Chapter 14. Acids and Bases

Transcription

1 Chapter 1. Acids and Bases

2 Reactions in aqueous solutions Precipitation reactions Acid-Base reactions xidation-reduction reactions Precipitation reaction : a reaction which results in the formation of an insoluble products, or precipitate. Acid-Base reaction : a reaction between an acid and a base. xidation-reduction reaction : intermolecular electron transfer reaction

3 Nature of Acids and Bases Properties of acids and bases Acids ave a sour taste. Vinegar owes its taste to acetic acid. Citrus fruits contain citric acid. React with certain metals to produce hydrogen gas. React with carbonates and bicarbonates to produce carbon dioxide gas Bases ave a bitter taste. Feel slippery. any soaps contain bases.

4 Nature of Acids and Bases Acid Definitions Base Arrhenius + producer - producer Brønsted-Lowry + donor + acceptor Lewis e - pair acceptor e - pair donor Defined only in aqueous solutions Development of concept acid base Cl(aq) + Na(aq) (l) + NaCl(aq) + (aq) + Cl - (aq) Na + (aq)+ - (aq) + (aq) + - (aq) (l)

5 Nature of Acids and Bases Acid Definitions Base Arrhenius + producer - producer Brønsted-Lowry + donor + acceptor Lewis e - pair acceptor e - pair donor Defined in both aqueous and non-aqueous solutions Development of concept base acid (l) Cl(aq) 3 + (aq) Cl - (aq) (hydronium ion) B + A B + + A - conjugate conjugate conjugate conjugate base 1 acid acid 1 base conjugate base: everything that remains of the acid molecule after a proton is lost. conjugate acid: formed when the proton is transferred to the base.

6 Nature of Acids and Bases Acid Definitions Base Arrhenius + producer - producer Brønsted-Lowry + donor + acceptor Lewis e - pair acceptor e - pair donor Defined in both aqueous and non-aqueous solutions Development of concept N 3 (g) + Cl(g) N Cl(s) N + Cl - B + A B + + A - conjugate conjugate conjugate conjugate base 1 acid acid 1 acid 1 conjugate base: everything that remains of the acid molecule after a proton is lost. conjugate acid: formed when the proton is transferred to the base.

7 Nature of Acids and Bases Acid dissociation (ionization) reactions Cl(aq) + (l) 3 + (aq) + Cl - (aq) A(aq) + (l) 3 + (aq) + A - (aq) [ 3 [ A [ 3 [ A [ [ A C[ A a [ 3 [ A [ A [ [ A [ A Acid dissociation constant In short, A(aq) + (aq) + A - (aq) Cl(aq) + (aq) + Cl - (aq) C 3 C(aq) + (aq) + C 3 C - (aq) N + (aq) + (aq) + N 3 (aq) C 6 5 N 3+ (aq) + (aq) + C 6 5 N (aq) [Al( ) 6 3+ (aq) + (aq) + [Al()( ) 5 + (aq)

8 Acid Strength A(aq) + (l) 3 + (aq) + A - (aq) A(aq) + (aq) + A - (aq) a [ 3 [ A [ A [ [ A [ A

9 Acid Strength A(aq) + (l) 3 + (aq) + A - (aq) oxyacids : the acidic proton is attached an oxygen atom. S N 3 Cl 3 P S diprotic acid N Cl P triprotic acid S (aq) + (aq) + S - (aq) : strong S - (aq) + (aq) + S - (aq) : weak rganic acids with carboxyl groups : the acidic proton is carboxylic proton. Generally weak. R C C 3 C C 6 5 C thers: Cl. Cl...

