1. Auto-ionization of water and pH Monday, April 16

2. September 5, 20152 Weak Acids Most acids are weak and only partially ionize. HA (aq) + H 2 O (l)  H 3 O + (aq) + A - (aq) or simply our equations: HA (aq)  H + (aq) + A - (aq) You can eliminate the H 2 O – cuz of the (aq) behind HA, and H 3 O + is more easily shown as just H +

3. September 5, 20153 Equilibrium-constant expression K eq = [H + ] [A - ] [H 2 O] is solvent (omitted) [HA] We will use the subscript a on K a to denote that it is an equilibrium constant for the ionization of an acid; called acid-dissociation constant. Table 16.2 on page 628 in textbook gives K a values for several weak acids, another list is in Appendix D. The larger the K a value, the stronger the acid.. Notice that K a is typically less than 10 -3.

4. September 5, 20154 pK a is found by the equation: pK a = -log(K a ) In fact, a lower-case p in all this chapter means: -log

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6. September 5, 20156 Polyprotic Acids Acids with more than one ionizable H atom. – Sulfurous acid, H 2 SO 3 ionizes in two steps: H 2 SO 3 (aq)  H + (aq) + HSO 3 - (aq) K a1 = 1.7 x 10 -2 HSO 3 - (aq)  H + (aq) + SO 3 2- (aq) K a2 = 6.4 x 10 -8 The K a1 and K a2 labels refers to the removal of the first H + and the second H +.

7. September 5, 20157 In previous example, the K a2 is much smaller than K a1. It is always easier to remove the first proton from a polyprotic acid than the second. – Based on electrostatic attractions. Expect a H + to be lost more easily from the neutral H 2 SO 3 than from the negatively-charged HSO 3 - ion. Also, for an acid with three ionizable H +, the second proton is easier to remove than the third. K a values get successively smaller as protons are removed.

8. September 5, 20158 Page 635 or Appendix D (Page 1044)

9. September 5, 20159 Because K a1 is so much larger than subsequent K values, almost all the H + in the solution comes from the first ionization reaction. As long as the successive K a values differ by a factor of 10 3 or more, it is possible to obtain a satisfactory estimate of the pH of polyprotic acid solutions by considering only K a1.

10. September 5, 201510 What is “negligible acidity”? Substances, such as CH 4 and H 2, that contain H but do NOT demonstrate any acidic behavior in water are considered “negligible”. Their conjugate bases (CH 3 - and H - ), while forming in very small concentrations, are strong bases – gaining H + from water to form OH - ions. – CH 3 - + H 2 O  CH 4 + OH - – B A CA CB – H - + H 2 O  H 2 + OH - – B A CA CB

11. Amphoteric: capable of acting as either an acid or base Acts as a base when combined with something more strongly acidic than itself, and as an acid when combined with something more strongly basic than itself. September 5, 201511

12. September 5, 201512 HSO 3 -1 is amphoteric. Write an equation for a reaction with water, where HSO 3 - acts as an acid; – HSO 3 - (aq) + H 2 O (l)  SO 3 2- (aq) + H 3 O + (aq) – A B CB CA Write an equation for a reaction with water where HSO 3 - acts as an base: – HSO 3 - (aq) + H 2 O (l)  H 2 SO 3 (aq) + OH - (aq) – B A CA CB

13. September 5, 201513 HCO 3 -1 is amphoteric. Write an equation for a reaction with water, where HCO 3 - acts as an acid; – HCO 3 - (aq) + H 2 O (l)  CO 3 2- (aq) + H 3 O + (aq) – A B CB CA Write an equation for a reaction with water where HCO 3 - acts as an base: – HCO 3 - (aq) + H 2 O (l)  H 2 CO 3 (aq) + OH - (aq) – B A CA CB

14. September 5, 201514 Water is Amphoteric!

15. September 5, 201515 Autoionization of WATER Water has the ability to act as either a Bronsted acid or a Bronsted base, depending on the circumstances. In fact, one water molecule can donate a proton to another water molecule Autoionization

16. September 5, 201516 Facts about autoionization Reactions are very rapid in both directions. At 25 0 C, only about 2 out of every 10 9 molecules is ionized at any given instant. Pure water consists of almost entirely H 2 O molecules and is a poor electrical conductor However, the autoionization is very important.

17. September 5, 201517 K eq = [H 3 O + ][OH - ] or [H + ][OH - ] Will use K w to refer specifically to water’s autoionization from now on; called the ion- product constant. At 25 0 C, K w = 1 x 10 -14 Considered valid for any dilute aqueous solution and it can be used to calculate either [H + ] if [OH - ] is known or [OH - ] if [H + ] is known.

18. September 5, 201518 When [H + ] = [OH - ] NEUTRAL In acidic solutions, [H + ] > [OH - ] In basic solutions, [H + ] < [OH - ] Calculate the values of [H + ] and [OH - ] in a neutral solution at 25 0 C [H + ][OH - ] = (x)(x) = 1.0 x 10 -14 x 2 = 1.0 x 10 -14 x = 1.0 x 10 -7 M = [H + ] = [OH - ]

19. September 5, 201519 Determine whether acidic, neutral, or basic??? [H + ] = 4 x 10 -9 M [OH - ] = 1 x 10 -7 M [OH - ] = 7 x 10 -13 M

20. September 5, 201520 Determine whether acidic, neutral, or basic??? [H + ] = 4 x 10 -9 M [OH - ] = 1 x 10 -7 M [H + ] = [OH - ] Neutral [OH - ] = 7 x 10 -13 M [OH - ] is greater than [H + ] Basic

21. September 5, 201521 Calculate the [H + ] when A. [OH - ] is 0.010 M K eq = [H + ][OH - ] = 1 x 10 -14 – [H + ] = 1 x 10 -14 = 1 x 10 -14 = 1.0 x 10 -12 M [OH - ] 0.010 B. [OH - ] = 1.8 x 10 -9 M – [H + ] = 1 x 10 -14 = 1 x 10 -14 = 5.6 x 10 -6 M [OH - ] 1.8 x 10 -9

22. September 5, 201522 Calculate [OH - ] when [H + ] is ten times greater than [OH - ] K eq = [H + ][OH - ] = 1 x 10 -14 = (10x)(x) = 1 x 10 -14 = 10x 2 = 1 x 10 -14 = x 2 = 1 x 10 -15 = x = (1 x 10 -15 ) 1/2 = 3.16 x 10 -8

23. September 5, 201523 The pH Scale pH = - log [H + ] or - log [H 3 O + ] pH of a neutral solution – = - log (1.0 x 10 -7 ) = -(-7) = 7 An acid solution is where [H + ] > 1.0 x 10 -7 Because of the negative sign, the pH decreases as [H + ] increases. A pH of 1 is more acidic than 3. A pH of 3 is more acidic than 5, and so on.

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