 Wednesday, May 4 th : “A” Day Agenda (1:05 release)  Collect Take-Home Quiz  Section 15.2: “Acidity, Basicity, and pH” Self-ionization constant of H.

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Wednesday, May 4 th : “A” Day Agenda (1:05 release)  Collect Take-Home Quiz  Section 15.2: “Acidity, Basicity, and pH” Self-ionization constant of H 2 O, K w, neutral, pH, indicator  Homework: Section 15.2 review, pg. 547: #1-3, 5, 7-12 Concept Review: “Acidity, Basicity, and pH” Looking Ahead: Titration Lab: Thursday, May 12 th Chapter 15 Webquest/concept review due: Wed, May 18 th

Self-Ionization of Water  Water can act as an acid and as a base.  This means that a water molecule can either give or receive a proton. Base Acid CA CB  A pair of water molecules are in equilibrium with two ions—a hydronium ion and a hydroxide ion—in a reaction known as the self-ionization of water.

Self-Ionization of Water  The equilibrium expression for this reaction is simply K eq = [H 3 O + ][OH − ]  This equilibrium constant, called the self- ionization constant, is so important that it has a special symbol, K w. Base Acid CA CB

Self-Ionization of Water  In pure water, the two ions must share the same concentration.  Experiments show that this concentration is 1.00 × 10 −7 M at 25°C. [H 3 O + ] = [OH − ] = 1.00 × 10 −7 M K w = [H 3 O + ] [OH - ] K w = 1.00 X 10 -14 Base Acid CA CB

Self-Ionization of Water  The product of these two ion concentrations is always a constant.  Anything that increases one of the ion concentrations decreases the other.  If you know one of the ion concentrations, you can calculate the other.

Self-Ionization of Water  The concentration of hydronium ions (H 3 O + ) in a solution expresses its acidity.  The concentration of hydroxide ions (OH - ) in a solution expresses its basicity.

Sample Problem A, pg. 541 Determining [OH - ] or [H 3 0 + ] using K w What is the [OH - ] in a 3.00 X 10 -5 M solution of HCl?  HCl is a strong acid and ionizes completely: HCl (g) + H 2 O (l) H 3 O + (aq) + Cl - (aq) 3.00 X 10 -5 M 3.00 X 10 -5 M  Because 1:1 ratio, [HCl] = [H 3 O + ] = 3.00 X 10 -5 M  K w = [OH - ] [H 3 O + ] = 1.00 X 10 -14  1.00 X 10 -14 = [OH - ] (3.00 X 10 -5 M) [OH - ] = 3.33 X 10 -10 M

Additional Practice What is the [H 3 O + ] in a solution of NaOH whose concentration is 3.75 X 10 -2 M?  NaOH is a strong base and ionizes completely: NaOH (s) Na + (aq) + OH - (aq) 3.75 X 10 -2 M 3.75 X 10 -2 M  Because 1:1 ratio, [NaOH] = [OH - ] = 3.75 X 10 -2 M  K w = [OH - ] [H 3 O + ] = 1.00 X 10 -14  1.00 X 10 -14 = 3.75 X 10 -2 M [H 3 O + ] [H 3 O + ] = 2.67 X 10 -13 M

pH and Acidity  When acidity and basicity are exactly balanced such that the numbers of H 3 O + and OH − ions are equal, we say that the solution is neutral.  Pure water is neutral because it contains equal amounts of the two ions.

Measuring pH  The letters p and H stand for “power of hydrogen”.  pH can be calculated by the following mathematical equation: pH = −log [H 3 O + ]

Measuring pH  Because of the negative sign, as the hydronium ion concentration increases, the pH will decrease.  A solution of pH 0 is very acidic.  A solution of pH 14 is very alkaline.  A solution of pH 7 is neutral.

Measuring pH  The pH equation may be rearranged to calculate the hydronium ion (H 3 O + ) concentration from the pH. [H 3 O + ] = 10 −pH

pH values of some common materials

Sample Problem B, pg. 544 Calculating pH for an Acidic or Basic Solution What is the pH of (a) a 0.00010 M solution of HNO 3, a strong acid, and (b) a 0.0136 M solution of KOH, a strong base? (a): HNO 3 is a strong acid and ionizes completely: HNO 3 (g) + H 2 O (l) → H 3 O + ( aq ) + NO 3 - (aq) 0.00010 M Because 1:1 ratio, [HNO 3 ] = [H 3 O + ] = 0.00010 M  pH = -log [H 3 O + ]  pH = -log (0.00010) pH = 4.00

Sample Problem B continued What is the pH of (a) a 0.00010 M solution of HNO 3, a strong acid, and (b) a 0.0136 M solution of KOH, a strong base? (b): KOH is a strong base and ionizes completely: KOH (s) → K + (aq) + OH - (aq) 0.0136 M 0.0136 M  Because 1:1 ratio, [KOH] = [OH - ] = 0.0136 M  K w = [OH - ] [H 3 0 + ] = 1.00 X 10 -14  1.00 X 10 -14 = (0.0136 M) [H 3 0 + ]  [H 3 0 + ] = 7.35 X 10 -13  pH = - log [H 3 0 + ] pH = - log (7.35 X 10 -13 ) pH = 12.13

Sample Problem C, pg. 545 Calculating [H 3 O + ] and [OH - ] from pH What are the concentrations of the hydronium and hydroxide ions in a sample of rain water that has a pH of 5.05?  [H 3 O + ] = 10 −pH  [H 3 O + ] = 10 -5.05 [H 3 O + ] = 8.91 X 10 -6 M  K w = [H 3 O + ] [OH - ] = 1.00 X 10 -14  8.91 X 10 -6 M [OH-] = 1.00 X 10 -14 [OH - ] = 1.12 X 10 -9 M

Additional Practice If the pH of a solution is 8.1, what is the [H 3 O + ] ? What is the [OH - ] in the solution?  [H 3 O + ] = 10 −pH  [H 3 O + ] = 10 -8.1 [H 3 O + ] = 7.9 X 10 -9 M  K w = [H 3 O + ] [OH - ] = 1.00 X 10 -14  7.9 X 10 -9 M [OH - ] = 1.00 X 10 -14 [OH - ] = 1.3 X 10 -6 M

Indicators  Certain dyes, known as indicators, turn different colors in solutions of different pH.  An indicator is a compound that can reversibly change color depending on the pH of the solution or other chemical change.

pH Meters  A pH meter is an electronic instrument equipped with a probe that can be dipped into a solution.  The probe has two electrodes, one of which is sensitive to the hydronium ion.  An electrical voltage develops between the two electrodes, and the circuitry measures this voltage.  The instrument converts the measurement into a pH reading, which is displayed on the meter’s screen.

Indicators vs. pH Meters  Indicators Quick and convenient Do not give very precise results  pH Meters Very precise More complicated and expensive

Homework  Section 15.2 review, pg. 547: #1-3, 5, 7-12  Concept Review: “Acidity, Basicity, and pH” Be ready for a quiz over this section next time…

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