Chapter Fourteen: ACIDS AND BASES

Water as an Acid and a Base “Auto-ionization” of water H 2 O + H 2 O  H 3 O + (aq) + OH - (aq) Or H 2 O  H + (aq) + OH - (aq) Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Water as an Acid and a Base “Auto-ionization” of water H 2 O (l)  H + (aq) + OH - (aq) At 25°C: K w = [H + ][OH – ] = 1.0 × 10 –14 So in pure water, [H + ] = [OH – ] = 1.0 × 10 –7 M Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Three Possible Situations [H + ] = [OH – ]; neutral solution [H + ] > [OH – ]; acidic solution [H + ] < [OH – ]; basic (alkaline) solution Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

The pH Scale pH = –log[H + ] pH decreases as [H + ] increases. [H+] = 10 -pH Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

pH Range pH = 7; neutral since in pure water, [H + ] = [OH – ] = 1.0 × 10 –7 M so pH = - log (1.0 × 10 –7 ) = 7.00 pH > 7; basic –Higher the pH, more basic (less acidic) pH < 7; acidic –Lower the pH, more acidic (more basic) Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

pOH pOH = - log [OH-] Remember that [H + ][OH – ] = 1.0 × 10 –14 So taking (–log) of both sides gives: pH + pOH = Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Models of Acids and Bases Arrhenius: Acids release H + ions in solution, bases release OH  ions. –Only NaOH, Ca(OH) 2, etc. Brønsted-Lowry: Acids are proton (H + ) donors, bases are proton acceptors. Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Brønsted-Lowry Acids and Bases Acids are proton (H + ) donors: HCl + H 2 O  Cl  + H 3 O + acid base An “ionization” reaction Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Brønsted-Lowry bases are proton acceptors. NH 3 + H 2 O  NH OH- You can’t leave the water out of Brønsted base ionization reactions Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Reversible Ionization Reactions HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq) acid base conjugate conjugate acid base Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Acid / Base Strength Acid and base “strength” refers to its extent of ionization… the percentage of the molecules that ionize. Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Acid Strength Strong acid: –Ionization is essentially 100% Considered, therefore, a non-reversible reaction Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Acid Strength Strong acid: HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq) acid base conjugate conjugate acid base mathematics : straight limiting reactant stoichiometry Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Acid Strength “THE SIX” Common Strong Acids Hydrochloric acid HCl Hydrobromic acid HBr Hydroiodidic acid HI (Note that HF is NOT a strong acid) Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Acid Strength “THE SIX” Common Strong Acids Hydrochloric acid HCl Hydrobromic acid HBr Hydroiodidic acid HI Nitric acid HNO 3 Perchloric acid HClO 4 Sulfuric acid H 2 SO 4 (but only the 1 st ionization is strong) Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

H 2 SO 4 – A special case Sulfuric acid H 2 SO 4 H 2 SO 4  H + + HSO % HSO 4 -  H + + SO 4 2- ionizes less than 2%, so HSO 4 - is a weak acid (more on “polyprotic” acids later) Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

“THE SIX” Anions that CAN’T act as a base chloride Cl - bromide Br - iodide I - nitrate NO 3 - perchlorate ClO 4 - Bisulfate HSO 4 - (a.k.a. hydrogen sulfate) You must memorize either these, or “THE SIX” strong acids!! TONIGHT! Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Acid Strength Weak acids: –Ionization equilibrium lies far to the left –The weaker the acid, stronger its conjugate base –“equilibrium math” is necessary. –“Small K” approximation usually applies! Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Identifying Weak Acids –All aqueous “HA” species except the 6 strong acids –All Carboxylic acids: R-CO 2 H –“Ammonium-based” cations CH 3 CH 2 NH 3 + NH 4 + Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Bases Strong Bases: –Group 1 and 2 metal hydroxides –Arrhenius bases, in other words! Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Identifying Weak Bases 2 Main classes: 1. All anions except “THE SIX” F- + H 2 O HF + OH- Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Identifying Weak Bases 2. Neutrally charged “N-H” compounds NH 3, (CH 3 )NH 2, C 5 H 5 N (pyridine) →.. NH 3 + H 2 O  NH OH- Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Calculating pH of Acidic Solutions For strong acids, the [H+] is the same as the molarity of the acid solution. –The pH of a 0.25 M HNO 3 solution? [H+] = 0.25 since 100% of HNO 3 ionizes pH = -log(0.25) = 0.62 Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Calculating pH of Acidic Solutions For weak acids, an ICE chart must be prepared to determine the equilibrium [H+]… which will be far less (~1-4%) than the molarity of the acid solution. Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Calculating pH of Acidic Solutions The equilibrium constant for the general reaction: HA (aq) H + (aq) + A - (aq) is Ka HA = [H+][A-] [HA] The greater Ka is, the more acidic the weak acid is. Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Calculating pH of Acidic Solutions The “small K” simplification: –Most (but not all) weak acids and bases have K values low enough to result in ionizations of 5% or less. Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Calculate the pH of a 0.50 M aqueous solution of the weak acid HF. (K a = 7.2 x ) Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide 29 HF(aq)H + (aq) + F - (aq) Initial Change Equilibrium K a HF = 7.2 x 10 -4

