Strong and Weak Acids and Strong and Weak Acids and BasesBases

Pg 560-578Pg 560-578

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

Strong AcidStrong Acid

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

Weak AcidWeak Acid

An acid that only An acid that only slightly dissociates in slightly dissociates in a water solutiona water solution

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

Example: CHExample: CH33COOHCOOH

A A strong acidstrong acid essentially ionizes 100%. essentially ionizes 100%.

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

HClHCl(g)(g) + H + H22OO(l)(l) H H33OO++(aq)(aq) + Cl + Cl--(aq)(aq)

0.10 mol0.10 mol 0.10 mol0.10 mol 0.10 mol0.10 mol

100% ionization100% ionization

few moleculesfew molecules many ionsmany ions

An example of a weak acid is acetic acid, An example of a weak acid is acetic acid, CHCH33COOH.COOH.

CHCH33COOHCOOH(l)(l) + H + H22OO(l)(l) H H33OO++(aq)(aq) + CH + CH33COOCOO--

(aq)(aq)

0.10 mol0.10 mol << 0.10 mol << 0.10 mol << 0.10 mol << 0.10 mol

5% ionization at 255% ionization at 25CCmany moleculesmany molecules few ionsfew ions

Strong BaseStrong Base

A base that A base that dissociates almost dissociates almost completely into its completely into its ions.ions.

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

Ex: NaOHEx: NaOH

Weak BaseWeak Base

Most bases are weakMost bases are weak

They dissociate only They dissociate only slightly in a water slightly in a water solutionsolution

Example: NHExample: NH33

Strong acids are strong electrolytes and Strong acids are strong electrolytes and weak acids are weak electrolytes weak acids are weak electrolytes

A A strong basestrong base dissociates 100%. dissociates 100%.

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

NaOHNaOH(s)(s) + H + H22OO(l)(l) Na Na ++(aq)(aq) + OH + OH--

(aq)(aq)

0.10 mol0.10 mol 0.10 mol 0.10 mol 0.10 mol 0.10 mol

100% dissociation100% dissociationfew formula units (NaOH) many ionsfew formula units (NaOH) many ions

A weak base ionizes to a small extent. A weak base ionizes to a small extent.

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

NHNH3(g)3(g) + H + H22OO(l)(l) NH NH44++

(aq)(aq) + OH + OH--(aq)(aq)

0.10 mol << 0.10 mol << 0.10 0.10 mol << 0.10 mol << 0.10 molmol

5% ionization at 255% ionization at 25CC

many molecules few ionsmany molecules few ions

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

Examples of Strong Acids and Examples of Strong Acids and BasesBases

Strong AcidsStrong Acids Strong BasesStrong BasesHClO4 perchloric acidHClO4 perchloric acid LiOH lithium hydroxideLiOH lithium hydroxideHCl hydrochloric acidHCl hydrochloric acid NaOH sodium hydroxideNaOH sodium hydroxideHNO3 nitric acidHNO3 nitric acid KOH potassium hydroxideKOH potassium hydroxideH2SO4 sulfuric acidH2SO4 sulfuric acid RbOH rubidium hydroxideRbOH rubidium hydroxideHBr hydrobromic acidHBr hydrobromic acid CsOH cesium hydroxideCsOH cesium hydroxideHI hydriodic acidHI hydriodic acid Ca(OH)2 calcium hydroxideCa(OH)2 calcium hydroxide

Sr(OH)2 strontium Sr(OH)2 strontium hydroxidehydroxide

Ba(OH)2 barium hydroxideBa(OH)2 barium hydroxide

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

compare a strong acid to a weak acid of compare a strong acid to a weak acid of equal concentrationequal concentration

– more hydronium ions and anions will be more hydronium ions and anions will be present in the strong acid solutionpresent in the strong acid solution

compare a strong base with a weak base of compare a strong base with a weak base of equal concentrationequal concentration

– more hydroxide ions and cations will be present more hydroxide ions and cations will be present in the strong base solution in the strong base solution

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

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

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

c)c) react the two acids with a metal like magnesium react the two acids with a metal like magnesium (stronger acid will react faster, more bubbling as H(stronger acid will react faster, more bubbling as H22 is is formed)formed)

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

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

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

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

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

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

Dissociation EquationDissociation Equation

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

Example:Example:

HSOHSO44--(aq)(aq) + H + H22OO(l)(l) SO SO44

2-2-(aq)(aq) + H + H33OO++

(aq)(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 AcidStrong vs. Weak Acid

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

Comparing StrengthsComparing Strengths

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

Figure 14.12 in text page 563Figure 14.12 in text page 563

Strengths Of Conjugate Acid-Base PairsStrengths Of Conjugate Acid-Base Pairs

The stronger an acid, the weaker is its The stronger an acid, the weaker is its conjugate base.conjugate base.

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

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

Concentrated vs. DiluteConcentrated 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 PracticeReview and Practice

Page 558-559Page 558-559

# 1-2,4, 6-11# 1-2,4, 6-11