H+

HA HA HA HA

HA HA HA

H+ A - HA HA

HA HA H + A –

HA H + A – HA HA

H+ A- H+ A- H+ A- H+ A- HAA- H+ A- H+ A- H+ A- H+ A -

H+ A- HA H+ A- H+ A- H+ A-

A- H+ A- H+ A- H+ A- H+ A- H+ H+ A - H + A - H + A - HA H + A -

A- H+ A- H+ A- H+ A- H+ A–

H+ A- H+ A- H+ A- H+ A- H+ A- H+ A- H+ A- H+ A- H+ A-

HA A- H+ A- H+ A- H+ A- H+

HA HA H+ A- HA HA HA HA HA HA HA HA H+ A- H+ A- HA HA HA HA HA

HA HA H+ A- HA HA HA HA HA HA H+

A- HA HA H+ A- HA HA HA HA HA HA HA HA H+

A- HA HA H+ A- HA HA HA HA HA HA HA H+ A- HA HA HA

DILUTECONCENTRATED

STR

ON

GW

EAK

Acids: Concentration vs. Strength

Acid Dissociation Constant

Bases

• All information and equations reference bases also.

Strengths Of Conjugate Acid-Base Pairs• The stronger an acid, the weaker is its

conjugate base.

• The stronger a base, the weaker is its conjugate acid.

• An acid-base reaction is favored in the direction from the stronger member to the weaker member of each conjugate acid-base pair.

For biological systems:

• Ionization of a strong acid is TOO BIG!• Ionization of water itself is way TOO LITTLE!

• Ionization of a weak acid is JUST RIGHT!

35

Weak acids, their conjugate bases, and buffers…

• Weak acids have only a modest tendency to shed their protons (definition of an acid).

• When they do, the corresponding negatively charged anion becomes a willing proton acceptor, and is called the conjugate base.

• The properties of a buffer rely on a balance between a weak acid and its conjugate base.

• And a titration curve looks like this…

36

Buffers• A buffer is a solution of a weak acid and its

conjugate base that resists changes in pH in both directions—either up or down.

• A buffer works best in the middle of its range, where the amount of undissociated acid is about equal to the amount of the conjugate base.

• One can soak up excess protons (acid), the other can soak up excess hydroxide (base).

37

pKa = 4.76

38

pH 7 _

_

_

_

_

_

_

0 equiv. of NaOH 1.0added

Buffering range: only small pH changes

result from addition of base or acid

Titration of acetic acid with sodium hydroxide

50% dissociation