Chem208_Lecture16_Rev_Color - Today: Polyprotic Acids...

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Unformatted text preview: Today: Polyprotic Acids (16-6); Titrations (17-4) Tues, 3/25: Lewis Acids/Bases (16.9) Solubility Equilibria (Chapter 18) We will not cover material in 16-8. PS6 due by 2:00 Fri, March 14 (tomorrow) Pick up or download PS7 today Question #2: 50.0 mL of the aqueous solution are titrated 1 Polyprotic Acids/Bases Acids containing more than one hydrogen capable of dissociation Common examples: H2SO4, H2S; H3PO4 Polybasic bases: H2NNH2 (hydrazine); H2NCH2CH2NH2 (ethylene diammine) Dissociation or protonation is a step-wise process 2 Calculate the pH, [HCO3–], and [CO32–] at equilibrium in a solution initially 0.014 M in carbonic acid, “H2CO3” Ka1 = 4.4 x 10–7 and Ka2 = 4.7 x 10–11 For “H2CO3”. 3 Titrations An analytical tool used to experimentally determine the concentration of an unknown. Used in both acid/base (proton transfer) and REDOX (electron transfer) reactions. Acid/base titrations involve the neutralization reaction Acid + Base ⇔ Salt + Water 4 Acid/Base Titrations Three Possibilities: 1. Strong acid + strong base - pH determined by excess acid or base. - No equilibrium constants necessary for calculations 2. Weak acid with strong base: - Text: Acetic acid with NaOH - Today: Hypochlorous acid (HOCl) with NaOH 3. Weak base with strong acid: - PS 7: Problem #4 5 All three titration reaction types: Strong Acid/Strong Base Weak Acid/Strong Base Weak Base/Strong Acid Proceed to completion because K for each is very large 6 Titration The determination of the concentration of an unknown acid or base involves neutralization using known quantities of base or acid. At the Equivalence point: moles acid = moles base (stoichiometric) (MaVa = MbVb) At the Endpoint: an indicator undergoes a color change. (visual) 7 Titration of 25.0 mL of 0.10 M HCl with 0.10 M NaOH mL NaOH added 0.0 5.0 10.0 15.0 20.0 23.0 24.0 25.0 26.0 30.0 35.0 40.0 50.0 pH 1.00 1.18 1.37 1.60 1.95 2.38 2.69 7.00 11.29 11.96 12.22 12.36 12.52 8 Colors of Indicators depend upon pH 9 Titration of 25.0ml of 0.10 M HCl Volume of 0.100M NaOH, mL Strong acid + strong base: pH = 7 at equivalence pt. 10 Best Indicator choice (Equiv. pt. = Endpoint) Poor Indicator choice (Equiv. pt. ≠ Endpoint) Volume of 0.100M NaOH, mL Choose indicator such that the endpoint coincides with the equivalence point. 11 Colors of Indicators depend upon pH 12 Titration of 25.0 mL of 0.10 M HOCl with 0.10 M NaOH (Ka = 2.9 x 10–8 for HOCl) mL NaOH added added 0.0 5.0 10.0 12.5 15.0 20.0 22.0 24.0 25.0 26.0 30.0 35.0 40.0 50.0 pH 4.27 6.94 7.36 7.54 7.72 8.14 8.41 8.92 10.12 11.29 11.96 12.22 12.36 12.52 13 ...
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This note was uploaded on 03/29/2009 for the course CHEM 2080 taught by Professor Davis,f during the Spring '07 term at Cornell University (Engineering School).

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