Freezing-point depression: Difference between revisions
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==Uses== |
==Uses== |
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The phenomenon of freezing point depression is used in technical applications to avoid freezing. In the case of water, [[ethylene glycol]] or other forms of [[antifreeze]] is added to cooling water in [[internal combustion engines]], making the mixture stay a liquid at temperatures below its normal freezing point. |
The phenomenon of freezing point depression is used in technical applications to avoid freezing. In the case of water, [[ethylene glycol]] or other forms of [[antifreeze]] is added to cooling water in [[internal combustion engines]], making the mixture stay a liquid at temperatures below its normal freezing point. |
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The use of freezing-point depression through "freeze avoidance" has also [[evolution|evolved]] in some animals that live in very cold environments. This happens through permanently high concentration of [[physiology|physiologically]] rather inert substances such as [[sorbitol]] or [[glycerol]] to increase the [[molality]] of fluids in cells and tissues, and thus decrease the freezing point. Examples include some species of [[arctic]]-living [[fish]], such as [[rainbow smelt]], which need to be able to survive in freezing temperatures for a long time. In other animals, such as the [[spring peeper]] frog (''Pseudacris crucifer''), the molality is increased temporarily as a reaction to cold temperatures. In the case of the peeper frog, this happens by massive breakdown of [[glycogen]] in the frog's liver and subsequent release of massive amounts of [[glucose]].<ref>L. Sherwood et al., ''Animal Physiology - From Genes to Organisms'', 2005, Thomson Brooks/Cole, Belmont, CA, ISBN 0-534-55404-0, p. 691-692</ref> |
The use of freezing-point depression through "freeze avoidance" has also [[evolution|evolved]] in some animals that live in very cold environments. This happens through permanently high concentration of [[physiology|physiologically]] rather inert substances such as [[sorbitol]] or [[glycerol]] to increase the [[molality]] of fluids in cells and tissues, and thus decrease the freezing point. Examples include some species of [[arctic]]-living [[fish]], such as [[rainbow smelt]], which need to be able to survive in freezing temperatures for a long time. In other animals,fuck such as the [[spring peeper]] frog (''Pseudacris crucifer''), the molality is increased temporarily as a reaction to cold temperatures. In the case of the peeper frog, this happens by massive breakdown of [[glycogen]] in the frog's liver and subsequent release of massive amounts of [[glucose]].<ref>L. Sherwood et al., ''Animal Physiology - From Genes to Organisms'', 2005, Thomson Brooks/Cole, Belmont, CA, ISBN 0-534-55404-0, p. 691-692</ref> |
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Together with formula above, freezing-point depression can be used to measure the degree of [[dissociation (chemistry)|dissociation]] or the [[molar mass]] of the solute. This kind of measurement is called '''cryoscopy''' ([[Ancient Greek|Greek]] "freeze-viewing") and relies on exact measurement of the freezing point. The degree of dissociation is measured by determining the [[van 't Hoff factor]] ''i'' by first determining ''m''<sub>B</sub> and then comparing it to ''m''<sub>solute</sub>. In this case, the molar mass of the solute must be known. The molar mass of a solute is determined by comparing ''m''<sub>B</sub> with the amount of solute dissolved. In this case, ''i'' must be known, and the procedure is primarily useful for organic compounds using a nonpolar solvent. Cryoscopy is no longer as common a measurement method as it once was. As an example, it was still taught as a useful analytic procedure in Cohen's ''Practical Organic Chemistry '' of 1910,<ref>Julius B. Cohen ''Practical Organic Chemistry'' '''1910''' [http://archive.org/details/PracticalOrganicChemistry Link to online text]</ref> in which the [[molar mass]] of [[naphthalene]] is determined in a so-called '''Beckmann freezing apparatus'''. |
Together with formula above, freezing-point depression can be used to measure the degree of [[dissociation (chemistry)|dissociation]] or the [[molar mass]] of the solute. This kind of measurement is called '''cryoscopy''' ([[Ancient Greek|Greek]] "freeze-viewing") and relies on exact measurement of the freezing point. The degree of dissociation is measured by determining the [[van 't Hoff factor]] ''i'' by first determining ''m''<sub>B</sub> and then comparing it to ''m''<sub>solute</sub>. In this case, the molar mass of the solute must be known. The molar mass of a solute is determined by comparing ''m''<sub>B</sub> with the amount of solute dissolved. In this case, ''i'' must be known, and the procedure is primarily useful for organic compounds using a nonpolar solvent. Cryoscopy is no longer as common a measurement method as it once was. As an example, it was still taught as a useful analytic procedure in Cohen's ''Practical Organic Chemistry '' of 1910,<ref>Julius B. Cohen ''Practical Organic Chemistry'' '''1910''' [http://archive.org/details/PracticalOrganicChemistry Link to online text]</ref> in which the [[molar mass]] of [[naphthalene]] is determined in a so-called '''Beckmann freezing apparatus'''. |
