Chapter 14 - Aqueous Equilibria. Acids and Bases

Chapter 14 - Aqueous Equilibria: Acids & Bases CHM152 GCC Why study acids & bases? Acids and bases are important in numerous reactions, from industrial processes to biological ones. Many household substances, including cleaning solutions and food/beverages that we consume, are acids or bases.  In the environment, the pH of rain, water and soil can also have significant effects. Acids Taste sour Corrode most metals Turn litmus paper red React with bases to form salt and water  examples: vinegar, lemon juice, gastric juice, soft drinks Bases Taste bitter, chalky Feel soapy, slippery Turn litmus paper blue React with acids to form salt and water examples: milk of magnesia, bleach, ammonia, detergents Acid-Base Concepts Bronsted Lowry reactions involve the transfer of 1 H+ ion • CHM 151 taught Arrhenius theory  Acids produce hydrogen ions in water  HA (aq)  H+ (aq) + A- (aq)  Bases produce hydroxide ions in water  MOH (aq)  M+ (aq) + OH- (aq) • 152 uses Brønsted-Lowry theory  Acid: Substance that can donate H+ Acid loses H+  Base: Substance that can accept H+ Base gains H+ • Identify the acid, the base, the conjugate acid and conjugate base. 1 Acid-Base Pairs Identify Conjugate Acid or Base a) What is the conjugate acid of CO32b) What is the conjugate base of H2PO4-? • Identify the acid-base pairs above. • Note: H2O is amphoteric – it can act as an acid or a base depending on what its reacting with. Strength of Acids/Bases Conjugate Acid-Base Pair Trends 1. A stronger acid loses its proton more readily than a weaker acid and a stronger base gains a proton more readily than a weaker base. 2. The stronger the acid, the weaker its conjugate base. Likewise, the stronger the base, the weaker its conjugate acid. 3. Proton transfer reactions proceed from the stronger acid to the stronger base. Thus, the equilibria lies on the side of the weaker acidbase pair. Strengths of Acids/Bases • Competition for protons (H+) e.g. HS-(aq) + HF(aq)  H2S(aq) + F-(aq) From Table 15.1 • Which acid is stronger? • Which base is stronger? • Which side does equilibria lie on? Strong Acids a substance that ionizes completely in water to form H3O+ ions (H+). SA’s are strong electrolytes. HNO3(aq) + H2O(l) → H3O+(aq) + NO3- (aq)  A one way arrow is used since this reaction is complete, all of the HNO3 molecules break apart to form H3O+ and NO3- ions.  These acids have very weak conjugate bases  For SA, eq lies very far to the right side! KNOW 7 Strong acids! Memorize these! HCl, HBr, HI, HClO4, HNO3, H2SO4 , HClO3 2 Weak Acids Strength of Acids an acid that ionizes only to a small extent in H2O. WA’s are weak electrolytes! E.g. HCN(aq) + H2O(l)  H3O+(aq) + CN-(aq) • A two-way arrow is used since this reaction is not complete but instead consists of an equilibrium mixture of HCN molecules, H3O+ and CN- ions • Equilibrium lies to the left since most of the acid molecules have not ionized. • Common WA’s: HF, HNO2, HCN, H3PO4, H2CO3, carboxylic acids like CH3COOH Strong Bases A substance that ionizes completely in water to form OH- ions. SB’s are strong electrolytes! SB dissociates in water; it doesn’t react with it: • HCl, H2SO4 • HNO2, HF • H2O Weak Base A base that ionizes only to a small extent in H2O. WB’s are weak electrolytes! NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq) NaOH(aq)  Na+ (aq) + OH- (aq) (100 % ions) •The reaction goes to completion – SB’s completely dissociate into ions. Strong Bases: These Group 1A & 2A hydroxides LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2, Ba(OH)2, Sr(OH)2 Memorize the 8 SB’s! Equilibrium lies to the left since most of the ammonia molecules have not accepted protons to form NH4+ and OH- ions. Common examples of WB’s are ammonia and amines (e.g. CH3NH2) SA, WA, SB, WB solutions Hydronium Ions Draw the following in water: HNO3, HF, Ca(OH)2, and NH3. Assume