lec15_whc - Last Time Autoprotolysis Kw = 11014...

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Last Time • Autoprotolysis – K w = 1·10 –14 • Concentration scales – pH and pOH ('p' means the negative logarithm in base ten) • Types of acids and bases
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OAc (aq) + H 2 O(l) OH - (aq) + HOAc(aq) Conjugate Confusion • Consider the following reactions: HOAc(aq) + H 2 O(l) H 3 O + (aq) + OAc (aq) (1) acid conjugate base (conjugate acid) (base) conjugate acid (conjugate base) base (acid) [H 3 O + ][OAc ] [HOAc] K a = [HOAc][OH ] [OAc ] K b = K a ·K b = K w –or±– pK a + pK b = 14
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HOAc(aq) + H 2 O(l) H 3 O + (aq) + OAc (aq) OAc (aq) + H 2 O(l) HOAc(aq) + OH (aq) Are pK a and pK b related? e.g. Acetic acid [H 3 O + ][OAc ] [HOAc] [HOAc][OH ] [OAc ] K a = K b = e.g. Sodium acetate But: K a × K b = [H 3 O + ] [OAc ] [HOAc] [OAc ] [HOAc] [OH ] × = K w K a x K b = K w –or ±– pK a + pK b = 14 More next time.
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Brønsted Acidity Bond strength to hydrogen HI HBr HCl HF 160.9 141.4 127.4 91.7/pm 297 368 431 569/kJ mol -1 10 9 10 8 1.3·10 6 6.6·10 -4 /K a strong weak • Oxidation number (H 2 S versus H 2 SO 4 ) • Rule of thumb: The more polar and weaker the HA bond, the stronger the acid.
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Strengths are determined relative to H 2 O HA(aq) + H 2 O(l) H 3 O + (aq) + A (aq) •I f 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) f H 2 O is a stronger acid than the conjugate acid HB, then B is a strong base.
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How to do pH Calculations: • Determine if it is a strong or weak acid or base and if it is at high concentrations relative to 10 -7 M • If the above is true, the calculation is trivial; they dissociate completely. • Weak acids and bases are tougher – use equilibrium principles and calculate [H 3 O + ] – equilibrium (ICE) tables
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Calculate pH Strong acid or base: Weak acid or base: pH = -log 10 [H 3 O + ] = -log 10 [HCl] initial HCl(aq) + H 2 O(l) Æ H 3 O + (aq) + Cl (aq) HC 2 H 3 O 2 (aq) + H 2 O(l) H 3 O + (aq) + C 2 H 3 O 2 (aq) K a = [CH 3 CO 2 H] [CH 3 CO 2 ][H 3 O + ] Æ equilibrium/ICE table!
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This note was uploaded on 06/13/2010 for the course CHEM 995940767 taught by Professor Topadakis during the Spring '10 term at UC Davis.

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lec15_whc - Last Time Autoprotolysis Kw = 11014...

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