10 Acid Strength A(aq) + (l) 3 + (aq) + A - (aq) a s of strong acids are not listed. Cl(aq) + (aq) + Cl - (aq) a [ [ Cl [ Cl too small to be measured accurately

11 Water as an Acid and a Base Acid Strength Water is amphoteric (it can behave either as an acid or a base). (l) + (l) 3 + (aq) + - (aq) autoionization N 3 + N 3 N + + N - dissociation constant (ion-product constant) w = [ + [ - = 1. x 1-1 at 5 o C ([ + =[ - =1. x 1-7 ) Aqueous solution 1. Neutral solution: [ + = [ - = 1. x 1-7 (ex, pure water). Acidic solution: [ + > [ - 3. Basic solution: [ + < [ - In any case, w = [ + [ - = 1. x 1-1 at 5 o C

12 Water as an Acid and a Base Acid Strength Ex) At 5 o C, a. 1. x b. 1. x c [ +, [ -? neutral, acid, basic? a. [ + = 1. x 1-9 (basic) b. [ + = 1. x 1-7 (neutral) c. [ + = 1. x 1-15 (acidic) Ex) At 6 o C, w = 1 x 1-13 a. 3 + (aq) + - (aq) endothermic, exothermic? b. [ +, [ - in neutral aqueous solution? a. At 5 o C, w = 1 x 1-1 T, w => endothermic b. [ + = [ - = (1x1-13 ) 1/ = 3 x 1-7

13 p Scale In aqueous solution, [ + is typically quite small, we use p scale. p log[ decimal places Neutral solution, [ + = 1. x 1-7 p log significant figures p in water usually ranges from to 1. p log[ p p w log p p 1 w [ log w [ log[ log[ Ex) At 5 o C, p of a sample of human bood is 7.1. p, [ +, [ - p [ [ 1 p

14 Calculating the p of Strong Acid Solutions Ex).1 N 3 (aq), p? 1. Find the major species in the solution. A(aq) + (aq) + A - (aq) a >>1 (l) + (aq) + - (aq) w = 1-1. Calculate p. N 3 (aq) + (aq) + N 3- (aq) Supplying.1 + and.1 N 3 - (l) + (aq) + - (aq) equilibrium, by Le Châtelier's principle, [ - << 1-7 ajor supplier of + :.1 N 3 => [ + =.1 => p =1. Ex) 1. x 1-1 Cl (aq), p? Cl(aq) + (aq) + Cl - (aq) Supplying 1.x1-1 + and 1.x1-1 Cl - (l) + (aq) + - (aq) Produced [ + by Cl is too small to affect water autodissociation. ajor supplier of + : => [ + = 1.x1-7 => p =7.

15 Calculating the p of Weak Acid Solutions 1. List major species in solution. A(aq) + (aq) + A - (aq) a << 1 (l) + (aq) + - (aq) w = 1-1. Choose species that can produce + and write reactions. 3. Based on values, decide on dominant equilibrium.. Write equilibrium expression for dominant equilibrium. 5. List initial concentrations in dominant equilibrium. 6. Define change at equilibrium (as x ). 7. Write equilibrium concentrations in terms of x. 8. Substitute equilibrium concentrations into equilibrium expression. 9. Solve for x the easy way. ( [A -x [A ) 1. Verify assumptions using 5% rule. 11. Calculate [ + and p. Ex) p of 1. F(aq)? a = 7. x 1-1. F,. F(aq) + (aq)+f - (aq) a = 7. x 1 - (l) + (aq)+ - (aq) w = 1. x F(aq) + (aq)+f - (aq) a = 7. x 1 -. a 5.[ F [ F [ [ [ F [ F ~9. F(aq) + (aq) + F - (aq) initial 1. eq 1.-x x x a 7.1 x.71 [ [ F [ F x 1. x x [F.71.7% p log[ log(.71 ) 1.57

