CHM120H5 Study Guide - Midterm Guide: Stability Constants Of Complexes, Cuvette, The Pipettes

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21 Mar 2015

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Spectrophotometric determination of the stability constant of a complex ion. Iron ions combine with ligands to produce a stable complex ion. Ligands have a lone pair of electrons which form a bond with the central ion. The stability of the complex ions is the stability constant and can be written as the equilibrium constant for the formation of the complex ion. The stability constant of the reaction between fe3 and scn and the formation of the complex ion, written by the equation: 2 is determined by using the absorption value of different mixtures. The solutions will be reacted with scn- and will make fe3+ the limiting reagent. The stability constant of the reaction will be expressed using the equation: Test tubes were filled with six separate solutions using graduated pipettes. The test tubes were sealed with parafilm and then mixed. The spectrophotometer was calibrated to a wavelength of 447 nm.

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 âtransferâ of one entity to another. In the case of BrÃ¸nsted-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 âhardnessâ defined as small highly charged species that are weakly polarizable (Lewis acids). In contrast, âsoftnessâ 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 âligandsâ 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 BrÃ¸nsted-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:
ÎG0 = -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-saltsâ 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 ÎG 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 ÎG 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 BrÃ¸nsted-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 BrÃ¸nsted-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:

âG = -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 ÎG 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 BrÃ¸nsted-Lowry acids and bases?

Show transcribed image text 47 EXPERIMENT 47 Report Sheet 1. The kind of fruit drink appl Jwic 2. Molarity of iodine solution ad 3. Titration results Sample 1 (100 mL) Sample 2 (20.0 mL) Sample 3 (30.0 mL) a. Initial buret reading b. Final buret reading c. Total volume of iodine ,',ã¼, mL u. 2 mL 2.5 mL 4.amL 10.5 mL 10.5 mL 20.5 mL solution used: (b)-(a) 4. The weight of vitamin C in the fruit drink sample: [(3c) x 1.76 mg/mL] 5. Concentration of vitamin C in the fruit drink (mg/100 mL): [(4)/volume of drink] x 100 O,20,30 5S 6. Average concentration of vitamin Cin the fruit drink 52 mg/100 mL
Post-Lab Questions 1. Use the average concentration of vitamin C from your data, and calculate the volume that would satisfy your minimum daily requirement of vitamin C. Show your work 2. Vitamin C is ascorbie acid. If you examine the structure of ascorbic acid given in the Background t. In the structure shown, the ascorbic acid is section (equation 2), there is no -COOH grou - as a lactone or cyclic ester. Write the equation for the hydrolysis (cleavage by addition of water) of E ste the lactone to the carboxylic acid. 3. Write the net ionic equation for the conversion of iodine, la, to iodide, I. What kind of reaction is this? 4. Refer to the Background section and examine the chemical reaction for the oxidation of ascorbic acid (equation 2) a. What happened to the hydrogen atoms that were present in the ascorbic acid? b. Acid was added in the titration, but it is not present in equation (2) in a stoichiometric amount. So, what is the purpose of the acid? 5. The iodine for this experiment is in the form of 15, and not 12 (see equation (1), Background section]. Why is this form of iodine used? 577
ts for General Organic and Blochemistry Bettelheim/Landesberg Vitamin C is oxidized by 12 (as 15) according to the following chemical reaction: HO-CH- CH2OH HO-CH-CH,OH O. . o 2HI Ð½ Ð¾Ð½ Ð¾Ð½ Oxidized product (MW 174) (MW 254) Vitamin C (MW 176) As vitamin C is oxidized by iodine, I2 becomes reduced to I. When the end point is reached (no vitamin C is left), the excess of 12 will react with a starch indicator to form a starch-iodine complex, which is blackish-blue in color. I2 +starch iodine-starch complex (blackish-blue) It is worthwhile to know that although vitamin C is very stable when dry, it is readily oxidized by air (oxygen) when in solution; therefore, a solution of vitamin C should not be exposed to air for long periods. The amount of vitamin C can be calculated by using the following conversion factor 41 mL of 12 (0.01 M) = 1.76 mg vitamin C OBJECTIVE To determine the amount of vitamin C that is present in certain commercial food products by the titration method. Pro to this laboratory, there are techniques that you should revieo. You uoll find the relevant techmiques in the following experiments: (a) Use of the Spectroline pipet filler is in Experiment 2; (b) buret use and titration techniques are im Experiment 19.
terps Tile logical antioxidant (reducing agent). It helps to keep the iron in the yme, prolyl hydroxylase, in the reduced form and, thereby, it helps to to prevent scurvy is ascorbic acid. It is a powerful Acave ingredient in fruits and intain th thesis of normal ions and broken blood vessels. e enzyme activity. Prolyl hydroxylase is essential for the collagen. In scurvy, the abnormal collagen causes skin Vitamin C cannot be synthesized tained from the diet ( in the human body and must be e.g., citrus fruits, broccoli, turnip greens, sweet ppers, tomatoes) or by taking synthetic vitamin C (e.g, vitamin C blets, "high-C" drinks, and other vitamin C-fortified commercial foods). he minimum recommended adult daily requirement of vitamin C to y is 60 mg. Some people, among them the late Linus Pauling, wice Nobel Laureate, suggested that very large daily doses (250-10,000 ng) of vitamin C could help prevent the common cold, or at least lessen he symptoms for many individuals. No reliable medical data support this At present, the human quantitative requirement for vitamin C is still revent scurv claim. In this experiment, the amount of vitamin C is determined quantita- tively by titrating the test solution with a water-soluble form of iodine, I5: controversial and requires further research. [I: 1:1: -(triioode ion) (1) :11:+:1:-- Expanded octets 569

CHM120H5 Study Guide - Midterm Guide: Stability Constants Of Complexes, Cuvette, The Pipettes

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