Chemical Equilibrium Chapter 6

Transcription

1 Chemical Equilibrium Chapter 6 "When a system is in chemical equilibrium, a change in one of the parameters of the equilibrium produces a shift in such a direction that, were no other factors involved in this shift, it would lead to a change of opposite sign in the parameter involved." Henri Louis Le Châtelier, 1888 Equilibrium and Thermodynamics, At constant temperature and pressure reactions can be characterized by their Gibbs free energy G = H T S = G o + RT ln Q At equilibrium G = 0and the product quotient Q becomes the equilibrium constant K so that Solubility Equilibria, 6-3 G o = RT ln K(T ) EX 1. What are the concentrations of calcium and fluoride ions in a saturated solution of CaF 2? K sp (CaF 2 ) = EX 2. If Ag 2 CrO 4 dissolves in water to the extent of mol L 1 estimate the solubility product for Ag 2 CrO 4?

2 The Approach to Equilibrium, G = G o + RT ln Q = RT ln K + RT ln Q = RT ln Q K Q < K => reaction moves right Q > K => reaction moves left EX 3. An emulsion of AgCl for photographic film is prepared by adding a soluble chloride salt to a solution of silver nitrate. 500 ml of a CaCl 2 solution with a chloride ion concentration of Misadded to 300 ml of a M solution of AgNO 3.Will a precipitate of AgCl(s) form when equilibrium is reached? K sp (AgCl) = EX 4. Asolution is simultaneously M Ag + and M Pb 2+.Can 99.9% of Pb 2+ be precipitated by CO 2 3 without precipitation of Ag +? K sp (PbCO 3 ) = , K sp (Ag 2 CO 3 ) =

3 -3- ACIDS AND BASES Chapter 6 "According to this theory strong acids and bases, as well as salts, are in extreme dilution almost completely dissociated into their ions, i.e. HCl into H + and Cl,NaOH into Na + and OH,and NaCl into Na + and Cl. Onthe other hand, water is hardly dissociated at all. The reaction of neutralization of a strong acid with a strong base... can therefore be expressed by... H + +OH. This equation is equivalent to the formation of water from its two ions, H + and OH,and is evidently independent of the nature of the strong acid and the strong base." Svante August Arrhenius, 1903 (Nobel Prize in Chemistry in 1903 "in recognition of the extrordinary services he has rendered to the advancement of chemistry by his electrolytic theory of dissociation".) The Nature ofacids and Bases: Acid-Base Theories Arrhenius acid - produces H + (aq) ions (protons) in aqueous solution base - produces OH (aq) ions in aqueous solution problem - acids only H + (aq), bases only OH (aq), need an aqueous solution Brønsted-Lowry -focus on hydrogen ion acid -substance that can donate ahydrogen ion (proton) base -substance that can accept ahydrogen ion little problem - acids can only donate H + (aq) Lewis - focus on electron pair acid - species that accepts a pair of electrons base - species that donates a pair of electrons advantage - no restriction on acids requiring an H atom Lewis coordinate covalent bond Brønsted Lowry conjugate acid base pairs A 1 + B 2 <=> A 2 + B 1 Arrhenius neutralization H + + OH H 2 O Brønsted-Lowry Acid-Base Theory, 6-5 acids - proton donors => form a species [acid - H + ]called conjugate base bases - proton acceptors => form a species [base + H + ]called conjugate acid EX 5. For each of the following acids write the formula of its conjugate base and for each of the bases write the formula of its conjugate acid. ACIDS - conjugate base BASES - conjugate acid HCl NH 3 H 2 O NaOH NH + 4 H 2 S CH 3 COOH O 2 SH CN

