Definition Arrhenius ( ) acid and base (1887) Acid A molecule ionizes in water to form a solution

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1 1 Acids and bases Definition Arrhenius ( ) acid and base (1887) Acid A molecule ionizes in water to form a solution containing H ions Base A molecule ionizes in water to form a solution containing H ions Brønsted acid and base (193 by Brønsted and Lowry) Acid: A proton (H + ) donor Base: A proton acceptor NH 3 (aq) + H (l) NH (aq) + H (aq) Lewis acid and base Acid: An electron-pair acceptor Base: An electron-pair donor E. BF 3 + NH 3 F 3 BNH 3 Lewis acid Lewis base 1

2 Acid strength Hydronium ion HA(aq) + H (l) H 3 (aq) + A (aq) acid base conjugate acid conjugate base conjugate ( 共軛 ) acid-base pair Can be represented as HA(aq) H (aq) + A (aq) Dissociation constant K a [ H3 ][A ] [H ][A ] [HA] [HA] HA is a strong acid when K a is large or equilibrium lies far to the right K a is a measurement of acid strength Strong acid gives weak conjugate base Weak acid gives strong conjugate base E. pk a = logk a H 15.7 NH H H S 3 HCl 6 ~ 7 HI 9.5 ~ 10

3 The ph ph = log[h + ] At neutral condition [H + ] = ph = 7.00 ph < 7.00 ph > 7.00 acidic basic Calculating ph Strong acid: complete dissociation The ph of a miture of weak acids E M HCN K a = M HN K a =.0 10 ph? H (l) H + (aq) + H (aq) K w = HN is the strongest acid, the other two can be neglected 3

4 HN (aq) H + (aq) + N (aq) ini (M) final K a small =.5 10 M = [H + ] ph = 1.35 Check: [H + ] is indeed small compared to 5.00 M (tolerance: ±5%) HCN (aq) H + (aq) + CN (aq) final (M) K a 10 (.5 10 ) (.5 10 ) = M = [CN ] Check: [CN ] is small indeed also [H + ] is mainly from HN

5 Percent dissociation amt dissociated 100% initial conc For weak acid: more dilute higher percent dissociation HA(aq) H (aq) + A (aq) K a [HA] o [HA] o percent dissociation K a [HA] o [HA] o 100% ( K a [HA] o )100% [HA] o percent dissociation Bases NaH(aq), KH(aq) dissociate completely in H Strong bases Ca(H) : similar but solubility is small does not produce high [H ] Slaked lime ( 熟石灰 ) Removal of Ca + in hard water Ca + Na C 3 (the soda lime method) Ca(s) + H (l) Ca(H) (aq) C 3 (aq) + H (l) HC 3 (aq) + H (aq) Ca(H) (aq) + Ca + (aq) + HC 3 (aq) CaC 3 (s) + H (l) in hard water 5

6 ther bases NH 3 (aq) + H (l) NH (aq) + H (aq) ammonia Amine bases pk a = 9.5 H 3 C N H H 3 C N N 3 C H C 5 H 5 H HCH C H 5 methylamine dimethylamine triethylamine N pyridine In general B(aq) + H (l) BH (aq) + H (aq) K b [BH ][H ] [B] K b base strength K w = K a K b In practice the base strength is often measured by the pk a of its conjugate acid pk a base strength 6

7 Polyprotic acids H S, H 3 P Dissociate stepwise H 3 P (aq) H (aq) + H P (aq) H P (aq) H (aq) + HP (aq) HP (aq) H (aq) + P 3 (aq) K a1 = K a = K a3 = In general K a1 >> K a >> K a3 E. For a 5.0 M H 3 P solution H 3 P (aq) H (aq) + H P (aq) K a ini (M) final 5.0- K a = M Within 5% error H 8 [ ][ HP ] ( 0. 19)[ HP ] [ HP ] [ H P ] ( 0. 19) M K a3 [H ][P [HP 3 ] (0.19)[P ] ( ] ) [P 3 ] = M 7

8 E. ph of a M H S Complete dissociation for the 1st step HS (aq) H (aq) + S (aq) ini (M) final K a 1.10 ( ) ( ) Assume >> = ( ) ( ) = 0 incorrect = M [H + ] = = ph = 1.80 Acid-base properties of salts Conjugate base of strong acid Conjugate acid of strong base no effect on ph Conjugate base of a weak acid NaC H 3, KCN, KF E M NaF(aq) K a for HF: K w = K a K b K b = F (aq) + H (l) HF(aq) + H (aq) ini (M) final K b = M = [H ] 8

9 Conjugate acid of a weak base E. Ammonium salt NH Cl NH (aq) H (aq) + NH 3 (aq) K a = A 0.10 M solution ph = 5.13 With highly charged metal ion 3+ E. Al(H ) 6 AlCl 3 (s) + 6H (l) Al(H ) 3+ 6 (aq) + 3Cl (aq) Al(H ) 3+ 6 (aq) Al(H)(H ) + 5 (aq) + H (aq) K a = A M solution ph = 3.3 Both ions affect the ph E. NH + C H 3 ammonium acetate Basic; K b = Acidic; K a = K a > K b K a < K b K a = K b acidic basic neutral E. NH + CN K b = basic 9

10 E. ph of a M NH CN NH (aq) H (aq) + NH 3 (aq) K a = CN (aq) + H (l) HCN(aq) + H (aq) K b = H (l) H + (aq) + H (aq) K w = NH (aq) + CN (aq) NH 3 (aq) + HCN(aq) [NH ][HCN] [NH ][H ] [HCN] K 3 K [NH ][CN ] [NH ] [H ][CN ] K 3 a(nh ) a(hcn) 0.90 The dominate process The effect of structure Brønsted-Lowry acid XH Potentially acidic Two major factors the strength: stronger bond weaker acid the polarity: more polar stronger acid E. In the same period HCH 3 HNH HH HF Least polar Weakest bond Acidity (polarity factor) Most polar Strongest bond 10

11 E. Down a group HF HCl HBr HI Most polar Strongest bond Least polar Weakest bond Fact: HI is the strongest acid in water Q: Is it due to bond strength factor? X F Cl Br I BE of H-X (kj/mol) H o solvation for X (kj/mol) S o solvation for X (J/K mol) electron affinity (kj/mol) HX (aq) G H (aq) + X (aq) G 1 G HX (g) H (g) + X (g) G 3 G 5 G 6 G : bond energy data G 3 : ionization data of H atom G 1 : reverse of the solvation of HX G : electron affinity data of X atom G 5 : solvation of proton e + H (g) G 6 : solvation of halide + e G X (g) G = G 1 + G + G 3 + G + G 5 +G 6 verall: the halide solvation entropy change is the determinant factor Note: in the same period the sizes of Xs are similar 11

12 yacids containing H--X structure E. H Cl H Cl HCl HCl K a ~10 7 ~ More oygen attached to the central atom more acidic E. HCl HBr HI K a With more electronegative atom attached to more acidic ides H--X can also behave as a base Releasing H : NaH, KH, (X is electropositive or -X is weak) Basic oide Ca(s) + H (l) Ca(H) (aq) Actually Ca + and 1

13 Acidic oide S 3 (g) + H (l) H S (aq) S More covalent H S H Based on Lewis acid-base model S 3 is a Lewis acid H is a Lewis base 13