16 Calculating the p of Weak Acid Solutions 1. List major species in solution. A(aq) + (aq) + A - (aq) a << 1 (l) + (aq) + - (aq) w = 1-1. Choose species that can produce + and write reactions. 3. Based on values, decide on dominant equilibrium.. Write equilibrium expression for dominant equilibrium. 5. List initial concentrations in dominant equilibrium. 6. Define change at equilibrium (as x ). 7. Write equilibrium concentrations in terms of x. 8. Substitute equilibrium concentrations into equilibrium expression. 9. Solve for x the easy way. ( [A -x [A ) 1. Verify assumptions using 5% rule. 11. Calculate [ + and p. Ex) p of.1 Cl(aq)? a = 3.5 x Cl,. Cl(aq) + (aq)+cl - (aq) a = 3.5 x 1-8 (l) + (aq)+ - (aq) w = 1. x Cl(aq) + (aq)+cl - (aq) a = 3.5 x 1-8. a [ Cl [ [ [ Cl [ Cl [ Cl ~9. Cl(aq) + (aq) + Cl - (aq) initial.1 eq.1-x x x a [ [ Cl [ Cl x x.1 x.1 5 x [Cl % p log[ log( ).3

17 Calculating the p of Weak Acid Solutions p of a ixture of Weak Acids Ex) p of the aq solution containing 1. CN ( a = 6. x 1-1 ) and 5. N ( a =. x 1 - )? [CN -? p? 1. CN, N,. CN(aq) + (aq)+cn - (aq) a = 6. x 1-1 N (aq) + (aq)+n - (aq) a =. x 1 - (l) + (aq)+ - (aq) w = 1. x N (aq) + (aq)+n - (aq) a =. x 1 -. a [ [ N.1 [ N 5~9. N (aq) + (aq) + N - (aq) initial x 1-7 ( ) eq 5.-x x x a.1 [ [ N [ N x.51 x 5. x x [N 11. p log[ [CN -? a % log( [ [ CN [ CN (.51 )[ CN 1. 8 [ CN 1.1 ) 1.35 CN(aq) + (aq) + CN - (aq) initial x 1-7 ( ) eq 1.-y ( 1.) x y

18 Calculating the p of Weak Acid Solutions Percent Dissociation amount dissociated () % Dissociation 1 % initial concentration () Ex) 1. C 3 C(aq) ( a = 1.8 x 1-5 ), % dissociation? C 3 C(aq) + (aq) + C 3 C - (aq) 5 x a 1.81 initial 1. 1.x1-7 ( ) 1. x eq 1.-x x x 3 x [.1 x 1. % dissociation 3 [ C3C [ C3C.1 1%.% [ C C 1. 3 Ex).1 C 3 C(aq) ( a = 1.8 x 1-5 ), % dissociation? C 3 C(aq) + (aq) + C 3 C - (aq) 5 x a 1.81 initial.1 1.x1-7 ( ).1 x eq.1-x x x 3 x [ 1.31 x.1 % dissociation 3 [ C3C [ C3C % 1.3% [ C C.1 3

19 Calculating the p of Weak Acid Solutions Percent Dissociation amount dissociated () % Dissociation 1 % initial concentration () Ex) 1. A(aq) C 3 C(aq) ( a = 1.8 x 1-5 ), % dissociation? + + A - (aq) A:weak acid [.1 3 A(aq) + (aq) + A - (aq) initial [A % dissociation eq [A.% -x x x a [ [ A [ A x [ A x x [ A Suppose the solution is diuluted 1 times at a time, Ex).1 C 3 C(aq) ( x [ a = 1.8 x 1-5 ), % dissociation? new [ A new 1 3 [ 1.31 [ A [ A new % dissociation 1.3% 1 [ new[ A new ( x /1) x a Q [ A [ A /1 1[ A 1 new Equilibrium => more dissociation

20 Acid Bases Definitions Base Arrhenius + producer - producer Brønsted-Lowry + donor + acceptor Lewis e - pair acceptor e - pair donor Strong and weak are used in the same sense for bases as for acids strong = complete dissociation (hydroxide ion supplied to solution) Na(s) Na + (aq)+ - (aq). strong bases all the hydroxides of Group 1A elements (Li, Na,, Rb, Cs) hydroxides of Group A elements (Ca(), Ba(), Sr() ) low solubility

