Unformatted text preview: Chapter 16 Acid-Base Equilibria
q Acids and bases are found in many common substances and are important in life processes. Which of the following are acids and which are bases? q Oven Cleaner q Sodium Carbonate q Vinegar q Windex q Vitamin C (Ascorbic Acid) q Soap Chapter 16 Acid-Base Equilibria
q There are many organic/biological acids and bases, and there are acids used industrially for many purposes. q Example: H2SO4 q Used in the production of fertilizers, paint dyes, plastics, pharmaceuticals, ... q The acid in lead storage batteries in most cars 16.1 Acids and Bases
q Acid: Base: tastes sour tastes bitter stings skin feels slippery corrosive to metals releases CO2 from carbonates turns litmus red turns litmus blue turns phenolphthalein colorless turns ph.ind. pink Arrhenius Theory
q Acid: substance that produces H+ in HCl aqueous solution q Base: substance that produces OH- in aqueous solution NaOH q Not realistic: H+ has a radius of 10-13 cm, which gives a very concentrated charge, so it associates with H2O as H(H2O)4+, which we usually simplify to H3O+ or H+ (aq) 16.2 Bronsted-Lowrey Acids and Bases
q Acid: donates a proton to another substance q Base: accepts a proton from another substance q Acids: HCl(aq) + H2O(l) H3O+ (aq) + Cl-(aq) h NH4+(aq) + H2O(l) h H3O+ (aq) + NH3(aq) q Bases: h NH3(aq) + H2O(l) h OH-(aq) + NH4+ (aq) h CO32-(aq) + H2O(l) h OH-(aq) + HCO3-(aq) Bronsted-Lowrey Acids and Bases
Transfer of a proton (hydrogen ion) Conjugate Acids and Bases
q Pair of substances differing only by one H+ q HF(aq) + H2O(l) h H3O+ (aq) + F-(aq) acid base acid base q CO32- (aq) + H2O(l) h HCO3-(aq) + OH-(aq) Which of these is a correct pairing of an acid and its conjugate base (in that order)? 1. HCO3-, H2CO3 2. NH3, NH4+ 3. H3PO4, H2PO44. H2PO4-, H3PO4 0%
4 Conjugate Acid-Base Pairs
q Determine the formula of the conjugate acid of each base. Base Conjugate Acid Amphoprotism
q Amphoprotic substances can act as either an acid or a base.
q HCO3-(aq) + OH-(aq) h CO32-(aq) + H2O(l) q HCO3-(aq) + HCl (aq) h H2CO3(aq) + Cl-(aq) h 1 Amphoprotism
q Write equations to describe the reactions of HPO42- with : q OHq HCl 1 Polyprotic Acids
q Some acids can donate more than one proton. q Mono- di- tri- ... polyq Diprotic: H2SO4, H2CO3, H2SO3, q Not all H atoms are acidic. Example: acetic acid, CH3CO2H q In oxoacids, the acidic hydrogens are bonded to oxygen. 1 Polyprotic Acids
q How many acidic hydrogens? H2SO4 H3PO4 CH3CH2CO2H 1 Acid Strengths
q We can measure the relative tendency for acids to donate protons (usually relative to the solvent). Acids are rated from strong to weak. q Strong acid: Complete donation of proton to water Examples
q HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq) q H2SO4(aq) + H2O(l) H3O+(aq) + HSO4-(aq) 1 Strong Acid: HNO3
q HNO3 is completely ionized into the ions H3O+ and NO3- 1 Acid Strengths
In water: q Any acid that is stronger than H3O+ (the conjugate acid of the solvent) will ionize completely to form H3O+ q H3O+ is the strongest acid we can find in H2O 1 Acid Strengths
q Weak acid: partial donation of protons to water. q HF(aq) + H2O(l) h H3O+(aq) + F-(aq)
weaker weaker acid base stronger acid stronger base q The reaction reaches a state of equilibrium in which the relative acid strengths determine the degree of ionization. 1 Weak Acid: HF
q HF is only partially dissociated into H3O+ and F- 1 1 Predicting Position of Equilibrium
q All acids above H3O+ in the table are strong acids, which ionize completely in aqueous solution. q All bases below OH- in the table are strong bases, which react completely in aqueous solution. q The table can be used to make predictions: The stronger acid reacts with the stronger base to form a weaker acid and a weaker base. 2 Predicting Acid-Base Reactions
q HCl + HSO3- h q stronger stronger acid base H2SO3 + Clweaker weaker acid base Which side is favored at equilibrium? 2 2 In the equilibrium given, will the product or reactant side be favored?
F- + HSO4- HF + SO42- 91% 1. 2. 3. 4.
Product Reactant Neither Can't Tell
4 2 16.3 Autoionization of Water
q Water undergoes autoionization: H2O + H2O h H3O+ + OHh K = [H O ][OH ] = 1.0 x 10 at 25oC q
w 3 + -14 Called the ion product of water q Since [H2O] is constant, it is omitted 2 16.3 Autoionization of Water
q Add an acid, [H3O+] > [OH-] q Example: q 0.10 M HCl (strong acid, completely ionized) [H3O+][OH-] = 1.0 x 10-14 (0.10)[OH-] = 1.0 x 10-14 [OH-] = 1.0 x 10-14 /0.10 = 1.0 x 10-13 M 2 Autoionization of Water
q Add a base, [H3O+] < [OH-] q Example: 0.010 M NaOH (strong base, completely ionized) [H3O+][OH-] = 1.0 x 10-14 [H3O+](0.010) = 1.0 x 10-14 [H3O+] = 1.0 x 10-14 /0.010 = 1.0 x 10-12 M q Acidic Solution [H3O+] > 10-7 M 2 Find the [H3O+] for the solutions given below.