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In principle, the boiling point elevation and the freezing point depression could be used interchangeably for this purpose. However, the [[cryoscopic constant]] is larger than the [[ebullioscopic constant]] and the freezing point is often easier to measure with precision, which means measurements using the freezing point depression are more precise. |
In principle, the boiling point elevation and the freezing point depression could be used interchangeably for this purpose. However, the [[cryoscopic constant]] is larger than the [[ebullioscopic constant]] and the freezing point is often easier to measure with precision, which means measurements using the freezing point depression are more precise. |
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== Freezing-point depression of a solute vs a solvent == |
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At the phase change of a solvent, [[kinetic energy]] remains constant, while [[potential energy]] decreases. In contrast, at the phase change of a solution, there may be a possibility of one of the substances' kinetic energy decreasing while the potential energy of the other substance decreases. This would mean that the potential energy of the solution would be decreasing ([[solidifying]]) and the kinetic energy would be decreasing ([[cooling]]) at the same time. |
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In order for a substance in the liquid phase to freeze, the molecules must begin to form a cluster of molecules which eventually grows into a solid. When a solute is added to a solvent, the solute particles hinder the solvent molecules from being able to cluster by effectively forcing the solvent molecules way from the cluster when they collide. In order for the solute molecules to reach the cluster and add themselves to the freezing solid, they must be slowed down, have a lower kinetic energy. This is achieved by lowering the temperature. Therefore, the freezing point of a solvent with a solute is lower than the freezing point of a pure solvent. Note: While a substance is going through a phase change, temperature remains constant. |
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==Calculation== |
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If the solution is treated as an ideal solution, the extent of freezing point depression depends only on |
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the solute concentration that can be estimated by a simple linear relationship with the [[cryoscopic constant]]: |
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''ΔT''<sub>F</sub> = ''K''<sub>F</sub> · ''m'' |
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where |
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* ''ΔT''<sub>F</sub>, the freezing point depression, is defined as ''T''<sub>F (pure solvent)</sub> - ''T''<sub>F (solution)</sub>. |
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* ''K''<sub>F</sub>, the cryoscopic constant, which is dependent on the properties of the solvent, not the solute. Note: When conducting experiments, a higher K<sub>F</sub> value makes it easier to observe larger drops in the freezing point. |
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This simple relation doesn't include the nature of the solute, so this is only effective in a diluted solution. For a more accurate calculation at a higher concentration, Ge and Wang (2010)<ref> X. Ge, X. Wang. Estimation of Freezing Point Depression, Boiling Point Elevation and Vaporization enthalpies of electrolyte solutions. Ind. Eng. Chem. Res. 48(2009)2229-2235. http://pubs.acs.org/doi/abs/10.1021/ie801348c (Correction: 2009, 48, 5123)http://pubs.acs.org/doi/abs/10.1021/ie900434h </ref> <ref> X. Ge, X. Wang. Calculations of Freezing Point Depression, Boiling Point Elevation, Vapor Pressure and Enthalpies of Vaporization of Electrolyte Solutions by a Modified Three-Characteristic Parameter Correlation Model. J. Sol. Chem. 38(2009)1097-1117.http://www.springerlink.com/content/21670685448p5145/</ref> proposed a new and easy-to-use equation: |
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''ΔT''<sub>F</sub> ={''ΔH''<sup>fus</sup><sub>T<sub>F</sub></sub> - ''2RT''<sub>F</sub>·''lna''<sub>liq</sub> - [''2ΔC''<sup>fus</sup><sub>p</sub>T<sub>F</sub></sub><sup>2</sup>''R·lna''<sub>liq</sub> + (''ΔH''<sup>fus</sup><sub>T<sub>F</sub></sub>)<sup>2</sup>]<sup>0.5</sup>} / [''2''(''ΔH''<sup>fus</sup><sub>T<sub>F</sub></sub>/ ''T''<sub>F</sub> + ''0.5ΔC''<sup>fus</sup><sub>p</sub>-''Rlna''<sub>liq</sub>)] |
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In the above equation, ''T''<sub>F</sub> is the normal freezing point of the pure solvent (0<sup>o</sup>C for water for example);''a''<sub>liq</sub> is the activity of the solution(water activity for aqueous solution); ''ΔH''<sup>fus</sup><sub>T<sub>F</sub></sub> is the enthalpy change of fusion of the pure solvent at ''T''<sub>F</sub>, which is 333.6 J/g for water at 0<sup>o</sup>C; ''ΔC''<sup>fus</sup><sub>p</sub> is the differences of heat capacity between the liquid and solid |
