All Your Acids and Bases are Belong to us...acid-base Equilibria: Ch Acid-Base Def ns. I. Properties

Transcription

1 Acid-Base Def ns. I. Properties Acids Taste sour Feel like water Corrosive ph<7 Conducts elec. II. There are three ways to define acids and bases. A. Arrhenius - Focused on what it dissolves into. 1. Acids: dissolves into H + or H3O + a. HC2H3O2 (aq) + H2O(l) H3O + (aq) + C2H3O2 - (aq) b. HCl(aq) H + (aq) + Cl - (aq) 2. Bases: dissolves into OH - a. NaOH (aq) Na + (aq) + OH - (aq) B. Bronsted-Lowry -Focused on proton (H + ). 1. Acids: proton (H + ) donor (shake) a. Ex: HF(aq) + H2O(l) H3O + (aq) + F - (aq) 2. Bases: proton (H + ) acceptor (take) a. Ex: NH3(aq) + H2O(l) NH4 + (aq) + OH - (aq) C. Lewis - Focused on lone pairs. 1. Acids: lone pair acceptor a. Ex: HF(aq) + H2O(l) H3O + (aq) + F - (aq) 2. Bases: lone pair donor a. Ex: NH3(aq) + H2O(l) NH4 + (aq) + OH - (aq) Bases Taste bitter Feel slippery Corrosive ph>7 Conducts elec. D. Water 1. It is neutral, ph=7 2. It is amphoteric or amphiprotic (it can act as an acid or as a base) III. Conjugate acids and bases: A. A pair of compounds that differ by a proton (H + ) B. They are always on the product side. C. An acid loses an H + to become its conjugate base. 1. HCl(aq) H + (aq) + Cl - (aq) D. A base gains an H + to become its conjugate acid. 1. NH3(aq) + H + (aq) NH4 + (aq) E. Label: 1. NH3(aq) + H2O(l) NH4 + (aq) + OH - (aq) 2. HF(aq) + H2O(l) H3O + (aq) + F - (aq)

2 Acid-Base Strength I. First, the acid dissociation constant, Ka A. Measures the degree in which acids dissociate. B. It is the equilibrium constant expression for an acid dissociating. C. Write the acid dissociation constant expression for: 1. HCl(aq) H + (aq) + Cl - (aq) 2. NH3(aq) + H + (aq) NH4 + (aq) 3. HF(aq) + H2O(l) H3O + (aq) + F - (aq) D. The higher the Ka or Kb, the more dissociation, the stronger the acid/base. II. Strong acids and bases dissociate completely A. ~100% dissociation B. Acids: HNO3, H2SO4, HCl, HBr, HI, HClO3, HClO4 C. Bases:LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2, Sr(OH)2, Ba(OH)2 D. Strong electrolytes III. Weak acids and bases do not dissociate completely A. ~1% dissociation B. Weak acids: C. Weak bases: ammonia, amines D. Weak electrolytes

3 IV. Comparing acid/base strength to conjugate acids/bases A. Strong acids/bases have weak conjugate acids/bases. B. Weak acids/bases have strong conjugate acids/bases.

4 V. Measuring acid/base strength A. Measured using ph and poh 1. ph = -log[h + ] = -log[h3o + ] 2. poh = -log[oh - ] 3. [H3O + ] = [H + ] = 10 -ph 4. [OH - ] = 10 -poh 5. Sig. figs with logs. a. When taking the antilog of a quantity, #of decimals of the quantity = # of sig figs of your answer. b. When taking the log of a quantity, # of sig figs of your answer = #of decimals of the quantity. B. The autoionization of water: 1. H2O(l) + H2O(l) H3O + (aq) + OH - (aq) 0r H2O(l) H + (aq) + OH - (aq) 2. The equilibrium conc. of H + and OH - is 1.00x10-7 M because [H + ] = [OH - ] when ph=7/neutral 3. Kc=Kw=[H + ][OH - ] 4. Kw=(1.00x10-7 M)(1.00x10-7 M) = 1.00x10-14 M 5. log K w = log [H + ] + log [OH ] = Therefore, pk w = ph + poh = VI. Calculating ph or poh A. To find the [H + ] or [OH - ] 1. for strong acids/bases, because they completely dissociate, [acid] or [base] = [H + ] or [OH - ] respectively a. Ex: [HCl] = 0.1M = [H + ] b. ph = -log(o.1m) = 1 c. What is the ph of a solution prepared by dissolving 1.00g of barium hydroxide into 1 L of water? 2. For weak acids and bases, because they do not dissociate completely, you usually will have to use the ICE chart method. a. You will need Ka/Kb and the constant exp. b. If ph or poh is given, you can find [H + ] or [OH - ] c. pka = -log Ka and pkb = -log Kb d. Examples...

