Announcements First Exam December 21---Christmas Break begins December 22. Silberberg Chapter 17-18, Skoog 2-7, 11-13

Transcription

1 Announcements First Exam December 21---Christmas Break begins December 22. Silberberg Chapter 17-18, Skoog 2-7, Conceptual Multiple choice questions that have little if any math 3-5 multiple piece questions that demonstrate problem solving --Error Analysis (I will give error propagation equations) --Mean, median, SD, RSD, CL and what they mean --Equilibrium Concepts and Problems (17 and 18) --ph and poh and inverse (Ka and Kb) --ppm, ppt, ppb, % w/w, %w/v to molarity to moles to grams describe making up solutions (Chapter 11 Skoog problem) --LeChatlier s principle --Hydrolysis Problem --Bronsted-Lowry, and Lewis Acid Base

2 What happens when we add ionic salts formed from acid-base reactions to water? Ionic Salt + Water ----->????? ph Hydrolysis Reactions (salt + water)

3 There are 4-Possible Acid-Base Neutralization Reactions 1. Strong acid + Strong base -----> Ionic salt + H 2 O HCl (aq) + NaOH (aq) -----> Na + + Cl - + H2O N 2. Strong acid + Weak base -----> Ionic salt + H 2 O HBr (aq) + NH3 (aq) -----> NH4 + + Br - + H2O A 3. Weak acid + Strong base -----> Ionic salt + H 2 O 2HCOOH (aq) + Ca(OH)2 (aq) <---> Ca HCOO - + 2H2O 4. Weak acid + Weak base -----> Ionic salt + H 2 O CH3COOH (aq) + NH3 (aq) <----> NH4 + + CH3COO - + H2O B?

4 1. Salts That Yield Neutral Solutions Salts contain an alkali metal or alkaline earth metal ions and the conjugate base of common strong acids (e.g. Cl -, Br -, I - and NO 3- ) give neutral solutions when added to water. NaCl(s) + H 2 O Na + + Cl + H 2 O Why? NaCl(s) + H 2 O HCl + OH The anion of a strong acid (Cl -, I -, Br - ) is a much weaker base than water and the cation of from the strong acid is a much weaker acid than water!

5 2. Salt That Yield Acidic Solutions: (1) the anion of a strong acid and the cation of a weak base (the cation acts as a weak acid); (2) highly charged metal cations; (3) cations of strong bases and anions of polyprotic acids with an ionizable proton. Case 1 NH 4 Cl(s) NH 4 + (aq) + H2 O(l) NH 4 + (aq) + Cl - (aq) NH 3 (aq) + H 3 O + (aq) Fe(NO 3 ) 3 (s) + 6H 2 O(l) Case 2 Fe(H 2 O) 3+ 6 (aq) + H2 O(l) Fe(H 2 O) 6 3+ (aq) + 3NO3 - (aq) Fe(H 2 O) 5 OH 2 + (aq) + H3 O + (aq) Case 3 NaH 2 PO 4 (s) H 2 PO 4 - (aq) + H2 O(l) Na + (aq) + H 2 PO 4 - (aq) HPO 4 2- (aq) + H3 O + (aq)

6 3. Salts That Yield Basic Solutions: Derived from a strong base and a weak acid. The anion of a weak acid and the cation of a strong base (the anion acts as a weak base) hydrolyzible CH 3 COONa (s) H 2 O Na + (aq) + CH 3 COO - (aq) CH 3 COO - (aq) + H 2 O (l) CH 3 COOH (aq) + OH - (aq)

7 4. Solutions in which both the cation and the anion hydrolyze: Both cation and anion react with water (hydrolyze)! which won wins (exerts its greatest affect on the ph is determined by the Ka vs Kb of the ions! Example: NH 4 HS NH + 4 (aq) + H 2 O NH 3 (aq) + H 3 O+ (aq) HS (aq) + H 2 O H 2 S(aq) + OH (aq) K a (NH 4 + ) = 5.7 x K b (HS - ) = 1.0 x 10-7 Since K b > K a, the solution is basic.

