Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Chemistry Olympiad and other competitions => Topic started by: vanderwaals on September 09, 2016, 03:47:15 PM
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2. What is the first change that occurs when I2(s) is heated slowly at one atmosphere pressure?
(A) The solid melts.
(B) The solid vaporizes.
(C) The solid breaks into atoms.
(D) The solid becomes darker in color.
I do not understand why the answer is (B) and not (A), because on a phase diagram of iodine, if you increase temperature but keep pressure constant (at 1 atm) by going horizontally, the solid seems to melt first. What am I missing?
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the first change ...heated slowly
You increase temperature slowly from RT (default), and up to 113.6 C iodine sublimes, though partial pressure of iodine vapors is much lower than 1 Atm.
The first change for ice heated from -20 C (up to 0.01 C - triple point) is also sublimation.
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Thank you! It's more clear to me now. However, why does the problem state that the change occurs at 1 atm if it actually occurs at a lower pressure?
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This is true for iodine up to 115 Atm (critical point). But then melting point is about 100 C (not at the triple point).
Boiling point of iodine is 184.3 C at 1 Atm. From the triple point temperature to boiling point phase changes from liquid to gas.
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IMHO question is poorly worded. "Heated at 1 atm" is ambiguous. If it is intended to mean "heated in an open vessel when the atmospheric pressure is 1 atm" it should state it clearly. It doesn't say what has the pressure of 1 atm. If you put a large iodine crystal in a closed vessel (no free space around) pressurized to 1 atm (say with a piston) there will be no sublimation.