Required Reading Material.

Transcription

1 JF Chemistry Introduction to Physical Chemistry: Acid Base and Solution Equilibria. Professor Mike Lyons School of Chemistry Required Reading Material. Kotz, Treichel and Weaver, 7 th edition. Chapter 17 (Chemistry of Acids and Bases) & Chapter18 (Principles of reactivity: other aspects of ionic equilibria), pp Chemistry 3, Burrows et al. Chapter 6, Acids & bases, pp Chemistry 3, 2 nd edition. Chapter 7, pp

2 Review : Kotz Chapter 3 for simple acid/base definitions. Lecture 10. Acid/base chemistry : Simple ideas: Arrhenius, Bronsted-Lowry, Lewis. Acid and Bases Kotz: section 3.7, pp Section 17.1, pp

3 Acid and Bases Acid and Bases 3

4 Arrhenius (or Classical) Acid-Base Definition An acid is a neutral substance that contains hydrogen and dissociates or ionizes in water to yield hydrated protons or hydronium ions 3 O +. A base is a neutral substance that contains the hydroxyl group and dissociates in water to yield hydrated hydroxide ions O -. Neutralization is the reaction of an + ( 3 O + ) ion from the acid and the O - ion from the base to form water, 2 O. These definitions although correct are limited in that they are not very general and do not Give a comprehensive idea of what acidity and basicity entails. Cl ( aq) Cl ( aq) NaO Na ( aq) O ( aq) Cl NaO NaCl 2O Arrhenius acid is a substance that produces + ( 3 O + ) in water. Arrhenius base is a substance that produces O - in water. 4

5 Acids and bases: Bronsted/Lowry definition. Bronsted/Lowry Acid (A): An acid is a species which donates a proton Bronsted/Lowry Base (B): A base is a species which accepts a proton. These definitions are quite general and refer to the reaction between an acid and a base. An acid must contain in its formula; NO 3 and 2 PO 4- are two examples, all Arrhenius acids are Brønsted-Lowry acids. A base must contain a lone pair of electrons to bind the + ion; a few examples are N 3, CO 3 2-, F -, as well as O -. Brønsted-Lowry bases are not Arrhenius bases, but all Arrhenius bases contain the Brønsted-Lowry base O -. In the Brønsted-Lowry perspective: one species donates a proton and another species accepts it: an acidbase reaction is a proton transfer process. Chemistry 3 section 6.1. pp Kotz 7 th ed. Section pp BL acid/base equilibria. BL base BL acid A(aq) + 2 O (l) 3 O + (aq) + A - (aq) Water can function Proton transfer both as an acid BL acid and a base BL base B (aq) + 2 O (l) B + (aq) + O - depending on the (aq) circumstances. Proton donation and acceptance are dynamic processes for all acids and bases. ence a proton transfer equilibrium is rapidly established in solution. The equilibrium reaction is described in terms of conjugate acid/base pairs. The conjugate base (CB) of a BL acid is the base which forms when the acid has donated a proton. The conjugate acid (CA) of a BL base is the acid which forms when the base has accepted a proton. A conjugate acid has one more proton than the base has, and a conjugate base one less proton than the acid has. If the acid of a conjugate acid/base pair is strong (good tendency to donate a proton) then the conjugate base will be weak (small tendency to accept a proton) and vice versa. Proton transfer Acid : proton donor Base : proton acceptor A (aq) + B (aq) B + (aq) + A - (aq) A B CA CB 5

6 An acid is a proton donor, any species which donates a +. A base is a proton acceptor, any species which accepts a +. base acid acid base base acid conjugate acid An acid-base reaction can now be viewed from the standpoint of the reactants AND the products. An acid reactant will produce a base product and the two will constitute an acid-base conjugate pair. conjugate base Table 18.4 The Conjugate Pairs in Some Acid-Base Reactions Conjugate Pair Acid + Base Base + Acid Conjugate Pair Reaction 1 F + 2 O F O + Reaction 2 COO + CN - COO - + CN Reaction 3 N CO 2-3 N 3 + CO - 3 Reaction 4 2 PO O - PO O Reaction 5 2 SO 4 + N Reaction 6 PO SO 2-3 SO N PO SO - 3 Copyright The McGraw-ill Companies, Inc. Permission required for reproduction or display. 6

