Geometrical isomers differ in the spatial arrangement of atoms or groups around a central atom. Geom — Organic Chemistry Chemistry Question
Preparation of trans-dichlorobis(ethylenediamine)-cobalt(III) chloride and kinetics of its acid hydrolysis
Geometrical isomers differ in the spatial arrangement of atoms or groups around a central atom. Geometrical isomers usually have distinctive physical and chemical properties. The cis- and trans-isomers of octahedral cobalt complexes are among the most well known examples. The complexes of Co(III) ion are sufficiently stable and occur in separable isomeric forms. For example, dichlorobis(ethylenediamine)cobalt(III) ion, [Co(en)2Cl2]+ can exist in either cis- or trans-form as shown below. Ethylenediamine (en) is a bidentate ligand, which coordinates to the metal through both of its nitrogen atoms. The cis complex is dark purple, whereas the trans complex is green.
[VISUAL]
At low concentrations the green trans-[Co(en)2Cl2]+ ion undergoes a first order acid hydrolysis reaction, and a red mixture of the cis and trans isomers of [Co(en)2(H2O)Cl]2+ complex ion are formed as shown below.
trans-[Co(en)2Cl2]+ + H2O → cis- and trans-[Co(en)2(H2O)Cl]2+
green red mixture
In this experiment, the chloride salt of trans-dichloro-bis(ethylenediamine) cobalt(III) complex will be prepared and the rate constant of its acid hydrolysis reaction will be determined.
Chemicals and reagents
* Ethylenediamine, H2NC2H4NH2, 10 % (v/v)
* Cobalt(II)chloride hexahydrate, CoCl2· 6 H2O
* Hydrochloric acid, HCl(aq), concentrated solution
* Sulfuric acid, H2SO4(aq), (c = 1.0 mol dm–3)
Apparatus and glassware
* Evaporating dish
* Steam bath
* Beakers, 25 cm3 (2)
* Pipette, 10 cm3
* Stirring rod
* Graduated cylinder, 25 cm3
* Colorimeter or UV-Vis spectrometer
* Cell for colorimeter
Procedures
A. Preparation of trans-dichlorobis(ethylenediamine)cobalt(III) chloride
1. In a fume hood, dissolve 1.6 g of cobalt(II) chloride hexahydrate in 5.0 cm3 of water in an evaporating dish.
2. Add 9.0 cm3 of 10 % v/v solution of ethylenediamine into the evaporating dish.
3. Place the dish on a steam bath and stir for 40 min maintaining the volume of the solution by adding small portions of hot water. During this process, Co(II) is oxidized to Co(III) by the oxygen of the air, therefore good agitation is necessary to promote the dissolution of oxygen in the solution.
4. Add 10 cm3 of concentrated HCl solution and continue heating and stirring (without adding water) until a thin slurry of crystals forms.
5. Dry the crystals on the steam bath to eliminate HCl and H2O.
6. Weigh and calculate the yield.
B. Kinetics of acid hydrolysis of trans-dichlorobis(ethylenediamine)cobalt(III) chloride
1. Dissolve 0.10 g of trans-[Co(en)2Cl2]Cl in 5.0 cm3 of water in a test tube.
2. Transfer about 2 cm3 of the solution into the absorbance cell using a pipette and record the absorbance of the solution at 620 nm using a colorimeter or spectrometer, with the help of your assistant or technician. Record absorbance value as Ao at to, 0.0 min into the Table given below.
3. Place 0.20 g of trans-[Co(en)2Cl2]Cl in a beaker and add 10.0 cm3 of H2SO4 solution (c = 1.0 mol dm-3) into the beaker.
4. Transfer about 2 cm3 of the solution into the absorbance cell using a pipette and with the help of your assistant or technician record the absorbance of the solution at 620 nm at intervals of about 10 min as ti for 90 minutes into the table given below.
Time Min Absorbance to 0.0 t1 t2 t3 t4 t5 t6 t7 t8 t9 t10
Treatment of Data
Calculate the percent yield of trans-dichloro-bis(ethylenediamine)cobalt(III) chloride.
Model Answer
Molar mass of trans-[Co(en)2Cl2]Cl = 335.49 g mol-1
Molar mass of CoCl2 . 6 H2O = 237.93 g mol-1
m(CoCl2 . 6 H2O) = 1.604 g
The mass of trans-[Co(en)2Cl2]Cl obtained after drying = 1.999 g
Theoretical yield:
Theoretical mass of trans-[Co(en)2Cl2]Cl = 1.604 g * (335.49 g mol-1 / 237.93 g mol-1) = 2.262 g
Yield (%) = (1.999 g / 2.262 g) * 100 = 88.4%
Plot ln At / Ao versus time.
Model Answer
time Absorbance ln(At/Ao)
0 1,392 0
10 1,379 -0,00938
20 1,372 -0,01447
30 1,364 -0,02032
40 1,36 -0,02326
50 1,348 -0,032
60 1,343 -0,03584
70 1,333 -0,04331
80 1,329 -0,04631
90 1,321 -0,05235
[VISUAL]
Estimate the first order rate constant from the plot.
Model Answer
Slope gives the first order rate constant: k = 5.6 · 10-4 min-1
Explain why absorbance values can directly be used for concentrations.
Model Answer
According to Beer’s Law, A = ε l c
Since, the molar absorbtivity coefficient (ε) and the path length (l) are constant during all of the measurements, the absorbance is directly proportional to concentration. This can be applied in this experiment.