010913Chem116lecture

010913Chem116lecture - Predicting acid strength from...

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Unformatted text preview: Predicting acid strength from molecular structure Look for the strength of the bond (are electrons fully shared with H?) increasing size weak acid strong acids H 2 S + H 2 O What’s more acidic, H 2 S or H 2 O? Relative strengths of acids and bases 1.0 7.5 x 10-3 3.5 x 10-4 1.8 x 10-5 4.3 x 10-7 9.5 x 10-8 6.2 x 10-8 5.7 x 10-10 5.6 x 10-11 2.2 x 10-13 1.0 x 10-14 very small very large K a H 2 S + H 2 O H 3 O + + HS- K a = [H 3 O + ][HS- ] [H 2 S] = 9.5 x 10-8 acid conjugate base base conjugate acid Recognizing Acid-Base Properties of Salt Solutions Salts made up of anions from strong acids together with cations from strong bases: • No acid-base behavior neutral solutions • Cations – group I: Li + , Na + , K + , Rb + , Cs + group II: Mg 2+ , Ca 2+ , Sr 2+ , Ba 2+ (not Be 2+ ) • Anions - Cl ¯ , Br ¯ , I ¯ , NO 3 ¯ , ClO 4 ¯ 1. Look for ions in tables of K a or K b values Will a 0.1 M solution of Mg(NO 3 ) 2 be acidic, basic, or neither? Relative strengths of acids and bases 1.0 7.5 x 10-3 3.5 x 10-4 1.8 x 10-5 4.3 x 10-7 9.5 x 10-8 6.2 x 10-8 5.7 x 10-10 5.6 x 10-11 2.2 x 10-13 1.0 x 10-14 K a 1.0 1.3 x 10-12 2.9 x 10-11 5.6 x 10-10 2.3 x 10-8 1.1 x 10-7 1.6 x 10-7 1.7 x 10-5 1.8 x 10-4 4.5 x 10-2 1.0 x 10-14 K b Salts made up of anions from weak acids together with cations from strong bases: • the anions from weak acids are themselves somewhat strong conjugate bases • conjugate base reacts with water, yields OH ¯ (basic solution) A ¯ ( aq ) + H 2 O( l ) HA( aq ) + OH ¯ ( aq ) examples/ Na CN , K 2 S , K F , Mg( OAc ) 2 , Ca( OCl ) 2 , Na 2 CO 3 2. Recognizing Acid-Base Properties of Salt Solutions Look for ions in tables of K a or K b values Will a 0.1 M solution of MgF 2 be acidic, basic, or neither? Relative strengths of acids and bases 1.0 7.5 x 10-3 3.5 x 10-4 1.8 x 10-5 4.3 x 10-7 9.5 x 10-8 6.2 x 10-8 5.7 x 10-10 5.6 x 10-11 2.2 x 10-13 1.0 x 10-14 K a 1.0 1.3 x 10-12 2.9 x 10-11 5.6 x 10-10 2.3 x 10-8 1.1 x 10-7 1.6 x 10-7 1.7 x 10-5 1.8 x 10-4 4.5 x 10-2 1.0 x 10-14 K b Salts made up of anions from strong acids together with cations from weak bases: • the cations from weak bases are themselves reasonably strong conjugate acids...
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This note was uploaded on 05/15/2011 for the course CHM 116 taught by Professor Matchett during the Fall '11 term at Grand Valley State.

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010913Chem116lecture - Predicting acid strength from...

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