Chemistry Lecture

Chemistry Lecture - Chemistry Lecture 2/26/2008 2:22:00 PM...

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Chemistry Lecture 26/02/2008 14:22:00 Relative strength of acids and bases By comparing actual reactions you can construct a table of relative strengths. The stronger the acid, the weaker the conjugate base. The stronger the base, the weaker the conjugate acid. Equilibrium favors. H+ transfer from the stronger acid to the stronger base.  PO43-(base) + H20(acid) <-> HPO42-(c. acid) + OH-(c. base) Equilibrium favors reactants because HPO42- is stronger acid and equilibrium  lies to the left. Types of acids Strong acid: complete transfer of H+ to H20. CB is not protonated. (CB does  not go in reverse direction to form more acid) Weak acid: transfers partially from H+ to H20. Exists as a mixture. CB has  strong tendency to be protonated. (will go back to form more acid) Negligible acidity: contain H+ but do not demonstrate any acidic behavior. (H+  cannot be removed). Weak Bases Kb is the base dissociation constant. NH3 + H20 <-> NH4+ + OH- Kb = [NH4+][OH-] / [NH3] The pH of a solution of a 0.100 M Ch3NH2 is 11.81. Determine Kb for  methylamine. o CH3NH2         CH3NH3+  +  OH- o I   0.100               0               0 o C -0.006457    +0.006457   +0.006457
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o E  0.0935           0.006457     0.006457 o Kb = (0.006457)(0.006457) / (0.0935) = 4.4 x 10^-4 Also important weak bases are the anions of the weak acids, i.e. HF o F-   +   H2O   <->   HF   +   OH- o Anion                 weak acid The strongest weak base has the larget K value. Ka and Kb  used for salt pH NH3 + H2O <-> NH4+ + OH-       Kb = [NH4+][OH-] / [NH3] NH4+ + H2O <-> NH3 + H3O+    Ka = [NH3][H3O+] / [NH4+] 2 H2O <-> H3O+ + OH-              Ka x Kb = [H3O+][OH-] = Kw Ka x Kb = Kw = 1 x 10^-14 Given either K, the other K can be found using the Kw. Acid base properties of salt in solution Salts can be acidic or neutral of basic Assume the salts are completely ionized. pH will be due to hydrolysis of the cations and anions. Hydrolysis: reaction of cations and anions with water to produce H+ and OH-. NaCl gives a neutral solution. Neither the cation nor the anion hydrolyzes. Strong base/strong acid salts. i.e. NaCl, think NaOH, HCl.
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NaF gives a basic solution. Only the anion hydrolyzes. Strong base/weak acid  salts. i.e. think NaOH, NaF F- + H2O <-> HF + OH- Kb(F-) = Kw / Ka(HF) NH4Cl gives an acidic solution. Only the cation hydrolyzes. Strong acid/weak  base salts. i.e HCl, NH3
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This note was uploaded on 04/09/2008 for the course CHEM 122 taught by Professor Zellmer during the Winter '07 term at Ohio State.

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Chemistry Lecture - Chemistry Lecture 2/26/2008 2:22:00 PM...

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