pKa : pKa
Strengths of Conjugate Acid-Base Pairs : Strengths of Conjugate Acid-Base Pairs strong medium weak very weak Acid strength increases HCl H2SO4 HNO3 H3O+ HSO4- H3PO4 HC2H3O2 H2CO3 H2S H2PO4- NH4+ HCO3- HPO42- H2O negligible very weak weak medium strong Base strength increases Cl- HSO4- NO3 H2O SO42- H2PO4- C2H3O2- HCO3- HS- HPO42- NH3 CO32- PO43- OH-
Conjugate Acid Strength : Conjugate Acid Strength Very
strong Strong Weak Very
weak Relative
acid
strength Relative
conjugate
base
strength Very
weak Very
strong Weak Strong HA H+ + A- pKa = [H+] [A-]
[HA] Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 508
Solutions of Acids and Bases: The Leveling Effect : Solutions of Acids and Bases: The Leveling Effect • No acid stronger than H3O+ and no base stronger than OH– can exist in aqueous solution, leading to the phenomenon known as the leveling effect.
• Any species that is a stronger acid than the conjugate acid of water (H3O+) is leveled to the strength of H3O+ in aqueous solution because H3O+ is the strongest acid that can exist in equilibrium with water.
• In aqueous solution, any base stronger than OH– is leveled to the strength of OH– because OH– is the strongest base that can exist in equilibrium with water
• Any substance whose anion is the conjugate base of a compound that is a weaker acid than water is a strong base that reacts quantitatively with water to form hydroxide ion
Weak Acids (pKa) : A(g) + 2 B(g) 3 C(g) + D(g) Weak Acids (pKa) Weak Acids – dissociate incompletely (~20%)
Strong Acids – dissociate completely (~100%) Equilibrium constant (Keq) = Keq = LeChatelier’s Principle
(lu-SHAT-el-YAY’s)
Slide 6 : Equilibrium constant Keq = = Ka = Acid dissociation constant Ka = 1.8 x 10-5 @ 25 oC for acetic acid
Slide 7 : HC2H3O2 H+ + C2H3O21- HCl H+ + Cl1- very large HNO3 H+ + NO31- very large H2SO4 H+ + HSO41- large 1.8 x 10-5 H2S H+ + HS1- 9.5 x 10-8 Ionization Constants for Acids Ka
Ionization of Acids : Ionization of Acids Acid Ionization Equation Ionization Constant, pKa Hydrochloric HCl H1+ + Cl1- very large
Sulfuric H2SO4 H1+ + HSO41- large
Acetic HC2H3O2 H1+ + C2H3O21- 1.8 x 10-5
Values of Ka for Some Common Monoprotic Acids : Formula Name Value of Ka* Values of Ka for Some Common Monoprotic Acids HSO4- hydrogen sulfate ion 1.2 x 10-2
HClO2 chlorous acid 1.2 x 10-2
HC2H2ClO2 monochloracetic acid 1.35 x 10-3
HF hydrofluoric acid 7.2 x 10-4
HNO2 nitrous acid 4.0 x 10-4
HC2H3O2 acetic acid 1.8 x 10-5
HOCl hypochlorous acid 3.5 x 10-8
HCN hydrocyanic acid 6.2 x 10-10
NH4+ ammonium ion 5.6 x 10-10
HOC6H5 phenol 1.6 x 10-10
*The units of Ka are mol/L but are customarily omitted. Increasing acid strength
Slide 10 : H2SO4 2 H+ + SO42- in dilute solutions...occurs ~100% One gram of concentrated sulfuric acid (H2SO4) is diluted to a 1.0 dm3 volume
with water. What is the molar concentration of the hydrogen ion in this solution?
What is the pH? x mol H2SO4 = 1 g H2SO4 Solution) First determine the number of moles of H2SO4 Sample 1) = 0.010 mol H2SO4 OVERALL: pH = - log [H+] pH = 1.69 0.010 M 0.020 M substitute into equation pH = - log [0.020 M] 98 g H2SO4 1 mol H2SO4
Slide 11 : A volume of 5.71 cm3 of pure acetic acid, HC2H3O2, is diluted with water at
25 oC to form a solution with a volume of 1.0 dm3. Step 2) Find the number of moles of acid. x mol acetic acid = 6.00 g HC2H3O2 = 0.10 mol acetic acid (in 1 L) M = 0.1 molar HC2H3O2 Step 3) Find the [H+]
Ka = Step 1) Find the mass of the acid Mass of acid = density of acid x volume of acid = 1.05 g/cm3 x 5.71 cm3 = 6.00 g Molarity: M = mol / L Substitute into equation M = 0.10 mol / 1 L What is the molar concentration of the hydrogen ion, H+, in this solution?
(The density of pure acetic acid is 1.05 g/cm3.) (From the formula of acetic acid,
you can calculate that the molar mass of acetic acid is 60 g / mol).
Slide 12 : Step 3) Find the [H+] Ka = 1.8 x 10-5 @ 25 oC for acetic acid Substitute into equation: x2 = 1.8 x 10-6 M x = 1.3 x 10-3 molar = [H+] HC2H3O2 H+ + C2H3O21- 0.1 M pH = - log[H+] pH = - log [1.3 x10-3 M] pH = 2.9 ? 0.1 M weak acid How do the concentrations of
H+ and C2H3O21- compare?
Slide 13 : Note: although the sulfuric acid is 10x less
concentrated than the acetic acid...
…it produces > 10x more H+ H+ Concentrations
…Strong vs. Weak Acid pH = - log[H+] 1.7 2.9 0.0200 M 0.0013 M
Slide 14 : 1a) What is the molar hydrogen ion concentration in a 2.00 dm3 solution
of hydrogen chloride in which 3.65 g of HCl is dissolved?
1b) pH
2a) What is the molar concentration of hydrogen ions in a solution
containing 3.20 g of HNO3 in 250 cm3 of solution?
2b) pH
3a) An acetic acid solution is 0.25 M. What is its molar concentration of
hydrogen ions?
3b) pH
4) A solution of acetic acid contains 12.0 g of HC2H3O2 in 500 cm3
of solution. What is the molar concentration of hydrogen ions? 1a) 0.0500 M 2a) 0.203 M 3a) 2.1 x 10-3 M 4) 2.7 x 10-3 M
1b) pH = 1.3 2b) pH = 0.7 3b) pH = 2.7 Practice Problems:
Weak Acids : Weak Acids Cyanic acid is a weak monoprotic acid. If the initial concentration of cyanic
acid is 0.150 M and the equilibrium concentration of H3O+ is 4.8 x 10-3 M,
calculate Ka for cyanic acid. HCN(aq) H+(aq) + CN1-(aq) H3O+(aq) 0.150 M 4.8 x 10-3 M Ka = [4.8 x 10-3 M] [0.150 M] [CN1-] [4.8 x 10-3 M] Ka = 1.54 x 10-4 How is [H3O+] determined? 4.8 x 10-3 M Measure pH of solution and work backwards
Weak Acid, pKa : Weak Acid, pKa Keys Weak Acid, pKa Weak Acid, pKa http://www.unit5.org/chemistry/AcidBase.html