Steps Toward Solving for pH Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide 30 HF(aq) H + (aq) + F - (aq) Initial0.50 M~ 0 0 Change-x+x Equilibrium0.50-xxx x 2 x 2 K a = 7.2 x = ≈ 0.50 – x 0.50 x = [H + ] = 1.9 x M so pH= 1.72 (5% test: 0.019/0.50 x 100 = 3.8% Therefore approximation is valid… but not by much!)

Special “pH” Rounding Rule A pH value gets as many decimal places as there are significant figures in the [H+] An [H+] calculated from pH gets as many sig figs as there were decimal places in the pH. Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

A solution of 8.00 M formic acid (HCHO 2 ) is 0.47% ionized in water. Calculate the K a value for formic acid. Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

A solution of 8.00 M formic acid (HCHO 2 ) is 0.47% ionized in water. HForm(aq)  H + (aq) + Form - (aq) Initial 8.00 M~ 0 0 Change Equilibrium Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Bases (Reminder) Strong Bases: –Group 1 and 2 metal hydroxides Weak Bases –All anions except “THE SIX” –Neutrally charged “N-H” compounds NH 3, (CH 3 )NH 2, C 5 H 5 N (pyridine).. Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

pH of Basic Solutions Strong Bases: –There is no “reaction” needed to produce OH- ions from an ionic hydroxide –Use [OH-] to calculate pOH, then subtract from to get pH –Watch out… 0.35 M Mg(OH) 2 has [OH-] = 0.70 M Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

pH of Basic Solutions Weak Bases –Use K b of the base in the equilibrium expression The K b values for the neutral “N-H” compounds are listed in your book. The K b values for anions are a bit of an issue… they are NOT listed… more on this shortly Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Calculate the pH of a 1.0 x M solution of calcium hydroxide. Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Salts When dissolved in water, ionic compounds dissociate, which may or may not affect pH –Anions have been discussed All are basic except “THE SIX” Of “THE SIX”, only HSO 4 - is acidic Only 5 anions are “pH neutral” Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide 38

Salts –Cations (most are pH neutral… and NONE are basic) Positively charged “N-H + ” compounds are acidic Zn 2+ and all cations with a +3 or greater charge are acidic… these are called “Lewis Acids”. Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide 39

K b of Anions 1.0 x = K a ∙ K b Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

K b of Anions 1.0 x = K a ∙ K b So the K b of a basic anion = K w K a Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide Of the base’s conjugate acid

Acidic and Basic Salts Are the following salts acidic, basic or neutral? –KClO 4 –NH 4 NO 3 –CaSO 4 Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide 42

Acidic and Basic Salts What about this one? –NH 4 F It depends! Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide 43

NH 4 F: Acidic, Basic or Neutral? NH 4 +  NH 3 + H + vs. F - + H 2 O  HF + OH- Ka NH 4 + = 5.6 x Kb F- = Kw / Ka HF = 1E-14 / 7.2E-4 = 1.4 x So NH 4 + produces more H + than F - produces OH-, meaning the salt will produce an acidic solution Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide 44

Exercise: What is the pH of a 0.35 M NaNO 2 solution? Evaluate the dissociated ions to see if either affects pH… then “react” that ion with water and set up an ICE chart and go from there. Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Exercise: What is the pH of a 0.35 M NaNO 2 solution? Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Exercise: What is the pH of a 0.35 M NaNO 2 solution? NO H 2 O HNO 2 + OH - K b NO 2 - = 2.5 x = x 2 ≈ x – x 0.35 x = 3.0 x M OH - so pOH = 5.53 therefore pH = – 5.53 = 8.47 Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Concept Check Arrange the following 1.0 M solutions from lowest to highest pH. HBr NaOH NH 4 Cl NaCN NH 3 HCN NaCl HF Be able to justify your answer. Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Polyprotic Acids More than one ionizable hydrogen per molecule. Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Let’s look at phosphoric acid, H 3 PO 4. H 3 PO 4  H + + H 2 PO 4 - K a1 = 7.5 x H 2 PO 4 -  H + + HPO 4 2- K a2 = 6.2 x HPO 4 2-  H + + PO 4 3- K a3 = 4.8 x Note that the 2 nd and 3 rd ionizations have substantially lower Ka values… thus the amount of H + from them is negligible. Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

