Friday, April 26 th : “A” Day Monday, April 29 th : “B” Day Agenda  Section 15.1 Quiz: “What are Acids and Bases?”  Begin Section 15.2: “Acidity, Basicity, and pH” Self-ionization constant of H 2 O, K w, neutral, pH, indicator  Homework: Practice pg. 541: #1,2,4,5 Practice pg. 544: # 1-4 Concept Review: “Acidity, Basicity, and pH” Next time: Finish section 15.2/work day!

Section 15.1 Quiz “What are Acids and Bases”  This will be a “stack behind the back” quiz.  Get in groups of 2-4.  Working together, everyone in the group will complete their quiz.  Once you’re done, I’ll shuffle them behind my back and sing a song until you tell me to stop.  The quiz on top is the only one I’ll grade and everyone in that group will get that grade.

Self-Ionization of Water  Water can act as either an acid or a base.  This means that a water molecule can either give or receive a proton. Base Acid Conjugate Acid Conjugate Base  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 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.  The self-ionization of water reaction is an additional reaction that runs in the background. Base Acid Conjugate Acid Conjugate Base

Self-Ionization of Water  In pure water, the two ions have 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 Since K w = [H 3 O + ] [OH - ], then K w = 1.00 X Base AcidConjugate Acid Conjugate Base

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

Sample Problem A, pg. 541 Determining [OH - ] or [H ] using K w  What is the [OH - ] in a 3.00 X 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 M 3.00 X M Because 1:1 ratio, [HCl] = [H 3 O + ] = 3.00 X M K w = [OH - ] [H 3 O + ] = 1.00 X X = [OH - ] (3.00 X M) [OH - ] = 3.33 X M

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

Acidity/Basicity  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.

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 + ]

Calculating pH Example #1  What is the pH if [H 3 O + ] = 3.5 X M? pH = - log [H 3 O + ] pH = - log (3.5 X ) pH = 3.5 (2 sig figs)

Calculating pH Example #2  What is the pH if [OH-] = 5.0 X M? pH = - log [H 3 O + ] K w = [OH - ] [H ] = 1.00 X X = (5.0 X M) [H 3 O + ] [H 3 O + ] = 2.0 X M pH = - log (2.0 X ) pH = 9.7 (2 sig figs)

Sample Problem B, pg. 544 Calculating pH for an Acidic or Basic Solution  What is the pH of (a) a M solution of HNO 3, a strong acid, and (b) a 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) M M Because 1:1 ratio, [HNO 3 ] = [H 3 O + ] = M  pH = -log [H 3 O + ]  pH = -log ( ) pH = 4.0 (2 sig figs)

Sample Problem B, continued  What is the pH of (a) a M solution of HNO 3, a strong acid, and (b) a M solution of KOH, a strong base? (b) KOH is a strong base and ionizes completely: KOH (s) → K + (aq) + OH - (aq) M M  Because 1:1 ratio, [KOH] = [OH - ] = M  K w = [OH - ] [H ] = 1.00 X  1.00 X = ( M) [H ]  [H ] = 7.35 X  pH = - log [H ] pH = - log (7.35 X ) pH = 12.1 (3 sig figs)

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 basic (alkaline).  A solution of pH 7 is neutral.

pH values of some common materials

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 or

Homework  Practice pg. 541: #1,2,4,5  Practice pg. 544: # 1-4  Concept Review: “Acidity, Basicity, and pH” Next time: Finish section 15.2 and work day!