Sublimation refers to the uncommon process of a material converting from the solid phase directly to the gas phase without first forming a liquid. Scientists classify this as an endothermic process because it corresponds to the compound absorbing heat from its surroundings. Scientists can measure the amount of heat necessary for this transformation and then express the result as the “heat of sublimation,” usually in units of joules of heat per gram of substance, J/g, or sometimes joules per mole of substance, J/mol.
Set up your calorimeter according to its usage instructions.
Calculate the temperature change, deltaT, of the water by subtracting the final water temperature from the initial water temperature. Thus, if the temperature of the water in the calorimeter dropped from 55.0 degrees Celsius to 22.6 degrees Celsius, then deltaT = 22.6 - 55.0 = -32.4 degrees Celsius.
Calculate the heat lost by the water, Q, according to the equation Q = m * c * deltaT, where m represents the mass of water and c represents water’s specific heat capacity, or 4.184 joules per gram degree Celsius. Note that 1 milliliter of water weighs about 1 gram. Therefore, if the calorimeter was filled with 200 mL of water, which would weigh 200 g, then Q = 200 * -32.4 * 4.184 = -27,100 joules of heat. The negative sign in front of the value indicates that the heat was lost by the water. The heat gained by the sublimed substance will be equal in quantity but opposite in sign to the heat lost by the water.
Calculate the heat of sublimation of the substance by dividing the heat absorbed by the substance, as calculated in step 2, by the mass of substance in grams. For example, if 47.5 g of substance was placed in the calorimeter, then the heat of sublimation would be 27,100 / 47.5 = 571 J/g.
Things You'll Need
- Northwestern University: Sublimation
- University of California Davis: Dept. of Chemistry: ChemWiki: Heat of Sublimation
- “Journal of Chemical Education”; Measuring the Heat of Sublimation of Dry Ice with a Polystyrene Foam Cup Calorimeter; A.W. Burgstahler et al.; April 1991
- Georgia State University, Dept. of Physics and Astronomy: Calorimetry
- West Valley College: Heat and Temperature
- hand calculator image by MateiA from Fotolia.com