8 Salts from Neutralization Impact Can be Acidic or Basic

9 Predicting the relative acidity of salt solutions PROBLEM: Predict whether aqueous solutions of the following compounds are acidic, basic, or neutral (write an equation for the reaction of the appropriate ion with water to explain ph effect). (a) potassium perchlorate, KClO 4 (b) sodium benzoate, C 6 H 5 COONa (c) chromium trichloride, CrCl 3 (d) sodium hydrogen sulfate, NaHSO 4 PLAN: Consider the acid-base nature of the anions and cations. Strong acid-strong base combinations produce a neutral solution; strong acid-weak base, acidic; weak acid-strong base, basic.

10 Predicting the relative acidity of salt solutions (a) potassium perchlorate, KClO 4 (b) sodium benzoate, C 6 H 5 COONa (c) chromium trichloride, CrCl 3 (d) sodium hydrogen sulfate, NaHSO 4 SOLUTION: (a) The ions are K + and ClO 4 -, which come from a strong base (KOH) and a strong acid (HClO 4 ). The salt solution will be neutral. (b) Na + comes from the strong base NaOH while C 6 H 5 COO - is the anion of a weak organic acid. The salt solution will be basic. (c) Cr 3 + is a small cation with a large + charge, so its hydrated form will react with water to produce H 3 O +. Cl - comes from the strong acid HCl. The salt solution will be acidic. (d) Na + comes from a strong base. HSO 4 - can react with water to form H 3 O +. The salt solution will be acidic.

11 Predicting the behavior of acid-base salts 0.1M solutions of the following are acid/ base/ neutral or amphoteric? HCl NaCl NaCN HCN Na 2 S Na 3 PO 4 NH 4 Cl

12 Kw relates Ka of and acid and Kb of the conjugate base Determine the ph of a M solution of the salt NaF if the Ka of HF is given as 6.80 X 10-4.

13 Kw relates Ka of and acid and Kb of the conjugate base Determine the ph of a M solution of the salt NaF if the Ka of HF is given as 6.80 X I C E F (aq) + H 2 O HF(aq) + OH (aq) 0.1M x x x (0.1-x)! 0.1 x x [B] initial > 100 = M > 100 K b poh=5.92 ph=8.08

14 Determining the ph of a solution of A - Sodium acetate (CH 3 COONa, abbreviated NaAc) has applications in photographic development and textile dyeing. What is the ph of a 0.25 M aqueous solution of NaAc? The K a of acetic acid (HAc) (conjugate acid of Ac) is 1.8 x PLAN: Write balanced equation knowing that sodium salts are completely soluble in water so [Ac - ] = 0.25 M. Write the equilibrium expression--looking for ph. Be organized set up ICE table and fill in the unknowns Use K a to find K b and recall ph + poh = 14.

15 What is the ph of a 0.25 M aqueous solution of NaAc given K a of acetic acid (HAc) (conjugate acid of Ac) is 1.8 x SOLUTION: concentration Ac - (aq) + H 2 O(l) HAc(aq) + OH - (aq) initial change -x - +x +x equilibrium x - x x K b = [HAc][OH- ] [Ac - ] = K w K a K w K a = K b = 1.0 x x 10-5 = 5.6 x 10-10

16 [Ac - ] = 0.25 M - x! 0.25 M (since K b is small) Check assumption: [Ac - ]in / Kb > 100 =.25/ 5.6 x > 100 [HAc][OH - ] K b = [Ac - ] 5.6 x 10-10! x 2 /0.25 M [OH - ] = x! 1.2 x 10-5 M [H 3 O + ] = K w /[OH - ] [H 3 O + ] = K w /[OH - ] = 1.0 x /1.2 x 10-5 = 8.3 x M ph = -log (8.3 x M) = 9.1

17 Molecular Structure and Its Connection to the Strength of an Acid

18 Acidity Depends On Molecular Structure and Strength The strength of an acid (base) depends on its tendency to ionizie or donate a proton (accept a proton)! Two properties generally dictate acid strength: electronegativity of and bond strength. Which property dominates depends on the compounds and whether we move across the periodic table or down a group!

