How to Calculate the Heat of Sublimation of PI3

Use a calculator to calculate the heat of sublimation of PI3.

Heat of sublimation, or enthalpy of sublimation, describes the amount of energy used to cause an amount of a substance to transfer directly from solid phase to gaseous phase. You can calculate a substance's heat of sublimation from its specific heat capacity, its heat of fusion and its heat of vaporization. Chemists use phosphorus triiodide, or PI3, in reactions that substitute iodine for hydroxyl groups, creating alkyl halides from alcohols.

Things You'll Need

• Heat of sublimation equation
• Heat of fusion of PI3
• Heat of vaporization of PI3
• Specific heat capacity of PI3
• Molar mass of PI3
• Starting temperature (or melting point)
• Ending temperature (or boiling point)
• Calculator

Instructions

1. Learn to understand phase changes. Although sublimation, the change from solid to gas, takes place differently than the slower changes from solid to liquid to gas, the energy involved remains equal. Therefore, the enthalpy of sublimation equals the total of all energy changes taking place from the starting temperature of the solid phase to the final temperature of the gaseous phase, according to the University of California, Davis.

2. Convert all temperatures to Kelvin. Johns Hopkins gives the conversion from Celsius to Kelvin as simply adding 273.15 degrees, and the conversion from Fahrenheit to Kelvin as first multiplying the Fahrenheit temperature by 0.555556 and then adding 255.37 degrees to the result.

   If the problem gives the heat of vaporization and heat of fusion in kilojoules per mole, make sure to convert figures using quantities listed in masses to moles as well, using the mass to mole conversion equation:

   (mass in grams) * 1/(molar mass of phosphorus triiodide)

3. Find the amount of energy required to take PI3 from its initial temperature to the final temperature. If the sample starts at or near the melting point and ends at or near the boiling point, or the problem does not specify the temperatures, multiply the temperature difference between melting and boiling point in Kelvin by the specific heat capacity of liquid PI3.

   If the sample starts at a temperature far below the melting point, also multiply the specific heat capacity of solid PI3 by the difference between the starting temperature and the melting point and add this to the previous product.

4. Check to see that all of the temperature changes and phase transfers are accounted for and in the same units.

5. Add the heat of fusion, the temperature change(s) previously calculated and the heat of vaporization. The total gives the heat of sublimation.

Tips & Warnings

• Watch for units conversions. One of the easiest ways to reach an incorrect solution to a phase transfer problem is to forget that calculating energy changes from specific heat capacities sometimes gives results in different units than the units in which the problem gives phase transfer energies.

References

• University of California Davis: ChemWiki: Heat of Sublimation
• Johns Hopkins: Conversion Factors Used in Meteorology
• University of Memphis: The Mole and Molar Mass