Reaction of potassium iodide with carbon tetrachloride and CS2
Answers
Iodine is not completely insoluble in water, but it’s not very soluble either. It takes something like 3.5 liters of water to dissolve 1 gram of iodine at 20ºC.
At any rate, it’s true that iodine is much more soluble in CCl4 (carbon tetrachloride) than it is in water. Why is that?
If this is a question for school, then the answer you’re probably meant to give is this: because iodine and CCl4 are both non-polar, while water is polar. Non-polar substances dissolve better in non-polar solvents, while polar substances dissolve better in polar solvents.
But just in case you’re interested in carrying your learning a bit further; read on.
In chemistry, a substance is said to be polar if its molecules naturally exhibit an uneven distribution of electric charge. Without a full-on discussion of VSEPR theory and molecular geometry, it’s difficult to demonstrate why some molecules are polar and others aren’t. If this is a question for school, I assume you’ve already discussed the topic of molecular polarity, so I’ll not write a textbook here.
Polar molecules are sort of like magnets, except instead of having north and south poles, they have positive and negative poles. Like magnets, polar molecules are strongly attracted to other polar molecules. They are also attracted a little bit to non-polar molecules, but the attraction is much weaker. You see, even a non-polar molecule can become temporarily polar in the presence of a polar molecule, but this temporary polarity is much weaker than the permanent polarity of a molecule like, say, water. So the attraction between two polar molecules is strong, while the attraction between polar and non-polar molecules, or between two non-polar molecules, is relatively weak.
A solute dissolves in a solvent if two things are true:
The attraction between the solvent and solute particles is stronger than the attraction between the solvent particles, and…
The attraction between the solvent and solute particles is stronger than the attraction between the solute particles.
When iodine is placed in water, there is a slight attraction between the polar water molecules and the non-polar iodine molecules; however, it’s not as strong as the attraction between the water molecules. The water molecules simply will not separate enough to allow the iodine molecules to enter solution. You can think of water as being a very tight-knit community that is slow to accept non-polar outsiders like iodine.
In carbon tetrachloride, the intermolecular forces are considerably weaker. (This also explains why CCl4 boils at only 77ºC, compared to the 100ºC boiling point of water.) The attraction between CCl4 molecules is weak enough that they will part and allow I2 molecules to join their company. Ergo, iodine dissolves in CCl4.
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