# lec16 - 6 H 5 NH 2 or NH 2 NH 2 ? Relative Strengths HA(aq)...

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1 Announcements • Practice, Practice, Practice • Learning Skills Center Last Time • Autoprotolysis – K w = 1x10 –14 • Concentration scales – pH and pOH • Types of acids and bases Weak Acids • Proton transfer equilibrium HOAc(aq) + H 2 O(l) H 3 O + (aq) + OAc (aq) K a = [H 2 O] K c = • Proton donating strength : pK a = – logK a • For HOAc, K a = 1.8 x 10 –5 and pK a = 4.75 [H 3 O + ][OAc ] [HOAc] Weak Bases • Proton transfer equilibrium NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH (aq) K b = [H 2 O] K c = • Proton accepting strength : pK b = – logK b •F o r NH 3 , K b = 1.8 x 10 –5 and pK b = 4.75 [NH 4 + ][OH ]

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2 Demonstration • What happens when Mg(s) is placed in 6 M solutions of HCl and HOAc? Mg(s) + 2H 3 O + (aq) Mg(OH) 2 (aq) + 2H 2 (g) • Rate depends on [H 3 O + ] Conjugate Confusion • Consider the following reactions: HOAc(aq) + H 2 O(l) H 3 O + (aq) + OAc (aq) (1) OAc (aq) + H 2 O(l) HOAc(aq) + OH (aq) (2) K a = K b = K a x K b = K w –or– pK a + pK b = 14 [H 3 O + ][OAc ] [HOAc] [HOAc][OH ] [OAc ]
3 Example Which is a stronger base C

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Unformatted text preview: 6 H 5 NH 2 or NH 2 NH 2 ? Relative Strengths HA(aq) + H 2 O(l) → H 3 O + (aq) + A – (aq) • If H 2 O is a stronger base than the conjugate base A – , then HA is a strong acid. B – (aq) + H 2 O(l) → HB(aq) + OH – (aq) • If H 2 O is a stronger acid than the conjugate acid HB, then B – is a strong base. ← ← 4 Factors Affecting Acidity • Hydrogen bonding • Electronegativity – HF vs. NH 3 • HA bond strength – HF<HCl<HBr<HI • Rule of thumb – The more polar and weaker the HA bond the stronger the acid pH Calculations • Determine if it is a strong or weak acid or base • Strong acids and bases – trivial • Weak acids and bases … – use equilibrium principles for [H 3 O + ] – equilibrium tables 5 Example What is the pH of the 6M HOAc solution used in the demonstration given that the pK a is 4.75?...
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## This note was uploaded on 04/23/2008 for the course CHEM 2B taught by Professor Clemens during the Winter '08 term at UC Davis.

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lec16 - 6 H 5 NH 2 or NH 2 NH 2 ? Relative Strengths HA(aq)...

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