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CHM120H5 Lecture Notes - Lecture 1: Stability Constants Of Complexes, Cuvette, Spectrophotometry

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plumsardine237
14 Apr 2016
School
UTM
Department
Chemistry
Course
CHM120H5
Professor
Judith C Poe

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Document Summary

Spectrophotometric determination of the stability constant of a complex ion. Some common ligands are h2o, nh3, cl- and heavier halides, co, cn- and scn. Many transition metal cations form very stable complex ions with many of these ligands and the stability of these complex ions is expressed by the equilibrium constant for the formation of the complex ion. Such equilibrium constants are referred to as stability constants or formation constants. In this experiment an aqueous solution containing ferric ions is reacted with thiocyanate ion to form a complex ion. Because the concentration of water in dilute aqueous solution is essentially constant, we normally omit the waters in the equation and write a simplified version. Fe+3(aq) + scn-(aq) fe(scn)+2(aq) for which the stability constant is expressed as. The object of this experiment is to determine the value of this stability constant.

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Related Questions


Constants | Periodic Table

A complex ion is a species in which a central metal ion is attached to a group of surrounding ions or molecules by coordinate covalent bonds. The surrounding species are called ligands. Within a ligand, the atoms that are directly attached to the metal ion (or atom) are called donor atoms. Most ligands contain a single donor atom (monodentate), but some contain two donor atoms (bidentate), six donor atoms (hexadentate), or another combination of donor atoms. A complex ion that is associated with one or more counter ions is called a coordination compound. For example [PdCl4]2−is a complex ion, and Na[PdCl4] is a coordination compound. The following table lists some common ligands.
Ligand Number of
donor atoms
Cl− 1
F− 1
Br− 1
CO32− 2
CN− 1
gly− 2
EDTA4− 6
NH3 1
H2O 1
CO 1
The coordination number for a complex is the total number of donor atoms surrounding the central atom. The overall charge on a complex is the sum of the charges of the metal and all the ligands.

Part A

What is the coordination number for each of the following complexes or compounds?

[Co(NH3)4Cl2]+

[Cr(EDTA)]−

[Zn(H2O)4]2+

Na[Ag(CN)2]

Enter the coordination numbers, in the order that the complexes are listed, as four integers separated by commas (e.g., 1,2,3,4).

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Part B

What is the charge on each of the following complex ions?

hexaaquamanganese(II), [Mn(H2O)6]?

tris(carbonato)aluminate(III), [Al(CO3)3]?

amminepentachloroplatinate(IV), [Pt(NH3)Cl5]?

Express the charges numerically, in the order that the complexes are listed, separated by commas. For a nonzero charge, be sure to include the sign (e.g., +1,-2,+3).

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Part C

What is the oxidation number of the central metal ion in each of the following complexes or compounds?

[NiCl2Br2]2−

[Cr(H2O)2(NH3)4]3+

Na[Au(CN)2]

Express the oxidation numbers numerically, in the order that the complexes are listed, separated by commas. For a nonzero oxidation number, be sure to include the sign (e.g., +1,-2,+3).

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I want to find the Kn value of Cu(II) and EDTA. I was given the formula kn= [ML4]/[ML3][L]. 12.3 ml of edta was used to tritrate 0.1194 grams of Copper in 50 ml of water. The indicator used was murexide.

