Wednesday, May 4th: “A” DayAgenda (1:05 release) • Collect Take-Home Quiz • Section 15.2: “Acidity, Basicity, and pH” • Self-ionization constant of H2O, Kw, 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 12th Chapter 15 Webquest/concept review due: Wed, May 18th
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 AcidCA 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 Base Acid CA CB • The equilibrium expression for this reaction is simply Keq = [H3O+][OH−] • This equilibrium constant, called the self-ionization constant, is so important that it has a special symbol, Kw.
Self-Ionization of Water • In pure water, the two ions must share the same concentration. • Experiments show that this concentration is1.00 × 10−7 M at 25°C. [H3O+] = [OH−] = 1.00 × 10−7 M Kw = [H3O+] [OH-] Kw = 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 (H3O+) in a solution expresses its acidity. • The concentration of hydroxide ions (OH-) in a solution expresses its basicity.
Sample Problem A, pg. 541Determining [OH-] or [H30+] using Kw 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) + H2O (l) H3O+(aq) + Cl-(aq) 3.00 X 10-5 M 3.00 X 10-5 M • Because 1:1 ratio, [HCl] = [H3O+] = 3.00 X 10 -5 M • Kw = [OH-] [H3O+] = 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 [H3O+] 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 • Kw = [OH-] [H3O+] = 1.00 X 10 -14 • 1.00 X 10 -14 = 3.75 X 10 -2 M [H3O+] [H3O+] = 2.67 X 10-13 M
pH and Acidity • When acidity and basicity are exactly balanced such that the numbers of H3O+ 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 [H3O+]
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 (H3O+) concentration from the pH. [H3O+] = 10−pH
Sample Problem B, pg. 544Calculating pH for an Acidic or Basic Solution What is the pH of (a) a 0.00010 M solution of HNO3, a strong acid, and (b) a 0.0136 M solution of KOH, a strong base? (a): HNO3 is a strong acid and ionizes completely: HNO3 (g) + H2O (l) → H3O+ (aq) + NO3- (aq) 0.00010M 0.00010M Because 1:1 ratio, [HNO3] = [H3O+] = 0.00010M • pH = -log [H3O+] • pH = -log (0.00010) pH = 4.00
Sample Problem B continued What is the pH of (a) a 0.00010 M solution of HNO3, 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 • Kw = [OH-] [H30+] = 1.00 X 10 -14 • 1.00 X 10 -14 = (0.0136 M) [H30+] • [H30+] = 7.35 X 10 -13 • pH = - log [H30+] pH = - log (7.35 X 10 -13) pH = 12.13
Sample Problem C, pg. 545Calculating [H3O+] 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? • [H3O+] = 10−pH • [H3O+] = 10 -5.05 [H3O+] =8.91 X 10-6 M • Kw = [H3O+] [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 [H3O+] ? What is the [OH-] in the solution? • [H3O+] = 10−pH • [H3O+] = 10 -8.1 [H3O+] = 7.9 X 10-9 M • Kw = [H3O+] [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…