5 3. To calculate percent dissociation: a. Find [H + ] or [OH - ], divide by original conc. then multiply by 100% b. % diss. = [H + ] or [OH - ] x 100% orig. conc. B. Always keep your head in the beaker. VII. Polyprotic Acids: A. Acids that have more than one H + B. Monoprotic acid and dissociation: HCl(aq) H + (aq) + Cl - (aq) C. Diprotic acid and dissociation: H2SO4(aq) H + (aq) + HSO4 - (aq) Ka Ka1 HSO4 - (aq) H + (aq) + SO4 2- (aq) D. Triprotic acid and dissociation: H3PO4(aq) H + (aq) + H2PO4 - (aq) H2PO4 - (aq) H + (aq) + HPO4 2- (aq) HPO4 2- (aq) H + (aq) + PO4 3- (aq) Ka2 Ka1 Ka2 Ka3 E. Ka1 >> Ka2 >> Ka3 F. Example: Calculate the ph and concentration of all species of a 3.0-M solution of phosphoric acid. G. Ka/Kb of total reaction = the product of the Ka s/kb s of each reaction. 1. Ex. Ka=Ka1xKa2xKa3 for a triprotic acid. Acid-Base Prop s of Salts I. Salts, when dissolved in water, can create acidic/basic solutions. A. Salts of weak acids dissolve to form its conjugate base (ex. sodium acetate). 1. NaC2H3O2(aq) Na + (aq) + C2H3O2 - (aq) 2. But there s water in solution! 3. Don t worry about the sodium ion because it is the conjugate acid of a strong base so it s not going to react to form the strong base (because it dissociates 100%) 4. This is also happening: C2H3O2 - (aq) + H2O(l) HC2H3O2(aq) + OH - (aq) 5. Thus, a basic solution is formed! B. Salts of weak bases dissolve to form its conjugate acid (ex. ammonium chloride). 1. NH4Cl(aq) NH4 + (aq) + Cl - (aq) 2. Don t worry about the chloride ion because it is the conjugate base of a strong acid so it s not going to react to form the strong acid (because it dissociates 100%) 3. This is also happening: NH4 + (aq) + H2O(l) NH3(aq) + H3O + (aq) 4. Thus, an acidic solution is formed!

6 Calculating w/ Salts I. First a bit about water again... A. Kw=Ka x Kb, Kw=1.00x10-14 M B. Ka=Kw / Kb, Kb=Kw / Ka II. If you are given Ka for the reverse reaction, you will need to solve for Kb using the above equation and vice versa. III. This is valid only for conjugate pairs. IV. Example: What is the ph of a 0.140M solution of sodium acetate? IV. Example: What is the ph of a 0.140M solution of diethyl amino chloride? V. Predicting ph for a solution that has both acidic and basic properties. A. If Ka > Kb, ph < 7 (acidic) B. If Kb > Ka, ph > 7 (basic) C. If Ka = Kb, ph = 7 (neutral) D. Predict whether an aqueous solution of each of the following salts will be basic, acidic, or neutral a. NH4C2H3O2 b. NH4CN c. Al2(SO4)3

7 Structure Effects on Acidity/Basicity I. Acid Halides (halogens) A. Acidity depends on the ability to lose H +. B. If bond strength is high, proton is less likely to dissociate, making it a weak acid. C. If bond strength is low, proton is more likely to dissociate, making it a strong acid. II. Oxoacids A. Oxygen is very electronegative. B. The more oxygen you have, the more they will attract electrons away from the H. This makes the proton dissociate more easily, resulting in a stronger acid. C. All things being equal, the more O s there are, the stronger it is. D. Example: chlor... Rxn Prediction Review I. Soluble metal oxides + water --> metal hydroxide A. Na2O + H2O --> 2NaOH II. Nonmetal oxides + water --> acid B. P2O5 + 3H2O --> 2H3PO4