7 15.4 7

8 Strong and weak acids. Strong acids dissociate completely into ions in water: A (g or l) + 2 O (l) 3 O + (aq) + A - (aq) In a dilute solution of a strong acid, almost no A molecules exist: [ 3 O + ] = [A] init or [A] eq = 0 Q c = [ 3 O + ][A - ] [A][ 2 O] at equilibrium, Q c = K c >> 1 Nitric acid is an example: NO 3 (l) + 2 O (l) 3 O + (aq) + NO 3 - (aq) Weak acids dissociate very slightly into ions in water: A (aq) + 2 O (aq) 3 O + (aq) + A - (aq) In a dilute solution of a weak acid, the great majority of A molecules are undissociated: [ 3 O + ] << [A] init or [A] eq = [A] init Q c = [ 3O + ][A - ] [A][ 2 O] at equilibrium, Q c = K c << 1 The Extent of Dissociation for Strong and Weak Acids Key concept : Acid/base strength quantified in terms of extent or degree of dissociation. An acid or base is classified as strong if it is fully ionized in solution (e.g. Cl, NaO). An acid or base is classified as weak if only a small fraction is ionized in solution (e.g. C 3 COO, N 3 ). Complete ionization Partial ionization 8

9 Reactivity of strong and weak acids. 1M Cl(aq) 1M C3COO(aq) Classifying the Relative Strengths of Acids. Strong acids. There are two types of strong acids: The hydrohalic acids Cl, Br, and I Oxoacids in which the number of O atoms exceeds the number of ionizable atoms by two or more, such as NO 3, 2 SO 4, ClO 4 Weak acids. There are many more weak acids than strong ones. Four types, with examples, are: The hydrohalic acid F Those acids in which is bounded to O or to halogen, such as CN and 2 S Oxoacids in which the number of O atoms equals or exceeds by one the number of ionizable atoms, such as ClO, NO 2, and 3 PO 4 Organic acids (general formula RCOO), such as C 3 COO and C 6 5 COO. 9

10 Classifying the Relative Strengths of Bases. Strong bases. Soluble compounds containing O 2- or O - ions are strong bases. The cations are usually those of the most active metals: M 2 O or MO, where M= Group 1A(1) metals (Li, Na, K, Rb, Cs). MO or M(O) 2, where M = Group 2A(2) metals (Ca, Sr, Ba) [MgO and Mg(O) 2 are only slightly soluble, but the soluble portion dissociates completely.] Weak bases. Many compounds with an electron-rich nitrogen are weak bases (none are Arrhenius bases). The common structural feature is an N atom that has a lone electron pair in its Lewis structure. Ammonia (N 3 ) Amines (general formula RN 2, R 2 N, R 3 N), such as C 3 C 2 N 2, (C 3 ) 2 N, (C 3 7 ) 3 N, and C 5 5 N Strong Electrolyte 100% dissociation NaCl (s) 2 O Na + (aq) + Cl - (aq) Weak Electrolyte not completely dissociated C 3 COO C 3 COO - (aq) + + (aq) Strong Acids are strong electrolytes Cl (aq) + 2 O (l) NO 3 (aq) + 2 O (l) ClO 4 (aq) + 2 O (l) 2 SO4 (aq) + 2 O (l) 3 O + (aq) + Cl - (aq) 3 O + (aq) + NO - 3 (aq) 3 O + (aq) + ClO - 4 (aq) 3 O + (aq) + SO - 4 (aq) 10

