Ch12-TCF

Ch12-TCF - Quantitative Chemical Analysis Chapter 12: EDTA...

Info iconThis preview shows pages 1–12. Sign up to view the full content.

View Full Document Right Arrow Icon
Quantitative Chemical Analysis Quantitative Chemical Analysis Chapter 12: EDTA Titrations EDTA Titrations
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
1 Complexometric Titrations Many metal ions and ligands form coordination complexes … dilute in H 2 O Cu 2+ H = 2100 kJ/mole Cu OH 2 H 2 OO H 2 H 2 O H 2 O 2 2 + ( aq ) ( gas phase ) Strictly, or 2 + O H H . . ++ . . O H H . . . . + Cu 2+ H O H But usually just written in shorthand … O H H 2 + or 2 + OH 2 or OH 2 2 +
Background image of page 2
2 Ligands that bond to the metal via one atom are monodentate One ligand can bond to the metal by 2, 3, 4 … different atoms within the same ligand. These are called … bidentate , tridentate , tetradentate polydentate ligands. A very common bidentate ligand is 1,2-diaminoethane aka ethylenediamine , “ en ”. H 2 C H 2 C NH 2 2 Cu(H 2 O) 4 [Cu(H 2 O) 6 ] 2 + + 2 + H 2 N CH 2 2 2 .. .. en + Rh(H 2 O) 3 Cl 3 2 2 C 3 P(CH 3 ) 2 2 P(CH 3 ) 2 3 ) 2 H 2 C Me 2 P Me 2 P Rh PMe 2 Cl H 2 C 2 C A complex of a tridentate ligand … + 3H 2 O
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
3 Complexes of bidentate or polydentate ligands are called chelate complexes or chelates from the Greek word for “claw”. The most common chelating agent in analytical chemistry, by far, is EDTA N , N , N ’, N ’-ethylene diamine tetraacetic acid CH 2 N 2 2 2 HO 2 C 2 C N 2 2 CO 2 H 2 H + 2 N 2 2 2 2 C O 2 C N 2 2 2 H 2 H H + (formal but not realistic) EDTA has seven degrees of protonation: H 6 Y 2+ …Y 4 (the neutral molecule is zwitterionic)
Background image of page 4
4
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
5
Background image of page 6
6
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
7 Nonactin Molecular mechanics calculation of the stepwise progress of coordination of K + ion by nonactin.
Background image of page 8
8 The Chelate Effect Consider: [Co(NH 2 CH 3 ) 6 ] 3+ + 3en [Co(en) 3 ] 3+ + 6NH 2 CH 3 The electronic effects of NH 2 CH 2 CH 2 NH 2 and 2NH 2 CH 3 are very similar, but, 4 particles versus 7 particles translational entropy > 0 Example : Cd 2+ ( aq ) + 4NH 2 Me [Cd(NH 2 Me) 4 ] 2+ + 6H 2 O Cd 2+ ( aq ) + 2en [Cd(en) 2 ] 2+ + 2 O * T kJ/mole 2 Me 57.3 67.3 20.1 37.1 56.5 +14.1 +4.2 60.7 ( * J/mole·K) δ∆ S˚ = 24 kJ/mole; ~ +12 kJ/mole for each chelate for ethylenediamine
Background image of page 9

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
9 Another Example : [Ni(NH 3 ) 6 ] 2+ + 3en [Ni(en) 3 ] 2+ + 6NH 3 H˚ = 17 kJ/mole – very close to the extra stabilization expected between NH 3 and NH 2 Me (estimated from electronic spectra to be ~ 12 kJ/mole) That is, electronically , H 2 N-CH 2 -CH 2 -NH 2 H 2 N-CH 3 + CH 3 -NH 2 S˚ = +121 J/mole·K @ 25 ˚C, T S = 36 kJ/mole ~ +12 kJ/mole for each chelate for ethylenediamine
Background image of page 10
10 Now, back to EDTA : + CH 2 N 2 2 2 H O 2 C H O 2 C N 2 2 CO 2 H 2 H H H + 2 2 N 2 2 2 2 2 O 2
Background image of page 11

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 12
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 03/28/2011 for the course CHEM 300L at USC.

Page1 / 31

Ch12-TCF - Quantitative Chemical Analysis Chapter 12: EDTA...

This preview shows document pages 1 - 12. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online