21 Bases Definitions Acid Base Arrhenius + producer - producer Brønsted-Lowry + donor + acceptor Lewis e - pair acceptor e - pair donor Ex) p of 5. x 1 - Na solution? Na(aq) Na + (aq) + - (aq) Supplying 5. x 1 - Na + and - (l) + (aq) + - (aq) equilibrium, by Le Châtelier's principle, [ + << 1-7 ajor supplier of - : 5. x 1 - Na => [ - = 5. x 1 - w [ [ [ (5.1 ) [.1 13 p log(.1 13 ) 1.7

22 Bases weak = very little dissociation (or reaction with water) N 3 (aq) + (l) N + (aq) + - (aq) B(aq) + (l) B + (aq) + - (aq) base acid conjugate conjugate acid base b [ B [ [ B weak bases: many amines (R-N or R N)...

23 Bases Ex) p of 15. N 3 (aq) ( b = 1.8 x 1-5 )? ajor species: N 3, N 3 (aq) + (l) N + (aq) + - (aq) b = 1.8 x 1-5 (l) + (aq) + - (aq) w = 1. x 1-1 ajor - supplier: N 3 (aq) + (l) N + (aq) + - (aq) initial x 1-7 ( ) eq 15.-x x x b [ N [ [ N x 15. x x 1.61 [ N3 [ N 5% rule : [ N p log[ 3 3 p log(1.6 1 p 1. x % ) 1.8 5

24 Polyprotic Acids Polyprotic acids: can furnish more than one proton ( + ) to the solution. C 3 : diprotic acid C 3 (aq) + (aq) + C 3- (aq) C 3- (aq) + (aq) + C 3 - (aq) a1 a [ [ C [ C [ [ C3 [ C C (g) + (l) + (aq) + C 3- (aq) 3 P : triprotic acid 3 P (aq) + (aq) + P - (aq) P - (aq) + (aq) + P - (aq) P - (aq) + (aq) + P 3- (aq) a1 a a3 [ [ P [ P [ [ P [ P 3 [ [ P [ P 3

25 Polyprotic Acids Polyprotic acids: can furnish more than one proton ( + ) to the solution. a 3 a1 a In most cases, only the first disssociation step makes a important contribution to [ +.

26 Polyprotic Acids Ex) p of 5. 3 P (aq)? eq concentrations of 3 P, P -. P -, and P 3-? ajor species: 3 P, 3 P (aq) + (aq) + P - (aq) initial 5. eq 5.-x x x a1 a a3 [ [ P [ P 3 [ [ P [ P 3 [ [ P [ P a1 [ [ P [ P.19 5% rule : 5% 5. 1 [ x 1.91 [ P x x 5. x 5. [ p P x x a [ [ P [ P 1 (1.9 1 ) [ P [ P a [ [ P [ P 1 (1.9 1 ) [ P [ P

27 Polyprotic Acids Ex) p of 1. S (aq)? S (aq) + (aq) + S - (aq) S - (aq) + (aq) + S - (aq) a1 a [ [ S [ S [ [ S [ S very large 1.1 ajor species: +, S -, S - (aq) + (aq) + S - (aq) initial eq 1.-x 1.+x x a [ (1 x) x (1.) x x % rule : 5% x Q=, equilibrium x 1.1 p.

28 Ex) p of 1. x 1 - S (aq)? Polyprotic Acids S (aq) + (aq) + S - (aq) S - (aq) + (aq) + S - (aq) a1 a [ [ S [ S [ [ S [ S very large 1.1 ajor species: +, S -, S - (aq) + (aq) + S - (aq) initial.1.1 eq.1-x.1+x x (.1 x) x (.1) x a 1.1 x x.1.1 5% rule : 5%.1 (.1 x) x 3 a 1.1 x.71 or.51.1 x [.1 x p Q=, equilibrium