Solution A: 1.3 x 10-2M HNO3 Solution B: 0.020M NaOH + 3 A + 3 B 86% 1. [H O ] =1.3x10-2M, [H O ] =2.0x10-2M 2. [H3O+]A=7.7x10-13M, [H3O+]B=2.0x10-2M 3. [H3O+]A=1.3x10-2M, [H3O+]B=5.0x10-13M 4. [H3O ]A=7.7x10 M, [H3O+]B=5.0x10-13M
+ -13 11% 3%
2 3 4 2 16.4 The pH Scale
q pH scale defined to make it easier to refer to relative [H3O+] or [OH-] values q pH = -log [H3O+] q pOH = -log [OH-] q [H3O+] = 1.0 M q [H3O+] = 1.0 x 10-14 M q [H3O+] = 1.0 x 10-7 M pH = 0 pH = 14 pH = 7 2 16.4 The pH Scale
q pH scale is defined to make it easier to refer to relative [H3O+] or [OH-] values q pH = -log [H3O+] q pOH = -log [OH-] q [H3O+] = 1.0 M q [H3O+] = 1.0 x 10-14 M q [H3O+] = 1.0 x 10-7 M pH = 0 pH = 14 pH = 7 pOH= pOH= pOH= 2 16.4 The pH Scale
q pH scale is defined to make it easier to refer to relative [H3O+] or [OH-] values q pH = -log [H3O+] q pOH = -log [OH-] q [H3O+] = 1.0 M q [H3O+] = 1.0 x 10-14 M q [H3O+] = 1.0 x 10-7 M pH = 0 pH = 14 pH = 7 pOH=14 pOH=0 pOH=7 3 pH Scale
q pH + pOH = 14 = pKw q Neutral: pH=pOH = 7 q Acidic: pH < 7 pOH > 7 q Basic: pH > 7 pOH < 7 3 Fig. 16.5 3 16.5 Strong Acids and Bases
q The strong common acids are HCl, HBr, HI, HNO , HClO , HClO , and H SO . 3 3 4 2 4 q Strong acids are strong electrolytes. q All strong acids ionize completely in 3 Strong Acids and Bases
q Most ionic hydroxides are strong bases (e.g. NaOH, KOH, and Ca(OH)2). q Strong bases are strong electrolytes and dissociate completely in solution. q The pOH (and hence pH) of a strong base is obtained from the initial molarity of the base. Be careful of stoichiometry. 3 What are the pH and pOH of the following solutions?
1. A: pH=2, pOH=12; B: pH=9, pOH=5 Solution A: 0.20 M HCl 2. A:pH=0.7, pOH=13.3; B:pH=12.7, pOH=1.3 3. A:pH=0.7,pOH=13.3; B:pH=1.3,pOH=12.7
Solution B: 0.050 M NaOH 0%
3 3 pH Calcs
q When we substitute [HCl] for [H3O+], we assume that: [H3O+] from HCl >> [H3O+] from H2O 3 Changes in pH with Dilution
qWhat is the pH for each dilution?
q What is the pH of 1.0 M HCl? q pH = 0.00 q What is the pH of 0.10 M HCl (a 1:10 dilution)? q pH = 1.00 q What is the pH of 0.010 M HCl? q pH = 2.00 3 Changes in pH with Dilution
q What is the pH of 1.0 x 10-3 M HCl? q pH = 3.00 q What is the pH of 1.0 x 10- 4 M HCl? q pH = 4.00 q What is the pH of 1.0 x 10-5 M HCl? q pH = 5.00 q What is the pH of 1.0 x 10-6 M HCl? q pH = 5.996 3 Changes in pH with Dilution
q What is the pH of 1.0 x 10-7 M HCl? q pH = 6.791 q What is the pH of 1.0 x 10-8 M HCl? q pH = 6.996 q Why does the pH stop changing at a value of about 7? q Water has a pH of 7 due to autodissociation, so it is never possible to get a pH higher than 7 by addition of water. 3 Strong Acids and Bases
q Be careful of stoichiometry!!! q What is the pOH of 0.1 M Ca(OH)2? The pH? q pOH = 0.7, not 1.0; pH = 13.3 4 Strong Acids and Bases
q Which of the following solutions should have the highest pH? q 0.01 M NaOH q 0.01 M Ba(OH)2 4 How do we Measure pH?
q Litmus or pH paper q Color changes of indicators q Voltage generated by electrodes (pH meter) 4 pH Indicators 4 Concentration from pH
q pH = -log[H3O+] q Rearrange: q [H3O+] = 10-pH q Similarly: q [OH-] = 10-pOH 4 A 50-L solution has a pH of 9.5. What is [OH-], and how many moles of OHare there?
25% 25% 25% 25% 1. [OH-]=3.2 x 10-10 moles = 1.6 x 10-8 2. [OH-]=3.2 x 10-5 moles = 1.6 x 10-3 3. [OH-]=4.6 x 10-4 moles = 2.3 x 10-2 4. [OH-]=4.6 x 10-7 moles = 2.3 x 10-5 1 2 3 4 4 ...
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