Chapter 10Acids and Bases

(10.1) Acid-Base Strength

Strong Acid (SA)

• an acid that dissociates completely into ions in water• Ex.)all molecules of HCl(aq) dissociate into

H+ and Cl- ions• the H+ ions that are formed bond with H2O to form

H3O+

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

(aq)

Strong Acid (SA)

other SA’s are…..HBrHIHNO3

H2SO4

HClO4

Concentration

• the conc. of H3O+(aq) ions, [H3O+], in a dilute

solution of a SA is equal to the conc. of the acid, [acid]

[H3O+] = [strong acid]

• Ex.) In a sample of 1.0 mol/L HCl(aq)

[H3O+] = 1.0 mol/L

square bracketMolar Concentration

Recall: C = n V

Dissociation (Strong Acid)

Text p 381 Figure 10.6100% of hydrogen chloride molecules dissociate

Dissociation (Weak Acid)Text p 382 Figure 10.71% of acetic acid molecules dissociate. In solution

there are mostly acetic acid molecules and only some ions.

Weak Acid (WA)

• an acid that dissociates very slightly into ions in water

• Ex.) not all molecules of CH3COOH(aq) break apart, some remain intact

• only about 1% of acetic acid molecules dissociate• In a sample of 1.0 mol/L CH3COOH (aq) ,

[H3O+] <<< 1.0 mol/L

• That is, [H3O+] < [weak acid]

Strong Base (SB)

• a base that dissociates completely into ions in water

• Ex.) all molecules of NaOH(aq) dissociate into Na+ and OH- ions

• other SB’s are… KOHCa(OH)2

Ba(OH)2

Concentration

• the conc. of OH1-(aq) ions, [OH1-], in a dilute

solution of a SB is equal to the conc. of the base, [base]

[OH1-] = [strong base]• Ex.) In a sample of 1.0 mol/L NaOH(aq)

[OH1-] = 1.0 mol/L

Weak Base (WB)

• most bases are weak• WB dissociates very slightly into ions in

water• Ex.) ammonia

[OH1-] < [weak base]

Acid Dissociation

• few acids contain only a single H+ ion• monoprotic acid: produces one H+ ion • Ex.) HF(aq) + H2O(l) H3O+

(aq) + F-(aq)

(simplified) HF(aq) H+(aq) + F-

(aq)

Acid Dissociation

• many acids contain 2 or more H+ ions• diprotic acid: produces two H+ ions• Ex.) H2SO4 (aq) H+

(aq) + HSO4-(aq)

HSO4-(aq) H+

(aq) + SO42-

(aq)

H2SO4(aq) 2H+(aq) + SO4

2-(aq)

Acid Dissociation

• triprotic acid: produces three H+ ions• Ex.) H3PO4 (aq) H+

(aq) + H2PO4-(aq)

H2PO4-(aq) H+

(aq) + HPO42-

(aq)

HPO42-

(aq) H+(aq) + PO4

3-(aq)

H3PO4(aq) 3 H+(aq) + PO4

3-(aq)

Measuring Strength – pH Scale

• In pure water, [H3O+] = [OH-] = 1.0 x 10-7 mol/L

• In acidic sol’n, [H3O+] > [OH-]

• In basic sol’n, [H3O+] < [OH-]

Measuring Strength – pH Scale

• In any aqueous sol’n the product can be calculated,

[H3O+] x [OH-] = 1.0 x 10-14

• The values of [H3O+] range from 10 mol/L to 10-15 mol/L

pH Scale

• We use system of logarithms to handle the range of exponents for [H3O+]

• Ex.) log (1x102) = 2log (1x10-6)= - 6

• simply, use the exponent when the value is written as 1 x base 10

or • use your calculator

pH Scale

• Ex.) [H3O+(aq)] = 1.0 x10-4

pH = -log[1.0 x10-4]= +4

pH = -log[H3O+(aq)]

pH Scale

Calculate,1)pH if [H3O+

(aq)] = 3.8 x10-3 mol/L

(ans: 2.42)2) [H3O+

(aq)] if pH = 3.85

(ans: 1.4 x 10-4 mol/L)3) pH of 0.082 mol/L HCl(aq)

(ans: 1.09)

[H3O+(aq)] = 10-pH

pOH Scale

• Ex.) [OH-(aq)] = 1.0 x10-6

pH = -log[1.0 x10-6]= +6

pOH = -log[OH-(aq)]

pOH + pH = 14

Homework