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 youre done, Ill shuffle them behind my back and sing a song until you tell me to stop. The quiz on top is the only one Ill 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 ionsa hydronium ion and a hydroxide ionin 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 25C. [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 meters 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!