Acids I. and bases: -Bronsted definition: Acids are proton (H1+) donors Bases are proton (H1+) acceptors -What makes a X-H bond acidic? The hydrogen atom has to be bound to an electronegative atom. So, H-CH3 is not acidic, but H-Cl is acidic. -Conjugate acid/base pair differ by only one proton (H1+) HCl + H2O H3O1+ + Cl1acid base c.acid c.base Practice problems: 1. What is the conjugate acid of each base? a) HPO42- _______ b) OH1- ________ c) NH3 ________ d) HSO41- ________ 2. What is the conjugate base of each acid? a) H2CO3 ________ b) HPO42- ______ 1+ c) H3O ________ d) HNO2 _______ II. Relative acid/base strength: Strong acids have weak (stable) conjugate bases. How can we stabilize conjugate bases? a) The negative charge of the base is stabilized by a larger ion. H-F < H-Cl < H-Br < H-I (increasing acid strength) b) The negative charge on the base is also stabilized by electron withdrawing groups and resonance. c) Acid strength of polyatomic acids increases as the oxidation state of the central atom becomes more positive. HClO < HClO2 < HClO3 < HClO4 d) Acid strength of polyatomic acids increases as the electronegativity of the central atom increases. HClO is a stronger acid than HBrO Strong bases have electron donating groups such as methyl(-CH3). Practice problems: 3. Which is the stronger acid? HIO3 or HClO3 4. Which is the stronger acid? HBrO4 or HBrO
5. Which is the stronger base? NH3 or NHCl2
III. Acid dissociation constant (K a): Ka is the equilibrium constant for an acid reacting with water. Equation for the acid HF reacting with water: HF(aq) + H2O(l) F1-(aq) + H3O1+(aq) [ F 1 ][ H 3 O 1 ] Ka [ HF ] Stronger acids larger Ka Weaker acids smaller Ka Strong acids HI, (HClO4, HBr, HCl, H2SO4, HNO3) dissociate completely: Ka >>1 Practice problems: 1. Where on the Ka table are the strong acids? 2. Where on the Ka table are the strongest bases? 3. Consider the following acids: HCN, HNO2, HF, NH41+ a) What is the strongest acid? b) Which is the strongest conjugate base? IV. Extent of acid/base reactions: K a reacting acid K K a produced acid Reaction is extensive and can be written with a single arrow if K > 10 3 Practice problems: Calculate K and determine extent of reaction for the following reactions: 1. HCN + NH3 CN1- + NH41+
2. HF + CN1- F1- + HCN
3. Strong acids have WEAK/STRONG bonds.
4. If the reacting acid is stronger than the produced acid, the reaction will be EXTENSIVE/NOT EXTENSIVE.
V. Writing net acid/base reactions: 1. Any strong acid (Ka >>1) will form H3O1+ and its conjugate base (A1-) in water. 2. Weak acids are written as undissociated acids 3. Ionic compounds are written as dissociated ions. 4. The strongest acid and strongest base are most reactive and will react to form products Example: Solutions of NaC2H3O2 and HCl are mixed Species in solution: Na1+, C2H3O21-, H3O1+, Cl1Strongest acid and base: H3O1+ and C2H3O21Net equation: H3O1+ + C2H3O21- H2O + HC2H3O2 Practice problems: Write net acid/base equations for the following solutions: 1. Solutions of KHSO3 and Na2HPO4 are mixed.
2. Solutions of HClO4 and NO21- are mixed.
3. Solutions of H2S and H2O are mixed
VI. pH and pK a: Kw = [H3O1+][OH1-] = 1x10-14 In pure water: [H3O1+] = [OH1-] = 1x10-7 M If [H3O1+] solution more acidic. If [OH1-] solution more basic pH = -log [H3O1+]; pH basic; pH acidic
Practice problems: 7, 9, 13, 15, 21, 23, 25, 27, 29, 31, 43