you have 3 formula units or molecules for each substance. • HA (aq)  H+ (aq) + A- (aq) • H+ is very reactive and will bond with O in H2O to form H3O+ • H+ and H3O+ mean the same thing but H3O+ is more accurate 3 Dissociation of Water • H2O (l) + H2O (l)  H3O+ (aq) + OH- (aq) • Identify the acid-base pairs • This is the dissociation of water, an amphoteric substance. • Write an equilibrium expression for this eqn. • Kw is ion-product constant for water. • Calculate Kw if [H3O+] = 1.0 x 10-7 (experimentally determined for pure H2O) Acidic, Basic, Neutral Solns Dissociation of Water • • • • • • • Kw = [H3O+][OH-] = 1x10-14 We can compare [H3O+] & [OH-] for solutions. They’re equal in neutral solutions. Which one is greater in acidic solutions? Basic solutions? [H3O+] = 5.6 x 10-4 M. What is [OH-]? Is solution acidic or basic? pH Scale • Logarithmic scale (like Richter scale for earthquakes). • [H3O+] = 1x10-2 M has a pH of 2 • [H3O+] = 1x10-12 M has a pH of 12 • pH = -log [H3O+] [H3O+] = 10-pH • pOH = -log [OH ] [OH-] = 10-pOH • Kw = [H3O+][OH-] = 1x10-14 • pH + pOH = 14 pH Sig Figs 2 sig figs [H3 O+] = 5.6 x 10-4 M 2 decimal places Group Quiz #9 Calculate [H3O+], [OH-] and pH for a solution if the pOH is 10.325. • pH = -log (5.6 x 10-4) = 3.25 (3 related to exponent; tells us acidity) Find [H3O+], pH, and pOH if [OH-] = 9.8x10-9 M 4 pH of Strong Acids/Bases HNO3(aq) + H2O(l)  H3O+(aq) + NO3-(aq) Calculate pH of 0.103 M HNO3 solution. Sr(OH)2  Sr2+(aq) + 2OH-(aq) Calculate pH of 0.020 M Sr(OH)2 soln. Calculate [H3O+], [OH-], pH, and pOH of a solution made by adding 2.250 g of LiOH to 250.0 mL of water. pKa’s of Weak Acids Equilbria of Weak Acids • HA(aq) + H2O(l)  H3O+(aq) + A-(aq)  Write equilibrium expression. • pKa = -log Ka: stronger acid = lower pKa • % Ionization = [H3O+] / [HA]i x 100% • As acid strength increases, indicate if the following will be higher or lower:      [H3O+] pH % ionization Ka pKa Weak Acid Problems • Set up ICE tables & Ka • With small K values, assume x will be small to simplify math. WA problem types: • Given [HA]i and Ka, find all equilibrium concentrations and pH • Given [HA]i and pH, find all equilibrium concentrations and Ka • Given [HA]i and %dissociation, find eq concentrations, pH and Ka Find pH of Weak Acid Find the equilibrium concentrations for all substances and the pH of a 0.50 M solution of HOCl (hypochlorous acid). Ka = 3.5 x 10-8 ICE Table HOCl + H2O  Find Ka for weak acid • Find the Ka of a 1.25 M solution of nitrous acid, HNO2. The pH of this solution is measured to be 1.62. H3O+ + OCl- I C E Example 14.10, Problems 14.14, 14.15 5 Percent Ionization • % dissociation is the same calculation as checking assumption of a small x.  % dissociation = ([H3O+]eq / [HA]i) x 100 • Will the percent dissociated increase or decrease as an acid is diluted? • Calculate the percent dissociation from the previous problem. Dilution and % Dissociation • As we add water, we can dissociate more. More dilute = more dissociation. Find Ka for weak acid Group Quiz #10 • A 0.200 M solution of a weak acid is 9.4 percent dissociated. Using this information, calculate [H3O+], [A-], [HA]eq, and Ka. • The acid-dissociation constant for benzoic acid, HC7H5O2, is 6.3 x 10-5. Calculate the equilibrium concentrations of H3O+, C7H5O2-, and HC7H5O2 in the solution if the initial concentration of the acid is 0.50 M. Polyprotic Acids Weak Bases • H2C2O4(aq) + H2O(l)  HC2O4-(aq) + H3O+(aq)  Ka1 = [HC2O4-][H3O+] / [H2C2O4] = 5.9 x 10-2 • HC2O4-(aq) + H2O(l)  C2O42-(aq) + H3O+(aq)  Ka2 = [C2O42-][H3O+] / [HC2O4-] = 6.4 x 10-5 • In general, Ka1 > Ka2 > Ka3 … Why? • pH of a polyprotic acid solution primarily arises from [H+] formed in the 1st step. • Similar to WA problems but base hydrolysis reaction forms OH- (this is x!) and use Kb • NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq) Kb = [NH4+][OH-] / [NH3] • Table 14.4: Kb Values for Weak Bases • Why are amines (derivatives of NH3) basic? • Lone e- pair (d-) on N attracts H (d+) from water. N wins the competition and pulls H+ off of water. 