Auto-ionization of water and pH Monday, April 16

September 5, 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 +

September 5, 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

September 5, 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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September 5, 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 HSO 3 - (aq)  H + (aq) + SO 3 2- (aq) K a2 = 6.4 x The K a1 and K a2 labels refers to the removal of the first H + and the second H +.

September 5, 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.

September 5, Page 635 or Appendix D (Page 1044)

September 5, 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.

September 5, 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 H 2 O  CH 4 + OH - – B A CA CB – H - + H 2 O  H 2 + OH - – B A CA CB

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,

September 5, 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

September 5, 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

September 5, Water is Amphoteric!

September 5, 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

September 5, 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.

September 5, 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 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.

September 5, 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 x 2 = 1.0 x x = 1.0 x M = [H + ] = [OH - ]

September 5, Determine whether acidic, neutral, or basic??? [H + ] = 4 x M [OH - ] = 1 x M [OH - ] = 7 x M

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

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

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

September 5, The pH Scale pH = - log [H + ] or - log [H 3 O + ] pH of a neutral solution – = - log (1.0 x ) = -(-7) = 7 An acid solution is where [H + ] > 1.0 x 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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