At the core of general acid-base theories, there is the concept of ‘transfer’ of one entity to another. In the case of Brønsted-Lowry acid base theory, the transferred entity is the proton, and their transfer processes are widely exploited in chemical methods. In addition to proton transfer, electron pairs can also be donated/shared between donors and acceptors. In this case, an atom/molecule, which can accept a pair of electrons, is referred to as a Lewis acid while the entity donating the pair of electrons is referred to as a Lewis base. The most prevalent Lewis acid-base systems involve metal complexes with vacant nd-orbitals, as well as electron deficient systems including B, Si, Ge, etc. The Lewis acid base theory is also known as the Hard-Soft acid base theory (HSABT) with ‘hardness’ defined as small highly charged species that are weakly polarizable (Lewis acids). In contrast, ‘softness’ is described by species that are larger and highly polarizable species with low charge (Lewis bases).
In the case of metal chemistry, Lewis acid-base theory provides a means to describe the ability of ‘ligands’ to coordinate to metal ions as well as the stability of metal-ligand complexes. Recall that coordination bonds are the donation/acceptance of lone pairs of electrons to empty metal orbitals to another atom; the definition of Lewis acid-base chemistry! The stability of Lewis acid-base complexes can be described through the stability constant which is analogous to the Ka that describes Brønsted-Lowry acidities. Many metal ions can coordinate (bind) many Lewis base ligands in a step-wise fashion according to:
M + L = ML K1 = [ML]/[M][L] Eq. 1
ML + L = ML2 K2 = [ML2]/[ML][L] Eq. 2
ML2 + L = ML3 K3 = [ML3]/[ML2][L] Eq. 3
MLn-1 + L = MLn Kn = [ML4]/[ML3][L] Eq. 4
and for which the free energy for this dissociation reaction is given by:
ΔG0 = -RTLnKn Eq. 5.
where R is the gas constant and T is the temperature in Kelvin.
Figure 1: Left- EDTA. Right- Typical EDTA-transition metal complex.
The ethylene diamine tetraacetic acid ligand (EDTA, a Lewis base) provides a mechanism to probe the Lewis acid strength of various metal complexes (Figure 1). This multi-dentate ligand (multiple Lewis base sites on a single ligand) forms complexes with metals of the form:
Mn+ + Y4- = MYn-4 Eq. 6
K = [MYn-4]/[Mn+][Yn-4] Eq. 7
where Y4- is the EDTA ligand.
In this experiment, the Lewis acid strength for two different metals will be examined by measuring the stability constants for the metal-EDTA complex. Of specific interest is the how the Lewis acid strength of the metal ion correlates with:
1) atomic/ionic radii, 2) oxidation state, 3) electron configuration, and 4) polarizability.
You will determine the stability constants of both a Cu2+- and Zn2+-EDTA complexes as well as the corresponding free energy. Using these data, you will compare the relative Lewis acidities of both metals and describe differences in terms of the metal ion properties described above.
.
Chemicals
CuSO4
ZnSO4
EDTA
Murexide
Instruments
Buret
Experimental
You and your group will determine the concentration and ultimately the free energy of Cu(II) and Zn(II) using the hexadentate chelating ligand, EDTA.
You will be given a ~100 mM standardized solution of EDTA (check the label for exact concentration prior to beginning). You will need to weigh ~0.1 g per metal salt sample. Dissolve the metal salt into 50 mL of DI water. Add 2-3 drops of murexide to your metal. After properly preparing your buret with EDTA, titrate your metal with EDTA. Repeat for a total of 3 titrations per metal.
1. Approximately how much EDTA will you need for each titration assuming the metal-salts’ masses are accurate?
2. What type of indicator is murexide and what color will its endpoint be?
Post-Lab Questions
1.
a) Using the equations given in the introduction and your titration results, what are the Kn of the EDTA-Copper complex? Show calculations.
b) Using the equations given in the introduction and your titration results, what are the Kn of the EDTA-Zinc complex? Show calculations.
2. What are the corresponding ΔG values for the EDTA-metal complexes? Show your calculations
3. Would copper be considered a hard or soft lewis acid? What about zinc? Describe what makes these metals hard or soft.
4. What trends would you predict for EDTA-metal complex Kn and ΔG values when:
a) going down a row of transition metals of the same oxidation state?
b) of the same transition metal with increasing oxidation state? (For example, Iron can exist as Fe(II), Fe(III), Fe(IV), and higher oxidation states)
5. Discussion (formal report style): Compare and contrast the concepts of Lewis and Brønsted-Lowry acids and bases. Support these concepts with your data over the last two weeks and with research. Cite your sources.