6 Weak Bases • Calculate the pH of a 0.50 M dimethylamine ((CH3)2NH) solution. Kb = 5.4 x 10-4 Group Quiz #11 A 0.065 M solution of methylamine, CH3NH2, has a pH of 11.70. What is Kb for CH3NH2? Example 14.12, Problems 14.19, 14.20 Relation Between Ka and Kb • • • • • • Weak acid: Ka = [H+][A-] / [HA] Weak base: Kb = [HA][OH-] / [A-] Multiply: Ka * Kb Ka·Kb = [H+] [OH-] = Kw Ka·Kb = 10-14 True only if the acid and base are conjugate pairs! What is the Kb for hydrazine (N2H4) if Ka for N2H5+ is 1.1 x 10-8? Properties of Salts • Acid + Base  Salt + Water • Salts are ionic products of acid-base reaction. • Cation in salt: comes from the base • Anion in salt: comes from the acid • Salts can be neutral, acidic, or basic depending on the strength of the acid or base from which they’re made • Example 14.13, Problem 14.21 Hydrolysis Reactions • Many salts cause pH shifts when dissolved in water. These salts contain the conjugate acid (or base) of a weak base (or acid). NH4Cl: NH4+ + H2O  NH3 + H3O+ • pH < 7 NaCH3CO2: CH3COO- + H2O  CH3COOH + OH• pH > 7 • These reactions are called hydrolysis. General Ion Categories 1) Acidic ions: Most cations (except 1A & 2A) E.g. NH4+, Al3+, Cu2+ 2) Neutral Ions: cations from strong bases and anions from strong acids Cations: Group 1A & 2A (except Be) Anions: NO3-, Cl-, Br -, I-, ClO4-, ClO3-(except HSO4-) 3) Basic Ions: Most anions derived from weak acids. E.g. F-, CN-, CH3COO- , NO2- 7 Types of Salts • Neutral Salts  NaOH + HCl  NaCl + H2O  Acid and base are strong, but resulting salt is neutral.  Neutral ions don’t hydrolyze! Types of Salts Basic Salts • Salts made from weak acids and strong bases give basic salts • Anion from weak acid; cation from strong base • Give an example. • Acidic Salts  HCl + NH3  NH4Cl  strong acid + weak base  Ammonium salts are acidic (cation from weak base, anion from strong acid) Example Salt Problems • Is KF an acidic, basic or neutral salt?  Write the hydrolysis reaction and calculate the pH of a 0.10 M KF solution. Ka (HF) = 3.5x10-4 • Is NH4Cl acidic, basic, or neutral?  Write the hydrolysis reaction and calculate the pH of a 0.10 M NH4Cl solution. Kb (NH3) = 1.8 x 10-5 Group Quiz #12 • Classify the following salts as acidic, basic, or neutral. For acidic or basic salts, write the equation of hydrolysis.  NH4ClO3  Ba(NO2)2  CaI2 Acidic cations and basic anions  (NH4)2CO3: NH4+ acts as weak acid; CO32- acts as weak base. Have to compare Ka and Kb values in order to determine if its acidic or basic. Classify Salts • Classify the following salts as acidic, basic, or neutral. For acidic or basic salts, write the reaction of hydrolysis.  KBr  NaF  LiCN  NH4Cl Lewis Acids and Bases • Broadest acid base definition since Lewis acids don’t have to contain H+. • Looks at electron transfer instead of H+ transfer • Acids are electron-pair acceptors, bases are electron-pair donors  H+ + :NH3  NH4+  Ammonia donates the electron pair to H+ to make the bond; H+ accepts the electron pair This definition is used in CHM235/236 where you learn to push e- pairs! 8 Lewis Acids and Bases Metal cations= Lewis acids Small, highly charged metals also make acidic salts; Al(H2O)63+ (Ka = 1.4 x 10-5) High charge on metal  electrons on O are attracted to +metal cation. Metal acids • Al3+ forms acid solutions! Easier to break O-H bonds and form hydronium! Acid Strength Factors Acid Strength Factors • Why is HF weak but HCl, HBr, and HI strong? • Determined by strength and polarity of H-X bond. • H-F is strongest in series (H and F are small and close together); other bonds are weaker and break easily in water Acid Strength Factors • In the same row, bond strengths are relatively similar, but electronegativity changes are greater 9