I. The Dissociation of Water

Transcription

1 AP Chem Unit #12

2 I. The Dissociation of Water A. What is the process known as the autoionization of water? B. K w = C. What are the [H + ] and [OH ] in pure distilled water? D. Compare and contrast the [H + ] and the [OH ] in an acid and a base: E. Sample Exercise 16.2 calculate the [H + ] (aq) in (a) a solution in which the [OH ] = M; (b) a solution in which [OH ] = 2.0 x 10 9 M. (show work) F. What happens to the H + ion (proton) when it is in water? (diagram it!) Review Concept What is a coordinate covalent Bond and how does it fit in with this concept?

3 II. Theories of Acids and Bases A. Arrhenius definitions: 1. Acid 2. Base B. Bronsted-Lowry Definitions: 1. Acid 2. Base C. Lewis Definition: 1. Acid 2. Base D. Bronsted-Lowry Conjugate Acid-Base Pairs 1. What are Conjugate Acid-Base Pairs? How can you recognize them? 2. What is the conjugate Acid and the conjugate base in the following equation? HNO 3 (aq) + H 2 O (l) NO 3 (aq) + H 3 O + (aq)

4 3. Sample Exercise 16.3 (a) what is the conjugate base of each of the following: HClO 4, H 2 S, PH 4 +, HCO 3 (b) what is the conjugate acid of each of the following: CN, SO 4 2, H 2 O, HCO 3? (Show Work) 4. Sample Exercise 16.4 The hydrogen sulfite (bisulfite) ion, HSO 3, is amphoteric. (a) Write an equation for the reaction of HSO 3 with water in which the ion acts as an acid. (b) write an equation for the reaction of HSO 3 with water in which the ion acts as a base. In both cases, identify the conjugate acid-base pairs. 5. What is an amphoteric substance? 6. Conjugate Acid-Base Strengths What is the general rule? 1. List 6 strong acids and give their conjugate bases. 2. List 6 strong bases and give their conjugate acids.

5 III. The ph and poh Scale A. ph Scale B. poh Scale C. Sample Exercise 16.7 A sample of freshly pressed apple juice has a ph of Calculate [H + ]. Sample Exercise An antacid tablet produces a solution with a poh of 4.82, calculate [OH ] D. Measuring ph

6 E. Acid Base Indicators An acid base indicator is a substance whose color changes according to the ph of the solution. They are often weak acids where the ionized and the unionized form have different colors. HIn (aq) H + (aq) + In - (aq) COLOUR 1 COLOUR 2 This equilibrium will be disturbed by the addition of H + (aq) ions or OH - (aq) ions. For example a high concentration of H + (aq) ions (acid solution) will shift the equilibrium to the LHS giving COLOUR 1. In practice the indicator will change color over a small given range of ph. Examples Indicat or ph Methyl orange R R R R * * Y Y Y Y Y Y Y Y Methyl red R R R R R * * Y Y Y Y Y Y Y Litmus R R R R R R * * B B B B B B Phenolphthalein C C C C C C C C * * R R R R Thymolphthalein C C C C C C C C C C * * B B Where R = RED, B = BLUE, Y = YELLOW, C = COLOURLESS, * = the ph range where the color change takes place. IV. Strong Acids and Bases A. Strong Acids Sample Exercise 16.8 What is the ph of a M solution of perchloric acid?

7 B. Strong Bases Sample Exercise 16.9 What is the ph of a M solution of Ca(OH) 2? V. Weak Acids A. K a (the acid dissociation constant) Sample Exercise A student prepared a 0.10 M solution of formic acid, HCHO 2, and measured its ph using a ph meter. The ph at 25 C was found to be (a) Calculate K a for formic acid at this temperature. (b) What % of the acid is ionized in this 0.10 M solution?

8 B. Calculating ph of solutions with weak acids: Given: HC 2 H 3 O 2 (aq) H + (aq) + C 2 H 3 O 2 (aq) K a = 1.8 x 10 5 (at 25 C) 1 st Step Write the ionization equilibrium equation for the acid. 2 nd Step Write the equilibrium expression with the K a value that is given. 3 rd Step Express the equilibrium [ ] using variables derived from the initial [ ]. 4 th Step Substitute the variables that express the equilibrium [ ] back into the equilibrium expression for the acid and solve for ph once [H + ] is known. 5 th Step Find the % ionization % ionization = [equilibrium concentration of H + ] x 100% [initial concentration of Acid]

9 C. Sample Exercise Calculate the ph of a 0.20 M solution of HCN, K a = 4.9 x D. Sample Exercise Calculate the % of HF molecules ionized in (a) a 0.10 M HF solution; (b) a M HF solution.

10 E. Polyprotic Acids Sample Exercise The solubility of CO 2 in pure water at 25 C and 0.1 atm pressure is M. The common practice is to assume that all of the dissolved CO 2 is in the form of carbonic acid, H 2 CO 3, which is produced by reaction between the CO 2 and H 2 O: CO 2 (aq) + H 2 O (l) H 2 CO 3 (aq) What is the ph of a solution of H 2 CO 3?

11 VI. Weak Bases A. K b the base dissociation constant B. Sample Exercise Calculate the concentration of OH in a 0.15 M solution of NH 3. C. Types of Weak Acids D. Sample Exercise A solution is made by adding solid NaClO to enough water to make 2.00 L of solution. If the solution has a ph of 10.50, how many moles of NaClO were added to the water?

12 VII. Relation Between K a and K b A. The Equation B. Sample Exercise Calculate the (a) the base dissociation constant, K b, for the fluoride ion, F ; (b) the acid dissociation constant, K a, for the ammonium ion, NH 4 +. VIII. Acid Base Properties of Salt Solutions A. Hydrolysis

13 B. Sample Exercise Predict whether the salt Na 2 HPO 4 will form an acidic or basic solution on dissolving in water. C. Salt derived from a strong base and a strong acid D. Salt derived form a strong base and a weak acid E. Salt derived from a weak base and a strong acid F. Salt derived from a weak base and a weak acid