Chapter 17: Acids & Bases

Acids Arrhenius acid – A substance that produces H+ (also H3O+) ions in aqueous solution Brønsted-Lowry acid - a substance that donates H+ ions

Structure of an acid Chapter 15, Unnumbered Figure, Page 660

Chapter 15, Unnumbered Figure 1, Page 661 Hydrogens connected to more electronegative atoms are more likely to act as an acidic hydrogen High EN atoms: F, Cl, Br, I, O

Practice Decide if the following molecules are acids or not, based on their structures.

Strong acids Strong acid - an acid that completely ionizes or dissociates in water

Weak acids Weak acid - an acid that partially ionizes or dissociates in water

Chapter 15, Unnumbered Figure 2, Page 661

Chapter 15, Unnumbered Figure 4, Page 661

Bases Arrhenius base – A substance that produces OH- ions in aqueous solution Brønsted-Lowry base - a substance that accepts H+ ions Arrow pushing mechanism

Strong bases Strong base - a base that completely ionizes or dissociates in water

Weak bases NH3(aq) + H2O(l) NH4+(aq) + OH-(aq) Weak base - a base that partially ionizes or dissociates in water

Conjugate acid-base pairs Each acid has a conjugate base and every base has a conjugate acid

Chapter 15, Figure 15.12 Strength of Conjugate Acid–Base Pairs

Problems Identify the conjugate bases for the following acids HBr H2S H2CO3 Identify the conjugate acids for the following bases NO2- NH3 OH- Problems

Polyprotic acids Polyprotic acids - acids that can donate more than one H+

Acid ionization constant (Ka) Acid ionization constant (Ka) - the equilibrium constant for the ionization reaction of an acid with water

Questions Write the Ka expression for the dissociation of HCN (hydrocyanic acid) in water. Write the Ka expression for the dissociation of HClO2 (chlorous acid) in water. If the Ka for HCN is 4.9 x 10-10 and the Ka for HF is 3.5 x 10-4, which acid is stronger?

Base ionization constant (Kb) Base ionization constant (Kb) - the equilibrium constant for the ionization reaction of a base with water

Autoionization of water Also called the self ionization of water 1 out of 10 million water molecules form ions through self ionization All aqueous solutions contain both H3O+ and OH–

Ion Product Constant for Water (Kw) The numerical value obtained by multiplying the molar concentrations for hydronium and hydroxide ions present in pure water Kw = [H3O+][OH-] = 1.00 x 10-14 @ 25 oC the concentration of H3O+ and OH– are equal in pure water [H3O+] = [OH–] = 10-7M @ 25°C Ion Product Constant for Water (Kw)

H+ OH- [H+] vs. [OH-] Acid Base

Calculate the [OH] at 25°C when the [H3O+] = 1 Calculate the [OH] at 25°C when the [H3O+] = 1.5 x 10-9 M, and determine if the solution is acidic, basic, or neutral Calculate the [H3O+] at 25°C in a solution that has a [OH] 1.3 x 10-10 M and determine if the solution is acidic, basic, or neutral Calculate the [H3O+] at 25°C in a solution that has a [OH] 1.0 x 10-7 M and determine if the solution is acidic, basic, or neutral Questions

pH and pOH pH is a measure of the concentration of H+ or H3O+ in solution pOH is a measure of the concentration of OH- in solution

pH and pOH Acidic solutions Basic solutions Neutral solutions

Determining [H3O+] and pH of an acidic solution : Strong acids The two sources of [H3O+] in a solution are from the autoionization of water and the weak/strong acid Strong acids completely ionize in water, so the initial acid concentration is the [H3O+] Determining [H3O+] and pH of an acidic solution : Strong acids

Determining [H3O+] and pH of an acidic solution : Weak acids For weak acids we must use the Ka value to determine [H3O+] from initial [HA] Determining [H3O+] and pH of an acidic solution : Weak acids

x is small approximation [HA] [H3O+] [A-] Initial Change -x +x Equilibrium [HA] - x x If Ka is very small, the x value is very small since small amount of products made This allows simplification of denominator expression

Exceptions to x is small approximation Can only apply this rule if the [HA] is much larger than Ka If they are close in within a factor of 1 to 100, then the % error is large, so you can’t use this approximation Ka = 1x10-10 [HA] = 1 x 10-3 Can use approximation Ka = 1x10-4 [HA] = 1 x 10-3 Can’t use approximation If you can’t use approx. then you have to use quadratic formula like from before

Practice HCN(aq) + H2O(l) CN-(aq) + H3O+(aq) Determine the [H3O+] of a 0.100 M HCN which has a Ka value of 4.9 x 10-10? HCN(aq) + H2O(l) CN-(aq) + H3O+(aq) [HCN] [H3O+] [CN-] Initial Change -x +x Equilibrium [HCN] - x x approx

Percent ionization of weak acids A way to quantify the ionization of a weak acid Example [HA] initial = 0.120 M [H3O+] equilibrium = 0.00600M Percent ionization of weak acids

Finding pH for a mixture of acids If you have a mixture of a strong and a weak acid with relative concentrations, the weak acid becomes essentially negligible Calculate as if only the strong acid is present If you have a mixture of two weak acids with relative concentrations, consider the weaker of the two acids to be negligible, if differing by a magnitude of 103 Calculate as if only the stronger acid is present Finding pH for a mixture of acids

Finding pH for mixture of acids If both weak acids have relative concentrations and Ka values that are close… Calculate the [H+] of the stronger weak acid using ice table Then take the calculated [H+] from the ice table and use it as the initial concentration in a ice table with the weaker weak acid Finding pH for mixture of acids

Practice What is the pH of a mixture of 0.100 M Benzoic acid (HC7H5O2 , Ka1 = 6.5 x 10-5) and 0.050 M Acetic acid (HC2H3O2, Ka2 = 1.8 x 10-5)? HC7H5O2 (aq) + H2O (l)  H3O+ (aq) + C7H5O2-(aq) HC2H3O2 (aq) + H2O (l)  H3O+ (aq) + C2H3O2-(aq) Chapter 15, Unnumbered Table, Page 698

Conjugate acids and bases Anions can be conjugate bases of acids The Kb of a conjugate base is related to the Ka of the acid it came from Ka Kb = Kw = 1014 In general Stronger acid = weaker conjugate base Weaker acid = stronger conjugate base Conjugate acids and bases

Acid strength trends If a Hydrogen has more of a “+” charge, or is more easily donated, it is a stronger acid Weaker bond to H atom, stronger acid Higher electronegativity of atom connected to H, stronger acid The more oxygen atoms in close proximity, stronger acid

Acid strength: Bond energy Chapter 15, Unnumbered Table, Page 698

Acid strength: Electronegativity Chapter 15, Unnumbered Table, Page 698

Acid strength: Oxygens in close proximity Chapter 15, Unnumbered Table 2, Page 699

Acid rain Combustion of fossil fuels release SO2 and NO2 into the atmosphere 2SO2+O2+2H2O  2H2SO4 sulfuric acid 4NO2+O2+2H2O  4HNO3 nitric acid These acids + rain = acid rain Can cause building degradation, hazardous to aquatic life, hindrance to tree survivability, overall bad for the environment

Chapter 15, Figure 15.16 Acid Rain

Chapter 17 Review Acids and bases Strong vs weak acids/bases Conjugate acid/base pairs Poly protic acids Acid/base ionization constant Ka/Kb Kw ion product of water pH and pOH ICE tables for acids, x is small approx % ionization Acid strength trends and Acid rain Chapter 17 Review