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CHM120H5 Study Guide - Midterm Guide: Molar Attenuation Coefficient, Stability Constants Of Complexes

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School
UTM
Department
Chemistry
Course
CHM120H5
Professor
Judith C Poe

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Document Summary

Absorption vs concentration n o i t p r o s b. The approach to confirm the synthesized product included two reactions. Considering the data collected, the stability constant was about 244 and the molar absorptivity constant was 12. 24. The calculations for the above experiment are placed under appendix i. The original data sheet is placed under appendix ii. Appendix i calculations (1) percentageyield= actual mass. The concentrationat an absorbance of 0. 640is 0. 0441 mol/ l x= 640 x 10 9 0. 998 x 10 9. Determine the moles of cr, with volume of 50 ml moles=concentration volume moles=0. 0022067 moles moles=0. 0441 m x 0. 05 l. Determine the mass of cr, with its molar mass of 51. 996 g/mol mass=molar mass moles mass=51. 996 g mol masss=0. 1147 g. The mass of cr obtained is 0. 1147 g x 0. 0022067 moles. Percentweight = mass cr weight of sample x 100.

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Related Questions

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.

Lewis Acids and Bases

This is related to a post-lab of inorganic chemirsty.

The stability of Lewis acid-base complexes can be describedthrough the stability constant which is analogous to the Ka that describes Brønsted-Lowryacidities. Many metal ions can coordinate (bind) many Lewis base ligands in a step-wisefashion according to:

M + L = ML K1 = [ML]/[M][L] Eq. 1--Please note that the L in the denomnator is supposed to be the alpha value.

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:

∆G = -RTLnKn Eq. 5.

where R is the gas constant and T is the temperature in Kelvin.

The ethylene diamine tetraacetic acid ligand (EDTA, a Lewis base) provides amechanism to probe the Lewis acid strength of various metal complexes. This multi-dentate ligand (multiple Lewis base sites on a single ligand) forms complexes withmetals 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 beexamined by measuring the stability constants for the metal-EDTA complex. Of specificinterest 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 comparethe relative Lewis acidities of both metals and describe differences in terms of the metalion properties described above.

Experimental Procedure

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 forexact concentration prior to beginning). You will need to weigh ~0.1 g per metal saltsample. Dissolve the metal salt into 50 mL of DI water. Add 2-3 drops of murexide toyour metal. After properly preparing your buret with EDTA, titrate your metal with EDTA. Repeat for a total of 3 titrations per metal.

Results:

For Cu(II), see below:

Values for each titration (we had three tries for each salt sample).

.1114 g Cu(II)------4.8 mL EDTA solution-------pH 3.0 found after titration

.1054 g Cu(II)-------4.1 mL EDTA solution ------pH 2.8 found after titration

.1182 g Cu(II)------4.8 mL EDTA solution------pH 2.7 found after titration

-------------------------------------------------------------------------------------------------------------------------

For Zn(II), see below:

.099 g Zn(II)-----3.5 mL EDTA solution----pH 2.5

.0965 g Zn(II)----4.4 mL EDTA solution----pH 2.7

.0952 g Zn(II)-----3.6 mL EDTA solution---pH 2.01

Questions:

1) 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)

2. That are the differences between the concepts of Lewis and Brønsted-Lowry acids and bases?


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