Auto-ionization of water and pH

Monday, April 16

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Weak Acids

• Most acids are weak and only partially ionize.

• HA (aq) + H2O (l) H3O+ (aq) + A- (aq)

• or simply our equations:• HA (aq) H+ (aq) + A- (aq)• You can eliminate the H2O

– cuz of the (aq) behind HA, and

• H3O+ is more easily shown as just H+

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Equilibrium-constant expression• Keq = [H+] [A-] [H2O] is solvent (omitted)

[HA]We will use the subscript a on Ka 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 Ka values for several weak acids, another list is in Appendix D.

The larger the Ka value, the stronger the acid.. Notice that Ka is typically less than 10-3.

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pKa is found by the equation: pKa = -log(Ka)

In fact, a lower-case p in all this chapter means: -log

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Polyprotic Acids

• Acids with more than one ionizable H atom.– Sulfurous acid, H2SO3 ionizes in two steps:

• H2SO3 (aq) H+ (aq) + HSO3- (aq) Ka1 = 1.7 x 10-2

• HSO3- (aq) H+(aq) + SO3

2- (aq) Ka2 = 6.4 x 10-8

• The Ka1 and Ka2 labels refers to the removal of the first H+ and the second H+.

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In previous example, the Ka2 is much smaller than Ka1 .

• 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 H2SO3 than from the negatively-charged HSO3

- ion.• Also, for an acid with three ionizable H+, the

second proton is easier to remove than the third.

• Ka values get successively smaller as protons are removed.

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Page 635 or Appendix D (Page 1044)

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• Because Ka1 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 Ka values differ by a factor of 103 or more, it is possible to obtain a satisfactory estimate of the pH of polyprotic acid solutions by considering only Ka1.

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What is “negligible acidity”?• Substances, such as CH4 and H2, that contain H

but do NOT demonstrate any acidic behavior in water are considered “negligible”.

• Their conjugate bases (CH3- and H-), while

forming in very small concentrations, are strong bases – gaining H+ from water to form OH- ions.– CH3

- + H2O CH4 + OH-

– B A CA CB– H- + H2O H2 + 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.

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HSO3-1 is amphoteric.

• Write an equation for a reaction with water, where HSO3

- acts as an acid;– HSO3

- (aq) + H2O (l) SO32- (aq) + H3O+ (aq)

– A B CB CA

• Write an equation for a reaction with water where HSO3

- acts as an base:– HSO3

- (aq) + H2O (l) H2SO3 (aq) + OH- (aq)

– B A CA CB

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HCO3-1 is amphoteric.

• Write an equation for a reaction with water, where HCO3

- acts as an acid;– HCO3

- (aq) + H2O (l) CO32- (aq) + H3O+ (aq)

– A B CB CA

• Write an equation for a reaction with water where HCO3

- acts as an base:– HCO3

- (aq) + H2O (l) H2CO3 (aq) + OH- (aq)

– B A CA CB

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Water is Amphoteric!

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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

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Facts about autoionization

• Reactions are very rapid in both directions.• At 250C, only about 2 out of every 109

molecules is ionized at any given instant.• Pure water consists of almost entirely H2O

molecules and is a poor electrical conductor• However, the autoionization is very

important.

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Keq = [H3O+][OH-] or [H+][OH-]

• Will use Kw to refer specifically to water’s autoionization from now on; called the ion-product constant.

• At 250C, Kw = 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.

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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 250C

• [H+][OH-] = (x)(x) = 1.0 x 10-14

• x2 = 1.0 x 10-14

• x = 1.0 x 10-7 M = [H+] = [OH-]

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Determine whether acidic, neutral, or basic???

• [H+] = 4 x 10 -9 M

• [OH-] = 1 x 10 -7 M

• [OH-] = 7 x 10 -13 M

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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

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101101

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104.1107

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[OH-] is greater than [H+] Basic

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Calculate the [H+] when

• A. [OH-] is 0.010 M• Keq

= [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

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Calculate [OH-] when

• [H+] is ten times greater than [OH-]Keq

= [H+][OH-] = 1 x 10 -14

= (10x)(x) = 1 x 10-14

= 10x2 = 1 x 10 -14

= x2 = 1 x 10-15

= x = (1 x 10-15)1/2 = 3.16 x 10-8

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The pH Scale

• pH = - log [H+] or - log [H3O+]

• 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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