CHEM211 Lecture Notes - Lecture 28: Rate-Determining Step, Concentration Polarization, Reference Electrode

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turquoisewalrus441

23 Dec 2017

School

University of Alberta

Department

Chemistry

Course

CHEM211

Professor

Charles Lucy

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Document Summary

Large formation constants, increase with the charge of m^(n+) Kf involved specific deprotonated form of edta. Set up an ice-table, then write out kf" and use formation constants table for edta, and activity of edta. Set the equilibrium equal to kf" and solve! Kf" = a(y4-)kf (convert logkf to kf). Fast kinetics, and selectively controlled by ph. If dried, is a primary standard, and provides reliable methods for determining endpoint. Metal-edta complexes are very soluble in water. Study of interchange of chemical energy and electrical energy based on processes that involve reduction and oxidation reactions. Electroanalysis chemistry use of electrical measurements on chemical system for analytical. Conductivity bulk ionic concentrations (ion chromatography) Potentiometry measure electrical potential when essentially no current flows (ph electrode, Voltammetry measure current in response to varied electrical potential. Amperometry measure current resulting from constant applied electrical potential (glucose purposes. fluoride electrode). analyzer) Coulometric titration with electrons (redox titration)

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.

CHEM211 Lecture Notes - Lecture 28: Rate-Determining Step, Concentration Polarization, Reference Electrode

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