Several concepts of acid-base theory: The Arrhenius concept The Bronsted-Lowry concept The Lewis concept

Arrhenius Acids & Bases According to the Arrhenius concept of acids and bases An acid is a substance that, when dissolved in water, increases the concentration of Hydrogen ion, H + (aq) The H + (aq) ion, called the Hydrogen ion is actually chemically bonded to water, that is, H 3 O +, called the Hydronium ion The Hydronium Ion (H 3 O + ) actually exists as a hydrogen bonded H 9 O 4 + cluster.

According to the Arrhenius concept of acids and bases A base is a substance that when dissolved in water increases the concentration of the hydroxide ion, OH - (aq) HBr(aq)  H + (aq) + Br - (aq) Acid + H 2 O  Anion + H 3 O + HA + H 2 O  A – + H 3 O + HC 2 H 3 O 2(aq) + H 2 O  H 3 O + (aq) + C 2 H 3 O 2 − (aq) a.Metal Hydroxides NaOH (s)  Na + (aq) + OH – (aq) Mg(OH) 2(s)  Mg 2+ (aq) + 2OH – (aq) b. Molecular bases: NH 3(aq) +H 2 O  NH 4 + (aq) + OH - (aq)

Arrhenius acid-base neutralization Acid and base combine to produce water and salt. Ex. HCl (aq) + NaOH (aq)  H 2 O + NaCl (aq) H 3 O + (aq) + Cl – (aq) + Na + (aq) + OH – (aq)  2H 2 O + Cl – (aq) + Na + (aq) However, many reactions resemble this without forming H 3 O + or OH – in solution!!! 5

Gas Phase Acid-Base Not covered by Arrhenius definition Ex. NH 3 (g) + HCl (g)  NH 4 Cl (s)

Brønsted-Lowry Definition Acid = proton donor Base = proton acceptor Allows for gas phase acid-base reactions HCl + H 2 O H 3 O + + Cl – HCl = acid : Donates H + Water = base : Accepts H + 7

Bronsted-Lowry Acids & Bases Amphoteric Species: can act as either as acid or base Water Water can react as an acid by donating a proton to a base Water can react as a base by accepting a proton from an acid H+H+ H+H+

Learning Check Write a reaction that shows that HCO 3 – is a Brønsted acid when reacted with OH – HCO 3 – (aq) + OH – (aq) Write a reaction that shows that HCO 3 – is a Brønsted base when reacted with H 3 O + (aq) HCO 3 – (aq) + H 3 O + (aq) 9 H 2 CO 3 (aq) + H 2 O (ℓ) H 2 O (ℓ) + CO 3 2– (aq)

Lone pair binds H + Conjugate Acid-Base Pair

Learning Check conjugate baseconjugate acid HCl NH 3 HC 2 H 3 O 2 CN – HF 12 Identify the Conjugate Partner for Each Cl – NH 4 + C2H3O2–C2H3O2– HCN F–F–

The concentration of water is almost constant. K a very large K a very small

Electrolytes Non- electrolytes Solute undergoes Dissociation strong weak complete dissociation partial dissociation NaCl / H 2 O HCl / H 2 O HAc / H 2 O Sugar / H 2 O

Relative Strengths of Acid and Bases As acid strength decreases, base strength increases; the weaker the acid, the stronger its conjugate base. As base strength decreases, acid strength increases; the weaker the base, the stronger its conjugate acid.

Position of Acid-Base Equilibrium Acetic acid (HC 2 H 3 O 2 ) is weak acid – Ionizes only slightly in water HC 2 H 3 O 2 (aq) + H 2 O(ℓ) H 3 O + (aq) + C 2 H 3 O 2 – (aq) weaker acid weaker base stronger acidstronger base Hydronium ion – Better H + donor than acetic acid – Stronger acid Acetate ion – Better H + acceptor than water – Stronger base Position of equilibrium favors weaker acid and base 19

Calculate [H + ] or [OH - ] as required for each of the following solutions at 25 o C, and state whether the solution is neutral, acidic, or basic

As T is increased, the product concentration increases, Reaction shifts to the right, Forward reaction proceeds faster, Forward reaction is endothermic.

A common way to express acidity and basicity is with pH pH = - log [H 3 O + ] or [H 3 O + ]=10 -pH Acidity and Basicity The # of sig figs in the concentration = the number of decimal places in pH In an acidic solution, [H 3 O + ]= 1.25 x M at 25 o C pH =-log (1.25 x )= -(-3.903) = 3.903

UA GenChem What is the pH of Black Coffee, [H 3 O + ]= 1.0 x M? What is the H 3 O + concentration in sea water if pH=8.30? What is the OH ¯ concentration in acid rain if pH=5.25?

[H 3 O + ], [OH - ] and pH General Basic solution pH > 7 Basic solution pH > 7 Neutral pH = 7 Neutral pH = 7 Acidic solutionpH < 7 Acidic solutionpH < 7

Other pX Scales In generalpX = -log X and so pOH = - log [OH - ] Which solution is more basic, one that has a pH of 5.5 or one with a pOH of 8.5? If K w = [HO + ] [OH - ] = 1 x If K w = [H 3 O + ] [OH - ] = 1 x Taking the log of both sides -log ( ) = - log [HO + ] + (-log [OH - ]) -log ( ) = - log [H 3 O + ] + (-log [OH - ]) pK w = 14 = pH + pOH pK w = 14 = pH + pOH The pH of a sample of human blood was measured to be 7.41 at 25 o C. Calculate pOH, [H + ], and [OH - ] for the sample.

