Sublimation and Deposition

Advertisements
Advertisements

Related Posts:


This figure shows a test tube. In the bottom is a dark substance which breaks up into a purple gas at the top.
Figure 1. Sublimation of solid iodine in the bottom of the tube produces a purple gas that subsequently deposits as solid iodine on the colder part of the tube above. (credit: modification of work by Mark Ott)

Sublimation and Deposition (OpenStax Chemistry 2e)

Some solids can transition directly into the gaseous state, bypassing the liquid state, via a process known as sublimation. At room temperature and standard pressure, a piece of dry ice (solid CO2) sublimes, appearing to gradually disappear without ever forming any liquid. Snow and ice sublime at temperatures below the melting point of water, a slow process that may be accelerated by winds and the reduced atmospheric pressures at high altitudes. When solid iodine is warmed, the solid sublimes and a vivid purple vapor forms (Figure 1). The reverse of sublimation is called deposition, a process in which gaseous substances condense directly into the solid state, bypassing the liquid state. The formation of frost is an example of deposition.

Like vaporization, the process of sublimation requires an input of energy to overcome intermolecular attractions. The enthalpy of sublimation, ΔHsub, is the energy required to convert one mole of a substance from the solid to the gaseous state. For example, the sublimation of carbon dioxide is represented by:

Likewise, the enthalpy change for the reverse process of deposition is equal in magnitude but opposite in sign to that for sublimation:

Consider the extent to which intermolecular attractions must be overcome to achieve a given phase transition. Converting a solid into a liquid requires that these attractions be only partially overcome; transition to the gaseous state requires that they be completely overcome. As a result, the enthalpy of fusion for a substance is less than its enthalpy of vaporization. This same logic can be used to derive an approximate relation between the enthalpies of all phase changes for a given substance. Though not an entirely accurate description, sublimation may be conveniently modeled as a sequential two-step process of melting followed by vaporization in order to apply Hess’s Law. Viewed in this manner, the enthalpy of sublimation for a substance may be estimated as the sum of its enthalpies of fusion and vaporization, as illustrated in Figure 2. For example:

A diagram is shown with a vertical line drawn on the left side and labeled “Energy” and three horizontal lines drawn near the bottom, lower third and top of the diagram. These three lines are labeled, from bottom to top, “Solid,” “Liquid” and “Gas.” Near the middle of the diagram, a vertical, upward-facing arrow is drawn from the solid line to the gas line and labeled “Sublimation, delta sign, H, subscript sub.” To the right of this arrow is a second vertical, upward-facing arrow that is drawn from the solid line to the liquid line and labeled “Fusion, delta sign, H, subscript fus.” Above the second arrow is a third arrow drawn from the liquid line to the gas line and labeled, “Vaporization, delta sign, H, subscript vap.”
Figure 2. For a given substance, the sum of its enthalpy of fusion and enthalpy of vaporization is approximately equal to its enthalpy of sublimation. Source: OpenStax Chemistry 2e

Source:

Flowers, P., Theopold, K., Langley, R., & Robinson, W. R. (2019, February 14). Chemistry 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/books/chemistry-2e

Advertisements
Advertisements

Related Research

Research Article: In Situ Atomic‐Scale Observation of Kinetic Pathways of Sublimation in Silver Nanoparticles

Date Published: January 30, 2019 Publisher: John Wiley and Sons Inc. Author(s): Junjie Li, Zhongchang Wang, Yunping Li, Francis Leonard Deepak. http://doi.org/10.1002/advs.201802131 Abstract: Uncovering kinetics of sublimation atomically is critical to understanding both natural phenomena and advanced manufacturing technologies. Here, direct in situ atomic‐scale observations to understand the effects of size, surface, and defects in … Continue reading

Research Article: Selective crystallization of indigo B by a modified sublimation method and its redetermined structure

Date Published: November 01, 2011 Publisher: International Union of Crystallography Author(s): Florian Kettner, Lucie Hüter, Johanna Schäfer, Konstantin Röder, Uta Purgahn, Harald Krautscheid. http://doi.org/10.1107/S1600536811040220 Abstract: Good-quality single crystals of the title compound, indigo B [systematic name: 2-(3-oxoindolin-2-yl­idene)indolin-3-one], C16H10N2O2, have been prepared with high selectivity by a sublimation process. The previous structure of indigo B [Süsse … Continue reading

Research Article: Photosensitive and Flexible Organic Field‐Effect Transistors Based on Interface Trapping Effect and Their Application in 2D Imaging Array

Date Published: February 26, 2016 Publisher: John Wiley and Sons Inc. Author(s): Yingli Chu, Xiaohan Wu, Jingjing Lu, Dapeng Liu, Juan Du, Guoqian Zhang, Jia Huang. http://doi.org/10.1002/advs.201500435 Abstract: Flexible organic phototransistors are fabricated using polylactide (PLA), a polar bio­material, as the dielectric material. The charge trapping effect induced by the polar groups of the PLA … Continue reading

Research Article: 4-Bromo­seleno­anisole

Date Published: July 01, 2009 Publisher: International Union of Crystallography Author(s): Henning Osholm Sørensen, Nicolai Stuhr-Hansen. http://doi.org/10.1107/S160053680902296X Abstract: The title compound, 1-bromo-4-methyl­seleno­benzene, C7H7BrSe, was prepared by methyl­ation of 4-bromo­seleno­phenolate with methyl iodide, and crystals suitable for structure determination were obtained by sublimation. The mol­ecule is essentially planar; the Se—Me bond is rotated by only 2.59 (19)° … Continue reading