19 Molecular Structure and Strength 2-families of compounds: 1. Non-metal hydrides Binary acids like HF, HCl, HBr 2. Oxo-acids H O Z

20 Strengths of Non-metal hydrides Electronegativity increases, acidity increases Bond strength decreases, acidity increases

21 Non-metal Hydrides Non-metal hydrides: two factors determine acid strength, namely, the electronegativity of the central non-metal atom E and the strength of the E-H bond Electronegativity increases, acidity increases 6A(16) H 2 O H 2 S 7A(17) HF HCl Bond strength decreases, acidity increases H 2 Se H 2 Te HBr HI

22 Oxo-acids are Acid Ternary Compounds Covalent acidic compounds made up of a hydrogen atom bonded to an oxygen atom which is bound to a central atom labeled Z. electronegative central atom! -! + Z O H Z O - + H + IIA IVA VA VIA VIIA H3BO3 H2CO3 HNO3 H2SiO4 H3PO4 H2SO4 HClO3 H3AsO4 H2SeO4 HBrO3 H6TeO6 HIO3

23 Relative Strength of Oxo-Acids For oxoacids with different central atoms that are from the same group of the periodic table and that have the same oxidation number, acid strength increases with increasing electronegativity.

24 Relative Strength of Oxo-acids Oxoacids of Chlorine:

25 Electronegativity Increases...Acidity Increases H O I > H O Br > H O Cl!+!"!+!"!+!" Acid Strength: HIO < HOBr < HOCl O H O Cl!+!" << H O Cl!+!" O HClO 4 > HClO 3 > HClO 2 > HClO O number of O atoms increases, acidity increases

26 Sample Exercise Indicate whether each is a strong or weak acid, name each acid and its sodium salt, and predict the relative strengths of the following groups of oxoacids: a) HClO, HBrO, and HIO. b) HNO 3 and HNO 2. c) H 3 PO 3 and H 3 PO 4.

27 Lewis Acid-Base Definitions A Lewis Acid is an electron-pair acceptor. It is also a Bronsted-Lowry acid. These are generally cations and neutral molecules with vacant valence orbitals, such as H +, Al 3+, Cu 2+, BF 3. A Lewis Base is an electron-pair donor. It is also a Bronsted-base. These are generally anions and neutral molecules with available pairs of electrons, such as H 2 O, NH 3, O 2. Neutralization is defined when a Lewis acid reacts and forms a covalent bond with a Lewis base. The bond is called a coordinate bond.

28 A Lewis acid is an electron-pair acceptor. A Lewis base is an electron-pair donor. The adduct contains a new covalent bond. acid base adduct M(H 2 O) 4 2+ (aq) M 2 + H 2 O(l) adduct

29 The Lewis defintion broadens the scope of acids to cover compounds and ions that do not have a hydrogen ion to donate (as in Bronsted or Arrhenius definition) Lewis Acids with Electron-Deficient Atoms ROR + AlCl 3 R-O-R base acid AlCl 3 adduct Lewis acids contain (or can generate) a vacant orbital. Lewis Acids with Polar Multiple Bonds SO 2 + H 2 O H 2 SO 3 acid base adduct Metal Cations as Lewis Acids M H 2 O M(H 2 O) 2+ 4 acid base adduct Metal ions act as Lewis acids when dissolved in water.

30 The acidic behavior of the hydrated Al 3+ ion electron density drawn toward Al 3 + solvent H 2 O acts as base H 2 O H 3 O + Al(H 2 O) 6 3+ Al(H 2 O) 5 OH 2 + Figure Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

31 high charge and small size enhance acidity

32 Identifying Lewis Acids and Bases PROBLEM: Identify the Lewis acids and Lewis bases in the following reactions: (a) H + + OH - H 2 O (b) Cl - + BCl 3 BCl 4 - (c) K + + 6H 2 O K(H 2 O) 6 + PLAN: Look for electron pair acceptors (acids) and donors (bases). SOLUTION: acceptor (a) H + + OH - donor H 2 O donor (b) Cl - + BCl 3 BCl - 4 acceptor acceptor (c) K + + 6H 2 O K(H 2 O) + 6 donor