AP Chemistry Acids and Bases

Aqueous Equilibria: Acids and Bases Arrhenius Acids and Bases Acids cause [H+] to increase, bases cause [OH-] to increase Bronsted-Lowry Acids and Bases H + /proton Donor (acid) and H +/ proton Acceptor (base) Lewis Acid and Bases Acids accept electron pair Bases donate electron pair

Acid and Base Strengths Based on extent of dissociation. Strong Acids Dissociate nearly 100% If HA  H + + A - –A - is a very weak base. (the conjugate base) Acid and Conjugate Base explanation of strength. Pair of substances differing only by H + HF(aq) + H 2 O(l) H 3 O + (aq) + F - (aq) acid 1 base 2 acid 2 base 1 H 3 O+(aq) + OH - (aq) H 2 O(l) + H 2 O(l) acid 1 base 2 acid 2 base 1

Acid and Base Strengths Taken from State University of West Georgia Chemistry Dept.

Acid and Base Strengths Taken from State University of West Georgia Chemistry Dept.

Hydronium Ions

H5O2+H5O2+

Dissociation of Water H 2 O + H 2 O H 3 O 1+ + OH 1- The equilibrium expression is products over reactants. K = [H 3 O 1+ ] [OH 1- ] / [H 2 O] [H 2 O] The molarity for the water is a constant at any specific temperature. So K [H 2 O] [H 2 O] = [H 3 O 1+ ] [OH 1- ] The quantity on the right hand side of the equation is formally defined as Kw. The numerical vale for Kw is different at different temperatures. At 25oC Kw = x Kw = K[H 2 O] [H 2 O] or Kw = [H 3 O 1+ ] [OH 1- ]

Dissociation of Water Equilibrium constants exist then for both acid dissociation and base. (K a and K b ) The higher the K a, the stronger the acid and the higher the K b, the stronger the base. K a and K b are related by the previous equation. K w = K a K b

Dissociation of Water As K a gets larger the strength of the acid gets higher, but K b must fall. Therefore the stronger the acid, the weaker the conjugate base. It can now be said that the conjugate base (acid) of a weak acid (base) is a weak base (acid) and the conjugate base (acid) of a strong acid (base) is a worthless base (acid). The strength of an acid/base is usually given as a pK a value. As pK a is inversely related to K a, the higher the K a (the stronger the acid), the lower the pKa value. The same is true of bases.

Calculating pH -log [H+] Power of Hydronium (Hydrogen) P[OH-] = - log [OH]

The pH Scale

pH in Solutions of Strong Acids and Strong Bases Strong acids Certain acids are known as strong acids. These are acids that fully ionize when placed in water: HA + H 2 O  A - + H 3 O + Goes to completion and thus Ka = [A-][H3O+]/[HA] = infinity Some common strong acids are: HCl, hydrochloric acid HBr, hyrdobromic acid HI, hydroiodic acid H 2 SO 4, sulfuric acid HNO 3, nitric acid HClO 4, perchloric acid

pH in Solutions of Strong Acids and Strong Bases Strong Bases Certain bases are known as strong bases. These are bases that fully ionize when placed in water. Some common strong bases are: LiOH, lithium hydroxide NaOH, sodium hydroxide KOH, potassium hydroxide Ca(OH) 2, calcium hydroxide Sr(OH) 2, strontium hydroxide Ba(OH) 2, barium hydroxide Alkaline earth oxides. Lime (CaO)

Equilibrium in Solutions of Weak Acids HA(aq) + H 2 O(l)  A - (aq) + H 3 O + (aq) The equilibrium constant for a weak acid isequilibrium constant Ka = [H 3 O + ][A - ]/[HA] For a weak acid then Ka << 1 For a strong acid Ka >> 1 A common way to express the strength of an acid is the pK a, which is similar in form to the pHpH pK a = -log 10 K a

Calculating Equilibrium Concentrations in Solutions of Weak Acids Principle Reaction vs Subsidiary Reactions. If one of the equilibrium reactions is less than 100 x the extent of the other. Always check H 2 O(l) + H 2 O(l) H 3 O+(aq) + OH - (aq) K w = 1.0 x

Percent Dissociation in Solution of Weak Acids Percent dissociation = [HA] dissociated / [HA] initial x 100%

More Discussion Acid HA + S  HS + + A - Acid Solvent Conjugate acid Conjugate base Base HB+ + S  HS + + B Conjugate acid Solvent Acid Base B + H 2 O  HB+ + OH- Kb =

More Discussion K w = K a x K b Or pK a + pK b = pK w Carbonic acid (H 2 CO 3 ) (Data in H 2 O) 1. K a = 4.3 x 10-7pK a = K a = 5.61 x pK a = Explain what happens when the K a of an acid is smaller than the K a for H 2 O.

