Tài liệu Chapter 12: EDTA Titrations docx

Chapter 12: EDTA Titrations Review of Textbook for Chem115/116: Chapters 17 & 23: Complex (coordinate compound) Coordinate covalent bonds: a bond formed when both electrons of the bond are donated by one atom. Ag + + 2(:NH 3 )  [H 3 N: Ag :NH 3 ] + Electron configuration of Ag [Kr]4d 10 5s 1 5P 0 Ag + [Kr]4d 10 5s 0 5P 0 Sp hybrid orbitals: accommodate 2 pairs of electrons. Linear  Complex ion: A metal ion with Lewis base attached to it through coordinate covalent bond.  Complex (Coordinate compound): a compound consisting either of complex ions and other ions of opposite charge or of neutral complex species. I. Basic Concepts and Terms A. Complex-Formation Reactions:  some elements (mostly metal ions) can form coordination compounds with molecules/ions which have a "spare pair" of electrons.  The central atom, (usually) a metal cation, accepts the pair of electrons from the "donor molecule or ion" to form a coordinate covalent bond.  The empty orbitals of the cation and the orbital occupied by the bonding pair of electrons on the donor ion or molecule (ligand) form a new molecular orbital.  The number of coordinate covalent bonds that a cation tends to form with individual ligands (or functional groups on ligands) is known as its coordination number.  Typical values for coordination numbers are 2, 4 and 6.  The compounds formed can be neutral, positively or negatively charged depending on the charge of the reacting species. Example 12-1 Co 3+ + 6 NH 3 <==> Co(NH 3 ) 6 3+ Central atom= Co 3+ Ligand= :NH 3 Coordination number = 6  When a ligand has a single complexing or donor group in its structure, it is said to be unidentate ,  when there are two groups, it is bidentate ,  When there are three groups, it is tridentate ligand, etc.  When a bidentate (or higher number of donor groups present in the ligand) forms a coordinate covalent compound with a metal cation, we call the resulting compound a metal chelate .  As titrants, multidentate ligands, particularly those w ith 4 to 6 donor groups have the advantage that they usually react in a single step pro c e s s , and the ir re ac tio ns w ith the me tal c atio n are more c o mple te than their unidentate counterparts. Y 4- + Co 3+ <=> CoY - cobalt(III)-EDTA complex Example 12-2 Unidentate Ligand, :NH 3, :I [Co(NH 3 ) 6 ] 2+ , [CuI 2 ] - Bidentate Ligand, glycine HOOC-CH 2 CH 2 COOH \ / :N-CH 2 -CH 2 -N: / \ HOOC-CH 2 CH 2 COOH Hexadentate Ligand: EDTA=Y [CoY] - Structure of a Metal/EDTA Chelate Hexadentate Octahedron M n+ + Y 4- <=> MY (n-4)+ Complex of Metal/EDTA Titration Curves Any Questions? [...]... moles of EDTA = CEDTAV EDTA Determine which one is titrant (in this case, it is EDTA) 1) If moles of Ca2+ > moles of EDTA, it is pre-equivalence point (excess of Ca2+): Such as at (A) CCa2+ = 0.00500 M; CEDTA = 0.0100 M; V Ca2+ = 50.00 mL V EDTA =0.00 mL moles of Ca2+ =2.50x10-4 moles > moles of EDTA = 0 Hence: it is pre-equivalence point (B) CCa2+ = 0.00500 M; V Ca2+ = 50.00 mL CEDTA = 0.0100 M; V EDTA. .. > moles of EDTA = 1.00x10-4 moles 1) If moles of Ca2+ = moles of EDTA, it is at equivalence point (no excess of either Ca2+ or EDTA) : Such as at (C) CCa2+ = 0.00500 M; V Ca2+ = 50.00 mL CEDTA = 0.0100 M; V EDTA =25.00 mL moles of Ca2+ =2.50x10-4 moles = moles of EDTA = 2.50x10-4 moles Hence: 2) at equivalence point If moles of Ca2+ < moles of EDTA, it is post-equivalence point (excess of EDTA) : Such... VEDTA = 35.00 mL > Veq: After the equivalence point pCa: Beyond the equivalence point, [CaY2-] and [Y4-] are obtained directly, and the [Ca2+] is obtained from the equilibrium constant expression: 2- [CaY ] ≈ C CaY 2[Y4-]= α4 CT 0.05000Lx0.00500 M = = 2.94 x10 −3 M 0.05000 + 0.03500 L CT = excess of EDTA + dissociation from CaY2= excess of EDTA + [Ca2+] ([Ca+2] MY(n-4) K MY [MY... ]3 + K1K 2 [H + ]2 + K1K 2 K 3 [H + ] + K1K 2 K 3 K 4 [Y 4- ] = C T α 4 III Equilibrium Calculations Involving EDTA A Titration Curve: Mn+ + Y4- MY(n - 4)+ A titration curve for the reaction of a cation with EDTA: a plot of pM = (- log[Mn+]) on the y-axis, versus volume (mL) of titrant (EDTA solution) on the x-axis Note in Table 14-1, that the KMY values are in the range of 107 to 1025 These reactions... . Chapter 12: EDTA Titrations Review of Textbook for Chem115/116: Chapters 17 & 23: Complex (coordinate compound) Coordinate. :N-CH 2 -CH 2 -N: / HOOC-CH 2 CH 2 COOH II. Titrations with Ethylenediaminetetraacetic Acid (EDTA) EDTA Disassociation EDTA Dissociation Constants: H 4 Y +