29 Salt : ionic compound Acid-Base Properties of Salts A salt in water produces anions and cations. Each ion behave as acids or bases. Salts That Produce Neutral Solutions Salts that consist of the cations of strong bases and the anions of strong acids have no effect on [ + when dissolved in water. => neutral solution NaCl(aq) Na + (aq) + Cl - (aq) Cl(aq) + (aq) + Cl - (aq) Na(aq) Na + (aq) + - (aq) no => Cl - no effect on [ + (or [ - ) no =>Na + : no effect on [ - (or [ + ) Examples) Cl, NaCl, NaN 3, N 3

30 Acid-Base Properties of Salts Salts That Produce Basic Solutions For any salt whose cation has neutral properties (such as Na + or + ) and whose anion is the conjugate base of a weak acid, the aqueous solution will be basic. NaC 3 C in an aqueous solution. => ajor species: C 3 C -, Na +, C 3 C - (aq) + (l) C 3 C(aq) + - (aq) a b for acetic acid 1.81 [ C3C[ [ C C C 3 C(aq) + (aq) + C 3 C - (aq) 5 3 [ C3C [ [ C C 3 no effect on [ + a [ b [ C3C [ [ C C [ 3 w [ C3C[ [ C C 3 For any weak acid and its conjugate base, a x b = w b w a basic solution

31 Acid-Base Properties of Salts Salts That Produce Basic Solutions For any salt whose cation has neutral properties (such as Na + or + ) and whose anion is the conjugate base of a weak acid, the aqueous solution will be basic. Ex) p of.3 NaF solution? ( a for F = 7. x 1 - ) ajor species in the solution: Na +, F -, Na + : neutral F - (aq) + (l) F(aq) + - (aq) [ -? F - (aq) + (l) F(aq) + - (aq) initial.3 1. x 1-7 ( ) eq.3-x x x b [ F[ [ F x.1 [ 6 w a x (5% rule : [ F x , p 1 p p 5.69 x.3 x 5% ) x.3

32 Acid-Base Properties of Salts Base Strength in Aqeous Solution In acid strength section, we said CN(aq) + (l) 3 + (aq) + CN - (aq) a = 6. x 1-1 CN: weak acid => CN - : strong base? CN - (aq) + (l) CN(aq) + - (aq) b = w / a = 1.6 x 1-5 stronger than (CN - competing with for + ) Actually b is small => CN - : weak base weaker than - (CN - competing with - for + ) base strength : - > CN - >

33 Acid-Base Properties of Salts Salts That Produce Acidic Solutions Salts in which the anion is not a base and the cation is the conjugate acid of a weak base produce acidic solution. Ex) p of.1 N Cl solution? ( b for N 3 = 1.8 x 1-5 ) ajor species in the solution: N +, Cl -, Cl - : neutral N + (aq) N 3 (aq) + + (aq) initial.1 1. x 1-7 ( ) eq.1-x x x a [ N3[ [ N x w b x (5% rule : [ N [ x 7.51 p x.1 x 6 5% ) x.1

34 Acid-Base Properties of Salts Salts That Produce Acidic Solutions Aqueous solution containing highly charged metal ion => acidic Al( ) 6 3+ (aq) + (aq) + Al()( ) 5 + (aq) Ex) p of.1 AlCl 3 aq solution? ( a for Al( ) 6 3+ = 1. x 1-5 ) ajor species in the solution: Al 3+, Cl -, Cl - : neutral Al( ) 3+ 6 (aq) Al()( ) + 5 (aq) + + (aq) initial.1 1. x 1-7 ( ) eq.1-x x x a [ Al( )( [ Al( x 3.71 ) ) x (5% rule : [ Al( ) [ [ 1.1 x 3.71 p x.1 x x.1 5% )

