Lab Question Determination of a Rate Law Dissociation of Ferroirn Fe2+ combines
ID: 1001390 • Letter: L
Question
Lab Question
Determination of a Rate Law Dissociation of Ferroirn Fe2+ combines with 1,10-phenanthroline: to form the complex ion Fe(phen)3 , called ferroin. The nonbonded electron pairs on the N atoms form bonds with Fe2+. Since Fe2* solutions are a very pale green, phenanthroline solutions are colorless and ferroin solutions are orange-red, the formation and dissociation of ferroin are particularly easy to study by measuring the absorbance of the orange-red solutions containing ferroin Ferroin is the subject of several experiments this semester. In the first experiment, you will determine the rate law for the dissociation of ferroin in acid solution. In another experiment, you will determine the equilibrium constant for the reaction. In a subsequent experiment, when we get to acids and bases, you'll measure the base dissociation constant for phenanthroline. One reason for wanting to establish the rate law for a reaction is that it allows us to develop a possible mechanism. We'll see how to do this in class toward the end of the kinetics chapter. Ferroin decomposes in the presence of acid, according to the net ionic reaction: Fe(phen)32+(aq) + 3H3 +(aq) Fe2+(aq) + 3 phenH+(aq) + 3H20(1) As the reaction progresses, the orange-red color of ferroin fades away. Because absorbance of light is directly proportional to concentration for this complex--see the video on your web page 2Explanation / Answer
1)
A) For Fe (phen)32+ (aq.)
We have before mixing M1 = 9.0 x 10–5 M and V1 = 100 mL
On mixing, M2 = ? and V2 = 100 + 20 mL (H2SO4) = 120 mL
Molar equation,
M2 x V2 = M1 x V1
M2 x 120 = 9.0 x 10–5 x 100
M2 x 120 = 9.0 x 10–3
M2 = (9.0 x 10–3)/120
M2 = 7.5 x 10–5 M
Initial concentration of Fe (phen)32+ (aq.) is 7.5 x 10–5 M.
B) Initial concentration of H2SO4
Before mixing, M1 = 3.0 M and V1 = 20.0 mL
On mixing, M2 =? V2 = 120 mL
Molar equation holds,
M2 x V2 = M1 x V1
M2 x 120 = 3.0 x 20.0
M2 x 120 = 60.0
M2 = 60.0/120
M2 = 0.5 M
Initial concentration of H2SO4 solution is 0.5 M
0.5 >>> 7.5 x 10–5
I.e. [H2SO4] >>> [Fe(phen)3]2+
I.e. H2SO4 is in very large excess over [Fe(phen)3]2+ is verified.
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2) We are supposed to prepare 20.0 mL 0.30 M H2SO4 solution using 3.0 M H2SO4 stock solution.
We need to find to find the volume of 3.0 M H2SO4 stock solution.
Let for a solution to be prepared, M1 = 0.30 M and V1 = 20.0 mL
And for Stock solution M2 = 3.0 M and V2 = ?
While diluting the solution to lower concentration molar equation holds.
Hence molar equation is,
M2 x V2 = M1 x V1
Let us put all known values and solve it for V2,
3.0 x V2 = 0.3 x 20
3.0 x V2 = 6.0
V2 = 6.0/3.0
V2 = 2.0 mL
It means volume of stock solution to be used is 2.0 mL
Preparation of 20.0 mL of 0.3 M H2SO4 solution.
Pipette out 2.0 mL of H2SO4 in 20.0 mL volumetric flask (Erlenmeyer flask). Rinse pipette with 2.0 mL distilled water 2/3 times and add it to volumetric flask containing 2.0 mL H2SO4 then add enough distilled water and make final volume 20.0 mL.
Hence the preparation of 20.0 mL of 0.3 M H2SO4 solution.
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3)
Cuvette is a cell used to put analyte (Ferroin complex here) in spectrophotometric analysis i.e. measurement of absorbance which is a function of concentration of absorbing species in sample.
In other words from absorbance value we can find the concentration of analyte (i.e. absorbing species ferroin here)) in sample.
If a cuvette is not clean i.e. if any absorbing species having max nearly same as that of analyte is adhered from outside it will add error to the absorbance measurement.
I.e. Absorbance measured will be more than expected and hence the concentration will be.
This error may lead to the faulty estimation of reaction order.
Hence cuvette must be cleaned properly from outside before placing in spectrophotometer.
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