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The term sublimation refers specifically to a physical change of state and is not used to describe the transformation of a solid to a gas in a chemical reaction. For example, the dissociation on heating of solid ammonium chloride into hydrogen chloride and ammonia is not sublimation but a chemical reaction.
In thermodynamics, the enthalpy of sublimation, or heat of sublimation, is the heat required to sublimate (change from solid to gas) one mole of a substance at a given combination of temperature and pressure, usually standard temperature and pressure (STP). It is equal to the cohesive energy of the solid.
For many substances, the formation reaction may be considered as the sum of a number of simpler reactions, either real or fictitious. The enthalpy of reaction can then be analyzed by applying Hess' Law, which states that the sum of the enthalpy changes for a number of individual reaction steps equals the enthalpy change of the overall reaction.
J.A. Dean (ed.), Lange's Handbook of Chemistry (15th Edition), McGraw-Hill, 1999; Section 6, Thermodynamic Properties; Table 6.4, Heats of Fusion, Vaporization, and Sublimation and Specific Heat at Various Temperatures of the Elements and Inorganic Compounds
Conditions are so chosen that the solid volatilizes and condenses as a purified compound on a cooled surface, leaving the non-volatile residual impurities or solid products behind. The form of the cooled surface often is a so-called cold finger which for very low-temperature sublimation may actually be cryogenically cooled.
This is an accepted version of this page This is the latest accepted revision, reviewed on 30 November 2024. This article is about the chemical element. For other uses, see Iodine (disambiguation). Chemical element with atomic number 53 (I) Iodine, 53 I Iodine Pronunciation / ˈ aɪ ə d aɪ n, - d ɪ n, - d iː n / (EYE -ə-dyne, -din, -deen) Appearance lustrous metallic gray solid ...
The reaction of molybdenum dioxide with the transporting agent iodine is an exothermic process, thus the MoO 2 migrates from the cooler end (700 °C) to the hotter end (900 °C): MoO 2 + I 2 ⇌ MoO 2 I 2 ΔH rxn < 0 (exothermic) Using 10 milligrams of iodine for 4 grams of the solid, the process requires several days.
This compound can be prepared by heating zirconium metal and an excess of iodine. [1] The solid is purified by sublimation (400 °C, 10-4 mm Hg).[4]2 I 2 + Zr → ZrI 4 ...