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 water If 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: CH 3 COOH

A

strong acid

essentially ionizes 100%. An example of a strong acid is hydrochloric acid, HCl (aq) HCl (g) 0.10 mol + H 2 O (l)  H 3 O + (aq) 0.10 mol + Cl (aq) 0.10 mol 100% ionization few molecules many ions

An example of a weak acid is acetic acid, CH 3 COOH.

CH 3 COOH (l) 0.10 mol + H 2 O (l)  H 3 O + (aq) + CH 3 COO (aq) << 0.10 mol << 0.10 mol 5% ionization at 25  C many 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: NH 3

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) 0.10 mol + H 2 O (l)  Na + (aq) + OH (aq) 0.10 mol 0.10 mol 100% dissociation few formula units (NaOH) many ions

A weak base ionizes to a small extent. An example of a weak base is NH3(g).

NH 3(g) + H 2 O (l)  NH 4 + (aq) + OH (aq) 0.10 mol << 0.10 mol << 0.10 mol 5% ionization at 25  C many molecules few ions

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

Examples of Strong Acids and Bases

Strong Acids

HClO4 perchloric acid HCl hydrochloric acid HNO3 nitric acid H2SO4 sulfuric acid HBr hydrobromic acid HI hydriodic acid

Strong Bases

LiOH lithium hydroxide NaOH sodium hydroxide KOH potassium hydroxide RbOH rubidium hydroxide CsOH cesium hydroxide 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 H 2 formed) is 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: HSO 4 (aq) + H 2 O (l)  SO 4 2 (aq) + H 3 O + (aq)

Acids: Concentration vs. Strength CONCENTRATED

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

HA

A H + H + A A -

HA

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

HA

A H + A H + H + H A A – + H + A A H

HA

A + H A H + + A H + A H A H + + A H + A H A H + + A H + A H + A H +

DILUTE

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

HA

A – H + A H + A A H + A -

HA

H + H + A H + A H +

HA HA

H + A -

HA HA HA HA HA

H + A H + A -

HA HA HA HA HA HA HA

H + A -

HA HA HA HA HA HA HA HA HA

H + A H + A -

HA HA HA HA HA HA HA HA HA HA

H + A H + A -

HA HA HA HA HA HA HA HA HA

H + A -

HA HA HA HA HA HA HA HA HA HA

H + A -

HA HA HA HA

H + A –

HA

H + A –

HA HA STRONG ACIDS Dissociate nearly 100% HA H 1+ + A WEAK ACIDS Dissociate very little HA H 1+ + A -

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 Dilute, strong acid 2.0 M HCl Concentrated, strong acid OR Dilute, strong, acid 12.0 M HCl Concentrated, strong acid 10.0 M CH 3 COOH Concentrated, weak acid

Review and Practice

Page 558-559 # 1-2,4, 6-11