Unformatted text preview: Acid-Base Proper<es of Salt Solu<ons Salts dissociate completely in H2O : strong electrolytes CH3COONa NH4Cl NaCl What is the pH of a solu<on formed by dissolving these salts in water? MJ Bojan Chem 112 1 Salt Solu9ons When a salt is added to water will the solution be acidic or basic? How do I know? 1. Salts dissociate completely in H2O 2. Does the cation act as an acid (donate a proton)? If so what is Ka? 3. Does the anion act as a base (accept a proton)? If so what is Kb? Hydrolysis: when a cation or anion reacts with H2O to form H3O+(aq) or OH-(aq) MJ Bojan Chem 112 2 Finding the pH of a salt solu<on What is the pH of 0.12 M CH3COONa? Ka (CH3COOH) = 1.8 x 10-5 If I know Ka, how do I get Kb? MJ Bojan Chem 112 3 If I know Ka how do I get Kb ? ACETIC ACID Acid: CH3COOH H+ + CH3COO- Base: CH3COO- + H2O CH3COOH + OH- [H ][CH COO ] K = + " a 3 Kb = [CH 3COOH ] [CH 3COOH ][OH " ] ------------------------------------------------------- ! H2O H+ + OH- Kw=
H CH COO ] [CH COOH][OH ] Ka Kb = [ ][ [CH COOH] [CH COO ]
" [CH COO ]
" 3 3 3 " 3 3 Ka Kb = [H+][OH-] = Kw = 1 x 10-14 ! ! pKa + pKb = 14 MJ Bojan Chem 112 4 Salt Solu9ons Acid + Base HCl+ NaOH Salt + water NaCl + H2O NH4Cl CH3COOH+ NaOH CH3COONa + H2O HCl + NH3 Are the salts acidic or basic? How do you know? (look at the anions/ca<ons) acid H+ OH- CH3COO- NH4+ Na+ Cl- MJ Bojan Chem 112 5 base Salt Solu9ons Is CH3COONa(aq) acidic or basic? CH3COONa CH3COO-(aq) + Na+(aq) Does CH3COO- or Na+ hydrolyze water? Is NH4Cl(aq) acidic or basic? NH4Cl(aq) NH4+(aq) + Cl-(aq) Does NH4+(aq) or Cl-(aq) hydrolyze water? MJ Bojan Chem 112 6 Salt Solu9ons Is NH4Cl(aq) acidic or basic? NH4Cl(aq) NH4+(aq) + Cl-(aq) Does NH4+(aq) or Cl-(aq) hydrolyze water? MJ Bojan Chem 112 7 Example: What is the pH of a 1.5 M NH4Cl solu9on? (pKb of NH3 = 4.74) MJ Bojan Chem 112 8 FYI: Salt Solu<ons Will a salt be acidic or basic? 1. Salt derived from a strong acid and a strong base Neutral solu<on (pH = 7) Example: NaCl (from NaOH and HCl) 2. Salt derived from a weak acid and a strong base Basic solu<on (pH > 7) Examples: NaClO (NaOH and HClO) ClO-(aq) + H2O HClO (aq) + OH-(aq) (CH3COO)2Ba (from Ba(OH)2 and CH3COOH) CH3COO-(aq) + H2O CH3COOH(aq) + OH-(aq) MJ Bojan Chem 112 9 FYI: Salt Solu<ons 3. Salt derived from a strong acid and a weak base Acidic solu<on (pH <7) Example: NH4Cl (NH3 and HCl) NH4+ + H2O NH3 + H3O+ 4. Salt derived from a weak acid and a weak base pH depends on acid/base involved Example: NH4CN (NH4+ and CN-) MJ Bojan Chem 112 10 Acids and Bases Why? Why do some compounds act as acids while others act as bases?
Na--O--H O N O O H Why are some protons acidic, when others are not? H H C H C O H O MJ Bojan Chem 112 11 Summarize: Factors Affec9ng Acid Strength 1. Strength of the bond: 2. Polarity of the bond: 3. Stability of the conjugate base:. MJ Bojan 12 Chem 112 What is the effect of Bond strength The stronger the H--X bond, the weaker the acid. The shorter the H--X the weaker the acid. Example: H--X Bond length H--F H--Cl H--Br H--I Which bonds are the longest? Strongest? Which acid(s) are the strongest? Weakest? WHY?
MJ Bojan Chem 112 13 bond strength Bond polarity How does bond polarity effect acid strength? The greater the bond polarity of the HX bond, the weaker the acid.
Examples: MJ Bojan Chem 112 14 Bond Polarity and Strength Trends in Hydrides Bond polarity is the major factor in acid strength for acids in the same row. Acidity as electronega9vity of X . Bond strength is the most important factor for binary acids (H-X) in the same GROUP. MJ Bojan Chem 112 15 Oxyacids Many acids consist of a central atom with several attached oxygen atoms. These are called oxyacids. E,g. HOClO3 HOClO2 HOClO HOCl Consider: If Y is a metal, the compound is a BASE. MJ Bojan Chem 112 16 Oxyacids If Y is a non-metal, as the electronega-vity of Y increases, so will the acidity of the substance. If Y is bonded to addi9onal electronega9ve atoms that pull electron density away from the O-H bond, the strength of the acid will increase. MJ Bojan Chem 112 17 Summary of Oxyacids: For oxyacids that have the same number of OH groups and the same number of Oxygen atoms: MJ Bojan Chem 112 18 Summary of Oxyacids: For oxyacids that have the same central atom Y: MJ Bojan Chem 112 19 Carboxylic Acids Hydrogen atoms in carboxylic acids are acidic because of the electronega9vity of oxygen. The stability of the conjugate base is increased by resonance (pair of electrons is shared): MJ Bojan Chem 112 20 Sample Problem Which of the following salts will give the most basic solution when dissolved in water? 1. KBrO4 2. KBrO3 3. KBrO2 4. KBrO 5. KClO4 MJ Bojan Chem 112 21 Sample Problem Of the following, __________ is the strongest acid.
A. B. C. D. E. F3C-COOH Cl3C-COOH Br3C-COOH Br2ClC-COOH H3C-COOH MJ Bojan Chem 112 22 Lewis Acids and Bases Lewis acids: electron pair Lewis bases: electron pair MJ Bojan 23 Chem 112 Metal Ions: Hydra9on WHY do metal ions behave like acids? Hydration is a Lewis acid/base interaction between the
metal cation and water. Mz+ - : O H + : H MJ Bojan Chem 112 24 Metal Ions: Hydra9on Hydra<on can lead to hydrolysis (a reac<on that dissociates water). Example of hydrolysis of a hydrated metal ion M(H2O)nz+ M(H2O)n-1(OH)(z-1)+ + H+ Fe(H2O)63+ Fe(H2O)5(OH)2+ + H+ H H H + H+
H H : O Mz+ : O H H Mz+ MJ Bojan Chem 112 : O : O 25 Metal Ions: Hydra9on Hydration increases with increasing charge/size ratio of the metal ions. Acidity of the hydrated ca<on INCREASES with charge and size. MJ Bojan Chem 112 26 To compare acidity of Lewis acids, first compare charge. If charge is the same then compare size. Charge/Size Ra9os Metal Ion Charge/Ionic radius Na+ 1.0 Li+ 1.5 Ca2+ 2.1 Mg2+ 3.1 Zn2+ 2.7 Cu2+ 2.8 Al3+ 6.7 Cr3+ 4.8 Fe3+ 4.7
MJ Bojan Chem 112 27 LEWIS CATIONS ...
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- Summer '07
- pH, Chem, Mj, Bojan Chem, Bojan