Strong and Weak Acids and Bases

Pg 560-578

The strength of an acid is determined by the extent to which it ionizes, its percent ionization, not the concentration of the acid, the concentration of its hydronium ions, or its ability to react with a metal

Strong Acid

An acid that nearly completely dissociates All molecules of the acid break up to form the ions soluble in waterIf more than one proton is being removed, not all steps need to be complete dissociation.

Weak Acid

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.

Example: CH3COOH

A strong acid essentially ionizes 100%.

An example of a strong acid is hydrochloric acid, HCl (aq)

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

0.10 mol 0.10 mol 0.10 mol

100% ionization

few molecules many ions

An example of a weak acid is acetic acid, CH3COOH.

CH3COOH(l) + H2O(l) H3O+(aq) + CH3COO-

(aq)

0.10 mol << 0.10 mol << 0.10 mol

5% ionization at 25Cmany molecules few ions

Strong Base

A base that dissociates almost completely into its ions.

All oxides and hydroxides of group 1 and 2 are strong bases.

Ex: NaOH

Weak Base

Most bases are weak

They dissociate only slightly in a water solution

Example: NH3

Strong acids are strong electrolytes and weak acids are weak electrolytes

A strong base dissociates 100%.

An example of a strong base is sodium hydroxide, NaOH.

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

(aq) 0.10 mol 0.10 mol 0.10 mol

100% dissociationfew formula units (NaOH) many ions

A weak base ionizes to a small extent.

An example of a weak base is NH3(g).

NH3(g) + H2O(l) NH4+

(aq) + OH-(aq)

0.10 mol << 0.10 mol << 0.10 mol

5% ionization at 25C

many molecules few ions

Strong bases are strong electrolytes and weak bases are weak electrolytes.

Examples of Strong Acids and Bases

Strong Acids Strong BasesHClO4 perchloric acid LiOH lithium hydroxideHCl hydrochloric acid NaOH sodium hydroxideHNO3 nitric acid KOH potassium hydroxideH2SO4 sulfuric acid RbOH rubidium hydroxideHBr hydrobromic acid CsOH cesium hydroxideHI hydriodic acid Ca(OH)2 calcium hydroxide

Sr(OH)2 strontium hydroxide

Ba(OH)2 barium hydroxide

To experimentally distinguish strong acids from weak acids; and strong bases from weak bases:

compare a strong acid to a weak acid of equal concentration– more hydronium ions and anions will be

present in the strong acid solution

compare a strong base with a weak base of equal concentration– more hydroxide ions and cations will be present

in the strong base solution

Therefore, we could compare a strong acid and a weak acid of equal concentration by:

a) use a conductivity apparatus test (light bulb will be brighter for a strong acid).

b) measure conductivity of solutions (strong acid will have a higher conductivity).

c) react the two acids with a metal like magnesium (stronger acid will react faster, more bubbling as H2 is formed)

d) measure the pH of the solutions using a pH meter or indicators (strong acid has a lower pH)

A strong base can be distinguished experimentally from a weak base of equal concentration by:

a) use a conductivity apparatus test (light bulb will be brighter for a strong base)

b) measure conductivity of solutions (strong base will have a higher conductivity)

c) react the two bases with a chemical and observe the rate of the reaction (stronger base will react faster)

d) measure the pH of the solutions using a pH meter or indicators (stronger base has a higher pH)

Dissociation Equation

A balanced chemical equation showing all ions produced when an ionic compound dissolves

Example:

HSO4-(aq) + H2O(l) SO4

2-(aq) + H3O+

(aq)

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

ST

RO

NG

WE

AK

STRONG ACIDS

Dissociate nearly 100%

HA H1+ + A-

WEAK ACIDS

Dissociate very little

HA H1+ + A-

Acids: Concentration vs. Strength

Strong vs. Weak Acid

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 508

Comparing Strengths

Tables tend to list strong acids towards the top, and strong bases towards the bottom

Figure 14.12 in text page 563

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.

Concentrated vs. Dilute

0.3 M HCl

2.0 M HCl

12.0 M HCl

10.0 M CH3COOHDilute, strong acid

Concentrated, strong acidOR Dilute, strong, acid

Concentrated, strong acid

Concentrated, weak acid

Review and Practice

Page 558-559

# 1-2,4, 6-11