35 Acid-Base Properties of Salts Salts Consting of Basic Anion and Acidic Cation Salts in which the anion is the conjugate base of a weak acid and the cation is the conjugate acid of a weak base produce either acidic or basic solution. a > b (p<7, acidic), a = b (p=7, neutral), a < b (p>7, basic) Ex) acidic, neutral, basic in solution? (a) N C 3 C, (b) N CN, (c) Al (S ) 3 (a) a for N + = 5.6 x 1-1, b for C 3 C - = 5.6 x 1-1 => neutral (b) a for N + = 5.6 x 1-1, b for CN - = w / a (CN) = 1.6 x 1-5 => basic (c) a for Al( ) 6+ = 1. x 1-5, b for S - = w / a (S - ) = 8.3 x 1-13 => acidic Ex) p of.1 N CN solution? ajor species in solution: N +, CN -, N + (aq) N 3 (aq)+ + (aq), a = 5.6 x 1-1 CN - (aq)+ (l) CN(aq)+ - (aq), b = 1.6 x 1-5 (l) + (aq)+ - (aq), w = 1. x 1-7 CN - (aq) + (l) CN(aq) + - (aq) initial.1 1. x 1-7 ( ) eq.1-x x x b 1.61 x 1.31 [ 3 5 [ CN[ [ CN x (5% rule : [ CN x , p.89 p 1 p x x.1 x % ).1

36 Acid-Base Properties of Salts Case Actually, alll depend on the relative magnitudes of a, b, and w (case 1) a << w and b << w => w dominant => neutral (case ) a << w => b dominant => basic (case 3) b << w => a dominant => acidic (case ) a > b => a dominant => acidic (case 5) a < b => b dominant => basic (case 6) a = b => neural

37 Effect of Structure on Acid-Base Properties Brønsted-Lowry (Acid) + donor (Base) + acceptor Any molecule containing a hydrogen atom is potentially an acid. C nonpolar polar What structural properties of a molecule cause it to behave as an acid or as a base? Bond strength (the lower, the more acidic) and bond polarity (the higher, the more acidic) -Cl Bond strength (kcal/mol) 1 13 Acidic? No Strong acid Bond polarity most least

38 Effect of Structure on Acid-Base Properties xyacids acidity - Bond Polarity Aqueous solution containing highly charged metal ion => acidic Al 3+ electron density

39 Effect of Structure on Acid-Base Properties xyacids Is --Y always acidic? No. We know Na, are basic. --Y Electronegativity of Y high very low -Y bond characteristics covalent and strong ionic In aqueous solution polar - bond broken polar -Y bond broken Releasing + and -Y - - and Y + Acidic, Basic? acidic basic

40 Acid-Base Properties of xides Acidic xides (Acid Anhydrides): xides with -Y bond is strong and covalent. => forming acidic solution in water. Ex) S 3, S, N, C S 3 (g) + (l) S (aq) S (g) + (l) S 3 (aq) C (g) + (l) C 3 (aq) N (g) + (l) N 3 (aq)+ N (aq) S N Basic xides (Base Anhydrides): xides with -Y bond is ionic. => forming basic solution in water. Ex), Ca Ca(s) + (l) Ca() (aq) (s) + (l) (aq) - (aq) + (l) - (aq) oxide ion has high affinity for proton.

41 Lewis Acid-Base odel Definitions Acid Base Arrhenius + producer - producer Brønsted-Lowry + donor + acceptor Lewis e - pair acceptor e - pair donor Brønsted-Lowry Lewis base acid acid base acid base A Brønsted-Lowry acid must contain at least one ionizable proton!

42 Lewis Acid-Base odel Al Al( ) 6 3+ BF 3 + N 3 BF 3 N 3 Lewis acid Lewis base Lewis acid Lewis base S 3 (g) + (l) S (aq) (S) Lewis acid () Lewis base ()Lewis base () Lewis acid Bronsted-Lowry acid S S A Brønsted-Lowry acid must contain at least one ionizable proton! A Lewis acid doesn't require it!

43 Solving Acid-Base Problems: Summary Don't memorize. ain questions are ajor species? What is their chemical behavior?