0.1 M < M pH =-log (0.1)= -(-1) = 1

1x M = M pH ≈-log (1x10 -7 )= -(-7) ≈7 ≈ M slightly smaller

Calculate the pH of 1.0 M HF solution. coco change c eq

< 5% For any weak acid: HA → H + + A - check If < 5% pH =-log(x)=-log (2.7x10 -2 )= 1.57

autoionization of water neglected Calculate the pH of a solution that contains 1.00 M HCN (K a =6.2× ) and 5.00 M HNO 2 (Ka= 4.0×10 -4 ). Also calculate the concentration of cyanide ion (CN) in this solution at equilibrium.

coco change c eq coco change c eq

Calculate the pH of 1.0 M HF solution. coco 100 change -xxx c eq 1-xxx amount dissociated of HF = x coco 1.0 M0.1 x 2.7x  2.7%8.1%

Calculate the percent dissociation of 1.0 M and 0.10 M HAc solution. coco coco 00 change -xxx c eq c o -xxx amount dissociated of HAc = x coco 1.0 M0.1 x 4.2x x10 -3  0.42%1.3% K a (Hac)=1.8x10 -5

In a M aqueous solution, lactic acid is 3.7% dissociated. Calculate the value of Ka for this acid. coco 100 change -xxx c eq 1-xxx

0.05 M < M pOH =-log (0.05)= -(-1.3) = 1.3 Calculate the pH of 5.0x10 -2 M NaOH solution. pH =14-pOH= = 12.7

NH 3(aq) + H 2 O  NH 4 + (aq) + OH - (aq) Weak Bases Partial Ionization Small K b Bases

Calculate the pH of 15.0 M NH 3 solution (K b =1.8x10 -5 ). NH 3(aq) + H 2 O  NH 4 + (aq) + OH - (aq) coco 1500 change -xxx c eq 15-xxx pH =14-pOH= = 12.2

Calculate the pH of 1.0 M methylamine solution (K b =4.38x10 -4 ) Check the 5% rule! triprotic acid

Calculate the pH of a 5.0 M H 3 PO 4 solution and the equilibrium concentrations of the species H 3 PO 4, H 2 PO 4 -, HPO 4 2- and PO 4 3-.

coco 500 change c eq coco change-y+y c eq 0.19-y0.19+yy

coco 6.2x change-z+z c eq 6.2x z0.19+zz

1 M coco 110 change-x+x c eq 1-x1+xx 1 M

0.01 M coco change-x+x c eq 0.01-x0.01+xx 0.01 M Calculate the pH of a 0.01 M H 2 SO 4. Approximation? Solve quadratic equation:

cation anion acidic 1+ and 2+ cations conjugate acid of weak bases NH 4 + No effect on pH 3+ cations acidic As  Size of Cation,  Acidity Smaller, more concentrated charge: Means  pull of e  density from O—H bond:  acidity of H +

cation anion basic conjugate base of strong acid Cl -, Br -, I -, NO 3 - No effect on pH conjugate base of weak acid Ac -, PO 4 3-

Acid dissociation constant x constant for conjugate base Water ionization constant =

coco change-x+x c eq 0.3-x+x pH =14-pOH= = 8.31

neutral basic acidic

Binary Acids = H n X X = Cl, Br, P, As, S, Se, etc. 1.Acid strength  from left to right within same period (across row) – Acid strength  as electronegativity of X  Ex. HCl is stronger acid than H 2 S which is stronger acid than PH 3 – or PH 3 < H 2 S < HCl

Trends in Binary Acid Strength Binary Acids = H n X X = Cl, Br, P, As, S, Se, etc. X: group VIIA, from bottom to top : I, Br, Cl, F Electronegativity inccreases Bond polarity higher Bond stronger (also because size smaller) More difficult to break the bond Dissociation less, acid weaker

Trends in Oxoacid Strength Oxoacids (H n YO m ) – Acids of H, O, and one other element – HClO, HIO 4, H 2 SO 3, H 2 SO 4, etc. 1.Acids with same number of O’s and differing Y a.Acid strength  from bottom to top within group HIO 4 < HBrO 4 < HClO 4 b.Acid strength  from left to right within period H 3 PO 4 < H 2 SO 4 < HClO 4  as electronegativity of central atom  55

why not or Positive Cl Positive O  Electronegativity of atom attached to O

Trends in Oxoacid Strength Oxoacids (H n YO m ) 2.For same Y – Acid strength  as number of O’s  H 2 SO 3 < H 2 SO 4 More oxygens, remove more electron density from central atom, weakening O—H bond and make H more acidic 57

Your Turn! Which of the following is the strongest organic acid? ABC D E

Oxides of nonmetalsOxides of metals Strong covalent bond NM-O acidic Acid anhydride basic base anhydride Ionic bond M-O