For the purification of the products, the reaction mixture was transferred to a test tube which was then kept in an ice-water bath until yellow crystals were formed. Then the test tube was centrifuged and the aqueous layer was removed. Then hot ethanol was added to the test tube to dissolve the crystals. Then the ethanol was removed, and some of the crystals were transferred to another test tube. 3 mL of water was added to the test tube and the solution was transferred to the cuvette for the UV-Vis scan. The same solution was scanned again after 60 minutes in order to study the decomposition of ICl3.
Results
During the stirring period, the black iodine powder gradually disappeared and the solution turned orange. After all the concentrated HCl was added, the reaction mixture became orange in color.
For the UV-vis spectra, please refer to Figure 1 and Figure 2. For Figure 1, the spectrum was obtained at zero minute. There is a small hump around 340 nm and a peak around 450 nm. For Figure 2, the spectrum was obtained at 48 minutes after the ICl3 crystal dissolving in water. The intensity of the small hump around 340 nm decreases, while the intensity of the peak around 450 nm increases.
Discussion
Compare Figure 1 and Figure 2, the intensity of the small hump around 340 nm decreases, while the intensity of the peak around 450 nm increases. The small hump around 340 nm is due to ICl3. The peak around 450 nm is due to ICl. The presence of ICl is due to the decomposition of ICl3. As time passed, ICl3 in the solution gradually decomposed to give ICl. This explained the increase in intensity of peak around 450 nm (representing the concentration of ICl) and the decrease in intensity of peak around 340 nm (representing the concentration of ICl3).
Compared with the standard spectra at zero minute (Figure 3), 30 minutes (Figure 4) and 1 hour (Figure 5), the spectra obtained in this experiment is a bit different from them. The spectra obtained are more similar to the standard spectrum of ICl3 at 1 hour and 30 minutes (Figure 6). This difference occurred because the reaction mixture was not scanned immediately after dissolving the ICl3 crystals in water. The crystals were added to the water at the time while waiting for the UV-Vis scan. At that time the decomposition started. During the time waiting for the UV-Vis scan, the decomposition of ICl3 continued. Thus the reaction mixture was not scanned at 0 minute (right after the crystals were dissolved in water). When the mixture was scanned for the first time, the decomposition has occurred for a while already. Hence in Figure 1, the peak at 450 nm (representing ICl) is more obvious and with high intensity than the one in the standard spectrum in Figure 3. The improvement for this is to bring the crystals and water to the spectrum bench first, then the crystals are added when UV-Vis scan is available. In this way, the reaction mixture can be scanned right away (0 minute).
There are some experimental difficulties encountered. For example, it was difficult to maintain the temperature of the water bath between 35 °C and 40 °C. The hot water was used from the tap, so as the experiment progressed, the temperature kept dropping and hot water needed to be replaced. If the temperature was not kept between 35 °C and 40 °C, the reaction may not be obtained or other reaction may take place. This would make the result inaccurate. Also, during the purification of the product, the reaction mixture should not be kept in the ice-water bath for too long, otherwise, the mixture would be frozen and it took extra time to dissolve the crystals again.
Questions
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Why are most interhalogens quite unstable? What makes ICl3 reasonably stable by comparison?
In interhalogens, there are two different halogens with different principal quantum number bonded together. The overlapping of the orbital between the atoms of different principal quantum is poorer than the one in homodiatomic halogens. Thus the bond between the two different halogen atoms is weak due to the poor overlapping. ICl3 is more stable than other XY3 compound because iodine has the largest atomic size compared with other halogens. Compared with the case which other halogen atoms act as central atom, when iodine acts as the central atom, the other halogen atoms can bond with it with less repulsion between the bond pair of electrons, as the bond pairs can be oriented further apart. Therefore, ICl3 is more stable than other XY3 compound.
- Why does Iodine form the largest variety of interhalogen compounds?
Iodine has the largest atomic size compared with other halogens. When it acts as the central atom in interhalogen compounds, it can bond with other halogen atoms with less repulsion between the bond pairs. Thus it can form more bonds with other halogens, compared with other central halogen atoms. For example, iodine can form maximum 7 bonds with 7 fluorine, while bromine can only form maximum 5 bonds with 5 fluorine. Therefore, it can form many different kinds of interhalogen. Moreover, the greater the difference of electronegativities between the central atom and the terminal atom is, the greater the number of terminal atom can be bonded to the central atom. As iodine has the lowest electronegativity among the halogens, the difference between iodine and the other terminal halogen atoms is always greater compared with other central halogen atoms. Thus it can act as the central atom in more kinds of higher halides than the others. Therefore, iodine forms the largest variety of interhalogen compounds.
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Speculate as to what the decomposition products of ICl3 might be.
The decomposition products of ICl3 might be iodine chloride (ICl) and chlorine gas (Cl2).
(ICl3)2 (s) → 2ICl (s)+ 2Cl2 (g)
References
1. Havinga, E. E., K. H. Boswijk and E. H. Wiebengaa. "Structures of Interhalogen Compounds and Polyhalides." Advances in Inorganic Chemistry. Vol. 3. Academic Pr, 1961. 133-69. Print.
Figure 3. Standard UV-Vis Spectrum of ICl3 at zero minute
Figure 4. Standard UV-Vis Spectrum of ICl3 at 30 minute
Figure 5. Standard UV-Vis Spectrum of ICl3 at 1 hour
Figure 6. Standard UV-Vis Spectrum of ICl3 at 1 hour and 30 minutes