Polyprotic Acids A polyprotic acid is one that has multiple ionizable protons, such as H 2 SO 4 or H 3 PO 4. –Each proton has its own equilibrium constant K a. For example, for a diprotic acid H 2 A, H 2 A(aq)  H + (aq) + HA - (aq) K a1 = [H + ][HA - ]/[H 2 A] HA - (aq)  H + (aq) + A -2 (aq) K a2 = [H + ][A -2 ]/[HA - ] In general, K a1 >> K a2 >> K a3. You can compute the K for the total ionization of the acid. If you add the above equations. –H 2 A (aq)  2H + (aq) + A - (aq) K total = K a1 *K a2

Polyprotic Acids Taken from University of Alberta chemistry dept. Ionization Constants of Aqueous Polyprotic Acids Common Formula Dissociation Constant pKa arsenic acid H 3 AsO 4 K1 = 5.65 x H 2 AsO 4 - K2 = 1.75 x HAsO 4 2- K3 = 2.54 x boric acid H 3 BO 3 K1 = 5.78 x carbonic acid H 2 CO 3 K1 = 4.35 x HCO 3 - K2 = 4.69 x chromic acid H 2 CrO 4 K1 = HCrO 4 - K2 = 3.36 x citric acid HOC(CH 2 COOH) 3 K1 = 7.42 x K2 = 1.75 x K3 = 3.99 x EDTA C 2 H 4 N 2 (CH 2 COOH) 4 K1 = 9.81 x K2 = 2.08 x K3 = 7.98 x K4 = 6.60 x

Polyprotic Acids Taken from University of Alberta chemistry dept. Common Formula Dissociation Constant pKa glycinium ion H3NCH2COOH+ K1 = 4.47 x (glycine) H2NCH2COOH K2 = 1.67 x hydrogen sulfide H2S K1 = 1.02 x HS- K2 = 1.22 x oxalic acid HOOCCOOH K1 = 5.40 x HOOCCOO- K2 = 5.23 x phthalic acid C6H4(COOH)2 K1 = 1.13 x K2 = 3.90 x phosphoric acid H3PO4 K1 = 7.11 x H2PO4- K2 = 6.23 x HPO42- K3 = 4.55 x succinic acid C(CH2)2COOH K1 = 6.21 x HOOC(CH2)2COO- K2 = 2.31 x sulfuric acid H2SO4 K1 > 1 negative - HSO4- K2 = 1.01 x sulfurous acid H2SO3 K1 = 1.71 x HSO3- K2 = 5.98 x

15.11 Polyprotic Acids

Equilibria in Solutions of Weak Bases Remember K w = K a K b BaseFormulaKbKb BH + Conjugate Acid KaKa Ammonia NH x NH x Aniline C 6 H 5 NH x C 6 H 5 NH x Dimethylamine (CH 3 ) 2 NH5.4 x (CH 3 ) 2 NH x Hydrazine N2H4N2H4 8.9 x N2H5+N2H x Hydroxylamine NH 2 OH9.1 x NH 3 OH x Methylamine CH 3 NH x CH 3 NH x

Relation Between K a and K b HA(aq) + H 2 O(l)  H 3 O + (aq) + A - (aq) K a A - (aq)+ H 2 O(l)  HA(aq) + OH - (aq) K b K w = K a K b = 1.0 x K a = K w / K b K b = K w / K a K net = K 1 x K 2 x K 3 ……

Acid/Base

Acid-Base Properties of Salts NeutralBasic Acidic? Acid Strength Base Strength Strong Weak Resulting Salt Solution

Acid-Base Properties of Salts CationAnionSolutionExample Neutral NaCl Neutral Conj. Base of a Weak Acid BasicNaF Conj. Acid of a Weak Base NeutralAcidicNH 4 Cl Conj. Acid of a Weak Base Conj. Base of a Weak Acid Depends on K a and K b Values NH 4 Cl

Acid-Base Properties of Salts Example #1 NaOH(aq) + HCl(aq)  NaCl(aq) + H 2 O Strong Base Strong Acid Neutral Salt

Acid-Base Properties of Salts Example #2 NaOH(aq) + HF(aq)  NaF(aq) + H 2 O Strong Base Weak Acid Basic Salt

Acid-Base Properties of Salts Example #3 NH 3 (aq) + HCl(aq)  NH 4 Cl(aq) Weak Base Strong Acid Acidic Salt

Acid-Base Properties of Salts Example #4 NH 3 (aq) + CH 3 COOH(aq)  NH 4 OOCCH 3 (aq) Weak Base Weak Acid ? Salt –Compare K a to K b –K a = 5.6 x of NH 4 + –K b = 5.7 x of - OOCCH 3 –Salt is Neutral or ?

Acid-Base Properties of Salts Example #5 2NH 3 (aq) + H 2 CO 3 (aq)  (NH 4 ) 2 CO3(aq) Weak Base Weak Acid Acidic Salt –Compare K a to K b –K a = 5.6 x of NH 4 + –K b = 1.8 x of CO 3 2- –Salt is Basic

Factors That Affect Acid Strength HA  H + + A - Extent of dissociation depends on H-A bond strength and Electronegativity (or stability of negative charge) on A. –This explanation works for Halogen acids, Organic oxoacids, or Inorganic oxoacids. –Oxidation # of the Halide is not necessary.

Lewis Acids and Bases Bronsted-Lowry Acid Proton Donor Lewis Acid Electron Pair Acceptor Bronsted-Lowry Base Proton Acceptor Lewis Base Electron Pair Acceptor

Lewis Acids and Bases Lewis Acid Anything with a vacant valence orbital Charged or Neutral –Other Examples –Fe +3 (aq) + 6CN - (aq) -> Fe(CN) 6 -3 (aq) –Cu +2 (aq) + 4NH 3 (aq) -> Cu(NH 3 ) 4 +2 (aq)