11 Weak Acids are weak electrolytes F (aq) + 2 O (l) NO 2 (aq) + 2 O (l) SO - 4 (aq) + 2 O (l) 2 O (l) + 2 O (l) 3 O + (aq) + F - (aq) 3 O + (aq) + NO - 2 (aq) 3 O + (aq) + SO 2-4 (aq) 3 O + (aq) + O - (aq) Strong Bases are strong electrolytes NaO 2 O (s) Na + (aq) + O - (aq) KO 2 O (s) K + (aq) + O - (aq) Ba(O) 2 O 2 (s) Ba 2+ (aq) + 2O - (aq) Weak Bases are weak electrolytes F - (aq) + 2 O (l) NO 2 - (aq) + 2 O (l) O - (aq) + F (aq) O - (aq) + NO 2 (aq) Conjugate acid-base pairs: The conjugate base of a strong acid has no measurable strength. 3 O + is the strongest acid that can exist in aqueous solution. The O - ion is the strongest base that can exist in aqeous solution

12 Representing Protons Both representations of the proton, + (aq) and 3 O + are equivalent. 5 O 2+ (aq), 7 O 3+ (aq), 9 O 4 + (aq) have been observed. We will use + (aq)! ( O O 7 2 ) 3 3 The hydrated proton is quite a complex entity. It is usually represented in shorthand form as + (aq). A better representation is in terms of the hydronium ion 3 O +. We will adopt this representation a lot. The real situation is more complex. The 3 O + ion binds to other water molecules forming a mixture of species with the general formula ( 2 O) n+. In fact the structural details of liquid water is still a hot item of research. What is + (aq)? + + O + O 2 O O+ + O 3 O + O 2 5 O O 9 O

13 An Arrhenius acid is defined as a substance that produces + ( 3 O + ) in water. A Brønsted acid is defined as a proton donor A Lewis acid is defined as a substance that can accept a pair of electrons. A Lewis base is defined as a substance that can donate a pair of electrons Yet more sophistication: Lewis acidity G.N. Lewis acid + O - base O + + N N acid base See Kotz section 17.9 pp

14 Electron-Pair Donation and the Lewis Acid-Base Definition The Lewis acid-base definition : A base is any species that donates an electron pair. An acid is any species that accepts an electron pair. Protons act as Lewis acids in that they accept an electron pair in all reactions: B + + B + The product of any Lewis acid-base reaction is called an adduct, a single species that contains a new covalent bond. A Lewis base has a lone pair of electrons to donate. A Lewis acid has a vacant orbital Lewis Acid/Base Reaction 14

15 Lewis Acids and Bases F F B F acid + N F B F F base N No protons donated or accepted! Lewis Acids & Bases Other good examples involve metal ions. Co 2+ ACID O Co 2+ BASE O 15

16 Lewis Acids & Bases The combination of metal ions (Lewis acids) with Lewis bases such as 2 O and N 3 leads to COMPLEX IONS Reaction of N 3 with Cu 2+ (aq) PLAY MOVIE PLAY MOVIE 16

17 The Lewis Acid-Base Chemistry of Nickel(II) Lewis Acids & Bases [Ni( 2 O) 6 ] N 3 [Ni(N 3 ) 6 ] 2+ + DMG DMG = dimethylglyoxime, a standard reagent to detect nickel(ii) 17

18 Lewis Acid-Base Interactions in Biology PLAY MOVIE The heme group in hemoglobin can interact with O 2 and CO. The Fe ion in hemoglobin is a Lewis acid O 2 and CO can act as Lewis bases eme group Lewis Acids & Bases Many complex ions containing water undergo YDROLYSIS to give acidic solutions. [Cu( 2 O) 4 ] O [Cu( 2 O) 3 (O)] O + 18

19 Lewis Acids & Bases This explains why water solutions of Fe 3+, Al 3+, Cu 2+, Pb 2+, etc. are acidic. This interaction weakens this bond Another 2 O pulls this away as + Amphoterism of Al(O) 3 19

20 Lewis Acids & Bases This explains AMPOTERIC nature of some metal hydroxides. Al(O) 3 (s) O + Al O ere Al(O) 3 is a Brønsted base. Al(O) 3 (s) + O - Al(O) - 4 ere Al(O) 3 is a Lewis acid. Al 3+ O - 20