Sublimation (phase transition)

Sublimation (phase transition)

Dark green crystals of nickelocene, sublimated from the round-bottom flask and freshly deposited on a cold finger

Sublimation is the transition of a substance directly from the solid to the gas phase without passing through the intermediate liquid phase.[1] Sublimation is an endothermic phase transition that occurs at temperatures and pressures below a substance's triple point in its phase diagram. The reverse process of sublimation is desublimation or deposition, in which a substance passes directly from a gas to a solid phase. Sublimation has also been used as a generic term to describe phase changes between solid and gas that avoid the liquid state without specifying the direction of the transition.[2]

At normal pressures, most chemical compounds and elements possess three different states at different temperatures. In these cases, the transition from the solid to the gaseous state requires an intermediate liquid state. The pressure referred to is the partial pressure of the substance, not the total (e.g. atmospheric) pressure of the entire system. So, all solids that possess an appreciable vapor pressure at a certain temperature usually can sublimate in air (e.g. water ice just below 0 °C). For some substances, such as carbon and arsenic, sublimation is much easier than evaporation from the melt, because the pressure of their triple point is very high, and it is difficult to obtain them as liquids.

Sublimation requires additional energy and is an endothermic change. The enthalpy of sublimation (also called heat of sublimation) can be calculated as the enthalpy of fusion plus the enthalpy of vaporization.

Contents

  • Examples 1
    • Carbon dioxide 1.1
    • Water 1.2
    • Other substances 1.3
  • Sublimation purification 2
  • Historical usage 3
  • Sublimation predictions 4
  • See also 5
  • References 6

Examples

Comparison of phase diagrams of carbon dioxide (red) and water (blue) showing the carbon dioxide sublimation point (middle-left) at 1 atmosphere. As dry ice is heated, it crosses this point along the bold horizontal line from the solid phase directly into the gaseous phase. Water, on the other hand, passes through a liquid phase.

Carbon dioxide

Small pellets of dry ice subliming in air

Solid carbon dioxide (dry ice) sublimates everywhere along the line below the triple point (e.g., at the temperature of −78.5 °C (194.65 K, −104.2 °F) at atmospheric pressure), whereas its melting into liquid CO2 can occur only along the line at pressures and temperatures above the triple point (i.e., 5.2 atm, −56.4 °C).

Water

Snow and ice sublimate, although more slowly, at temperatures below the freezing/melting point temperature line at 0 °C for most pressures; see line below triple point.[3] In freeze-drying, the material to be dehydrated is frozen and its water is allowed to sublimate under reduced pressure or vacuum. The loss of snow from a snowfield during a cold spell is often caused by sunshine acting directly on the upper layers of the snow. Ablation is a process that includes sublimation and erosive wear of glacier ice.

Other substances

Camphor subliming in a cold finger. The crude product in the bottom is dark brown; the white purified product on the bottom of the cold finger above is hard to see against the light background.

Iodine produces fumes on gentle heating. It is possible to obtain liquid iodine at atmospheric pressure by controlling the temperature at just above the melting point of iodine. In forensic science, iodine vapor can reveal latent fingerprints on paper.[4]

Naphthalene, a common ingredient in mothballs, also sublimates easily. Arsenic can also sublimate at high temperatures.

The term sublimation refers to a physical change of state and is not used to describe 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 rather a chemical reaction. Similarly the combustion of candles, containing paraffin wax, to carbon dioxide and water vapor is not sublimation but rather a chemical reaction with oxygen.

Sublimation purification

Crystals of ferrocene after purification by vacuum sublimation

Sublimation is a technique used by organic electronics industry, where very high purities (often > 99.99%) are needed to satisfy the standards for consumer electronics and other applications.

Historical usage

In ancient Rosarium philosophorum, as a process necessary for the completion of the magnum opus. Here, the word sublimation is used to describe an exchange of "bodies" and "spirits" similar to laboratory phase transition between solids and gases. Valentine, in his Triumphal Chariot of Antimony (published 1678) makes a comparison to spagyrics in which a vegetable sublimation can be used to separate the spirits in wine and beer.[7] Ripley uses language more indicative of the mystical implications of sublimation, indicating that the process has a double aspect in the spiritualization of the body and the corporalizing of the spirit.[8] He writes:

And Sublimations we make for three causes,
The first cause is to make the body spiritual.
The second is that the spirit may be corporeal,
And become fixed with it and consubstantial.
The third cause is that from its filthy original.
It may be cleansed, and its saltiness sulphurious
May be diminished in it, which is infectious.[9]

Sublimation predictions

The enthalpy of sublimation has commonly been predicted using the equipartition theorem. If the lattice energy is assumed to be approximately half the packing energy, then the following thermodynamic corrections can be applied to predict the enthalpy of sublimation. Assuming a 1 molar ideal gas gives a correction for the thermodynamic environment (pressure and volume) in which pV = RT, hence a correction of 1RT. Additional corrections for the vibrations, rotations and translation then need to be applied. From the equipartition theorem gaseous rotation and translation contribute 1.5RT each to the final state, therefore a +3RT correction. Crystalline vibrations and rotations contribute 3RT each to the initial state, hence −6RT. Summing the RT corrections ; −6RT + 3RT + RT = −2RT.[10] This leads to the following approximate sublimation enthalpy. A similar approximation can be found for the entropy term if rigid bodies are assumed.[11]

\Delta H_{sublimation} = -U_{lattice~energy} - 2RT

See also

To
Solid Liquid Gas Plasma
From Solid Solid-solid transformation Melting Sublimation
Liquid Freezing Boiling / evaporation
Gas Deposition Condensation Ionization
Plasma Recombination / deionization

References

  1. ^ http://www.merriam-webster.com/dictionary/sublimation
  2. ^ http://dictionary.reference.com/browse/sublime
  3. ^ Hydrol. Process. 18, 3481–3492 (2004)Estimating Alter-shielded gauge snowfall undercatch, snowpack sublimation, and blowing snow transport at six sites in the coterminous USA,S. R. Fassnacht,
  4. ^
  5. ^ R. B. King Organometallic Syntheses. Volume 1 Transition-Metal Compounds; Academic Press: New York, 1965. ISBN 0-444-42607-8.
  6. ^
  7. ^ Francis Barrett. The lives of alchemystical philosophers: with a critical catalogue of books in occult chemistry, and a selection of the most celebrated treatises on the theory and practice of the hermetic art. Printed by Macdonald and son, for Lackington, Allen, & co., 1815. p. 233
  8. ^ Barbara DiBernard. Alchemy and Finnegans wake. SUNY Press, 1980. p. 57, ISBN 0873953886.
  9. ^ George Ripley. Compound of Alchemy. 1591. Retrieved from http://www.levity.com/alchemy/ripgat8.html
  10. ^
  11. ^