4 -4- Conjugate Acid-Base Pairs: A 1 + B 2 A 2 + B 1 acid - proton donor HClO 3 (aq) + H 2 O(l) H 3 O + (aq) + ClO 3(aq) CH 3 COOH(aq) + H 2 O(l) <=> H 3 O + (aq) + CH 3 COO (aq) strong weak base - proton acceptor H (aq) + H 2 O(l) OH (aq) + H 2 (g) NH 3 (aq) + H 2 O(l) <=> OH (aq) + NH + 4 (aq) strong weak amphoteric nature of water autoionization of water (solvent) - Harris designates this as autoprotolysis H 2 O(l) + H 2 O(l) <=> H 3 O + (aq) + OH (aq) EX 6. Show how the following reaction can be viewed as a Brønsted-Lowry acid-base reaction. 2KOH(aq) + (NH 4 ) 2 SO 4 (aq) <=> K 2 SO 4 (aq) + 2H 2 O(aq) + 2NH 3 (g) Water and the ph Scale, 6-6 autoionization of water (at 25 o C): 2H 2 O(l) <=> H 3 O + (aq) + OH (aq) K w = [H 3 O + ][OH ] = => Temperature Dependence of K w EX 7. K w = at body temperature (98.6 o F=37.0 o C). Temp ( o C) K w a) What is the hydrogen ion concentration? b) What is the ph?

5 -5- ph scale ph = - log 10 [H 3 O + ] poh = - log 10 [OH ] pk w =-log 10 K w condition concentration only at 25 o C acidic [H 3 O + ]>[OH ] ph<7 neutral [H 3 O + ] = [OH ] ph=7 basic [H 3 O + ]<[OH ] ph>7 EX 8. Answer each of the following a) [H 3 O + ] = ,pH? b) ph =9.3, [H 3 O + ]? c) 0.40 moles of Ba(OH) 2 is dissolved in a liter of water, poh? d) ph =9.3, [OH ]? Acid/Base Strength, 6-7 (Table 6-2) strong/weak electrolytes seven strong acids to know soluble strong bases to know hydrochloric acid HCl lithium hydroxide LiOH hydrobromic acid HBr sodium hydroxide NaOH hydroiodic acid HI potassium hydroxide KOH perchloric acid HClO 4 rubidium hydroxide RbOH chloric acid HClO 3 cesium hydroxide CsOH sulfuric acid H 2 SO 4 barium hydroxide Ba(OH) 2 nitric acid HNO 3 strong bases - all Group I and Group II hydroxides except Be

6 -6- acid strength -determined by extent of reaction of acid with water to form H 3 O + (aq) -then for any acid HA HA(aq) + H 2 O(l) <=> H 3 O + (aq) + A (aq) K a = leveling effect of water (p. 253) Various Ways to Describe the Strength of an Acid, HA property strong acid weak acid value of K a K a large K a small position of the ionization (dissociation) equilibrium far toright far to left equilibrium concentration of H + compared with [H +] [HA] 0 [H + ] << [HA] 0 initial concentration of HA strength of conjugate base compared with that of A much weaker A much stronger water base than water base than water base strength -(aside from Group I and Group II hydroxides) determined by extent of reaction of base with water to form OH (aq) -then for any base B: B:(aq) + H 2 O(l) <=> OH (aq) + BH + (aq) K b = Various Ways to Describe the Strength of a Base, B: property strong base weak base value of K b completely, either K b small 1) dissociates to position of the ionization (dissociation) equilibrium give [OH ]ions to far tothe left the solution or equilibrium concentration of OH compared with 2) reacts with water [OH ] << [B: ] 0 initial concentration of B: in either case: strength of conjugate acid compared with that of BH + much stronger water [OH ]=[B:] 0 acid than water

7 -7- the stronger the acid the weaker is its conjugate base the stronger the base the weaker is its conjugate acid weak is conjugate to weak EX 9. K a for hydrosulfuric acid (H 2 S) is at 25 o C. a) What is its conjugate base? b) What is the basicity constant (K b )for its conjugate base? c) To which reaction does K b correspond? EX 10. For the reaction HNO 2 (aq) + CH 3 COO (aq) <=> NO 2(aq) + CH 3 COOH(aq) a) From the Brønsted-Lowry point of view, identify each acid and its conjugate base and each base and its conjugate acid. b) If K a (HNO 2 ) = and K a (CH 3 COOH) = what is K for the reaction? c) Identify the stronger acid and stronger base in part a)