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Chp8_chelate - Rxn table(note large Kf => reaction...

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A “Chelating agent” or simply a Chelate is a compound whose structure allows that molecule to bind another, usually a metal cation, via multiple bonding sites. The chelate literally wraps itself around the metal – hence the etymology of the Greek term meaning “claw”. A very common chelating agent is EDTA = ethylene diamine tetra-acetic acid. Although the acid-base chemistry of EDTA can be complex, for our purposes, we can consider the fully deprotonated form: EDTA^4- . Consider then, the reaction: ) ( } { ) ( ) ( 2 4 2 aq EDTA Zn aq EDTA aq Zn The equilibrium constant for this process is termed a “formation constant” and denoted K f . The value for this reaction is 3.0E16. So, suppose 50.0 mg Zn^2+ is mixed into 230 mL of 0.01 M EDTA^4-. Compute the concentration of the uncomplexed zinc ions. Step I: Compute molar amounts for rxn table 0.764 mmol Zn^2+ 2.30 mmol EDTA^4- Step II:
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Unformatted text preview: Rxn table (note, large Kf => reaction extensive): Zn^2+ + EDTA^4-=> {Zn-EDTA}^2-0.764 2.300-0.764-0.764 +0.764 ~0 1.536 0.764 Now, recall file on “Extensive Approximation”. We know that [Zn^2+] is not really zero, just small compared to the other species. Thus, solve Kf expression using eq values of other species viz: M mL mL K EDTA EDTA Zn Zn f 17 16 4 2 2 10 66 . 1 10 . 3 230 / 536 . 1 230 / 764 . ] [ Note that the volume, needed to convert mmol to M, cancel here, but only because of the 1:1 stoichiometry. For the Cu(NH3)4^2+ complex in WA, this would NOT be the case. Thus, it is usually best to include this step even if for some cases the volume ends up cancelling out. This sort of thing is important in, for example, treatments of heavy metal poisoning (the ions, not the music!)....
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