What is the pH of a 0.75 M H 3 PO 4 solution? H 3 PO 4  H + + H 2 PO 4 - K a1 = 7.5 x H 2 PO 4 -  H + + HPO 4 2- K a2 = 6.2 x HPO 4 -  H + + PO 4 3- K a3 = 4.8 x Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

What is the pH of a 0.75 M H 3 PO 4 solution? H 3 PO 4  H + + H 2 PO 4 - K a1 = 7.5 x H 2 PO 4 -  H + + HPO 4 2- K a2 = 6.2 x HPO 4 -  H + + PO 4 3- K a3 = 4.8 x Only the first ionization provides significant H +, so 7.5 x = x 2 graphed x = M H –x so pH = 1.15 Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Amphiprotic Species Water is amphiprotic: –Meaning that water can act as either a proton donor (Brønsted acid) or a proton acceptor (Brønsted base) Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Amphiprotic Species Water is amphiprotic: –Meaning that water can act as either a proton donor (Brønsted acid) or a proton acceptor (Brønsted base) Other amphiprotic species: –Any anion with an ionizable hydrogen (HA - ) except HSO 4 - Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Amphiprotic Species Water is amphiprotic: –Meaning that water can act as either a proton donor (Brønsted acid) or a proton acceptor (Brønsted base) Other amphiprotic species: –Any anion with an ionizable hydrogen (HA - ) except HSO 4 - Examples: HPO 4 2-, H 2 PO 4 -, HCO 3 - Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Amphiprotic Species Water is amphiprotic: –Meaning that water can act as either a proton donor (Brønsted acid) or a proton acceptor (Brønsted base) Other amphiprotic species: –Any acidic anion (HA - ) except HSO 4 - Examples: HPO 4 2-, H 2 PO 4 -, HCO 3 - (HSO 4 - is acidic, but not basic) Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Amphiprotic Species Are amphiprotic anions acidic or basic? Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide 57

Amphiprotic Species Are amphiprotic anions acidic or basic? It depends! Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide 58

Amphiprotic Species Are amphiprotic anions acidic or basic? It depends! Example: Is NaHCO 3 an acidic or basic salt? HCO H 2 O  CO H 3 O + K a HCO 3 - = K a 2 H 2 CO 3 = 5.6x HCO H 2 O  H 2 CO 3 + OH - K b HCO 3 - = K w /K a 1 H 2 CO 3 = 1.0 x x = 2.3 x Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide 59

Structures of Oxyacids Oxyacids: the hydrogens are bonded to the oxygens, regardless of how the molecular formula is written: H 2 SO 4 is Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Structures of Oxyacids The acid strengths of a series of oxyacids can be ranked by the number of oxygens… the more oxygen atoms the stronger the acid. For example: –HClOKa = 3.5 x –HClO 2 Ka = 1.2 x –HClO 3 Ka = 1.0 –HClO 4 Ka = undefined Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Structures of Oxyacids This is due to the “inductive effect” that the very electronegative oxygens have on the bond between the hydrogen and the oxygen it’s bonded to. When electron density is pulled away from the hydrogen, the bond weakens, allowing it to ionize more easily. Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Acidic and Basic Oxides All nonmetal oxides are acidic when dissolved in water: CO 2 + H 2 O  H 2 CO 3  H + + HCO 3 - (CO 2 is the source of our “naturally” acidic rain… unpolluted rainwater pH ≈ 5.3) NO 3 + H 2 O  HNO 3 + other “HNO” products (NO 3, NO x and SO x pollutants are the cause of “acid rain” that cause environmental damage) Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Acidic and Basic Oxides All metal oxides are basic when dissolved in water –Because they are ionic, and O 2-, like most anions, is basic: Na 2 O: O 2- + H 2 O  OH - + OH - Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Lewis Acids and Bases No longer covered on the AP Exam But I still require you to know that… Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide

Lewis Acids and Bases …this concept accounts for the fact that Zn 2+ and +3 or greater cations cause solutions to be acidic The pH of a 0.010M AlCl 3 solution is 3.43 Copyright © Houghton Mifflin Company. All rights reserved.Chapter 14 | Slide