Lecture14_S08

Lecture14_S08 - Feb. 20, 2008 Lecture 14, Chapter 16: Acids...

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Feb. 20, 2008 Lecture 14, Chapter 16: Acids and Bases Can measure relative acid/base strength by comparing Ka’s, pKa’s, and even pH’s but there are some trends that allow for qualitative predictions: H-F H-Cl H-Br H-I weak strong Acid Strength increases - H-X bond lengths increase with increasing size of X - H-X bond strength gets weaker with increasing size - basicity of X - decreases with increasing size Same is true for other periodic series : CH 4 NH 3 H 2 O HF Not weak neutral weak base acid SiH 4 PH 3 H 2 S HCl Not weaker weak strong base acid acid Bronsted acidity Di- and Polyprotic Acids: Undergo multiple successive ionizations Example: Phosphoric Acid, H 3 PO 4 H 3 PO 4 + H 2 O (l) H 2 PO 4 - + H 2 O (l) HPO 4 2- + H 2 O (l) H 2 PO 4 - + H + HPO 4 2- + H + PO 4 3- + H + K a1 = 7.5 x 10 -3 K a2 = 6.2 x 10 -8 K a3 = 4.2 x 10 -13 H 3 PO 4 + H 2 O (l) PO 4 3- + H + Each successive deprotonation gets harder because the conjugate bases get stronger with increasing negative charge and want to reform the acids K tot = K a1 *K a2 *K a3 K tot = 1.953 x 10 -22
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Lecture14_S08 - Feb. 20, 2008 Lecture 14, Chapter 16: Acids...

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