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phases at ''T''<sub>F</sub>, which is 2.11 J/g/K for water. |
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The solvent activity can be calculated from Pitzer model or modified TCPC model, which typically requires 3 adjustable parameters. For the TCPC model, these parameters are available at reference <ref> X. Ge, X. Wang, M. Zhang, S. Seetharaman. Correlation and Prediction of Activity and Osmotic Coefficients of Aqueous Electrolytes at 298.15 K by the Modified TCPC Model. J. Chem. Eng. data. 52 (2007) 538-547.http://pubs.acs.org/doi/abs/10.1021/je060451k </ref> <ref>X. Ge, M. Zhang, M. Guo, X. Wang. Correlation and Prediction of thermodynamic properties of Some Complex Aqueous Electrolytes by the Modified Three-Characteristic-Parameter Correlation Model. J. Chem. Eng. Data. 53(2008)950-958. http://pubs.acs.org/doi/abs/10.1021/je7006499 </ref> <ref>X. Ge, M. Zhang, M. Guo, X. Wang, Correlation and Prediction of Thermodynamic Properties of Non-aqueous Electrolytes by the Modified TCPC Model. J. Chem. Eng. data. 53 (2008)149-159.http://pubs.acs.org/doi/abs/10.1021/je700446q </ref> <ref>X. Ge, X. Wang. A Simple Two-Parameter Correlation Model for Aqueous Electrolyte across a wide range of temperature. J. Chem. Eng. Data. 54(2009)179-186.http://pubs.acs.org/doi/abs/10.1021/je800483q </ref> for a lot of single salts. |
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The equation is not only effective for aqueous solution, but also for non-aqueous solution. |
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==See also== |
==See also== |
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Revision as of 16:59, 6 December 2010
- This article deals with melting and freezing point depression due to mixture of another compound. For depression due to small particle size, see melting point depression.
Freezing-point depression describes the phenomenon in which the freezing point of a liquid (a solvent) is depressed when another compound is added, meaning that a solution has a lower freezing point than a pure solvent. This happens whenever a solute is added to a pure solvent, such as water. The phenomenon may be observed in sea water, which due to its salt content remains liquid at temperatures below 0 °C, the freezing point of pure water.
Uses
The phenomenon of freezing point depression is used in technical applications to avoid freezing. In the case of water, ethylene glycol or other forms of antifreeze is added to cooling water in internal combustion engines, making the mixture stay a liquid at temperatures below its normal freezing point.
The use of freezing-point depression through "freeze avoidance" has also evolved in some animals that live in very cold environments. This happens through permanently high concentration of physiologically rather inert substances such as sorbitol or glycerol to increase the molality of fluids in cells and tissues, and thus decrease the freezing point. Examples include some species of arctic-living fish, such as rainbow smelt, which need to be able to survive in freezing temperatures for a long time. In other animals,fuck such as the spring peeper frog (Pseudacris crucifer), the molality is increased temporarily as a reaction to cold temperatures. In the case of the peeper frog, this happens by massive breakdown of glycogen in the frog's liver and subsequent release of massive amounts of glucose.[1]
Together with formula above, freezing-point depression can be used to measure the degree of dissociation or the molar mass of the solute. This kind of measurement is called cryoscopy (Greek "freeze-viewing") and relies on exact measurement of the freezing point. The degree of dissociation is measured by determining the van 't Hoff factor i by first determining mB and then comparing it to msolute. In this case, the molar mass of the solute must be known. The molar mass of a solute is determined by comparing mB with the amount of solute dissolved. In this case, i must be known, and the procedure is primarily useful for organic compounds using a nonpolar solvent. Cryoscopy is no longer as common a measurement method as it once was. As an example, it was still taught as a useful analytic procedure in Cohen's Practical Organic Chemistry of 1910,[2] in which the molar mass of naphthalene is determined in a so-called Beckmann freezing apparatus.
Freezing-point depression can also be used as a purity analysis tool when analysed by differential scanning calorimetry.[3] The results obtained are in mol%, but the method has its place, where other methods of analysis fail.
This is also the same principle acting in the melting-point depression observed when the melting point of an impure solid mixture is measured with a melting point apparatus, since melting and freezing points both refer to the liquid-solid phase transition (albeit in different directions).
In principle, the boiling point elevation and the freezing point depression could be used interchangeably for this purpose. However, the cryoscopic constant is larger than the ebullioscopic constant and the freezing point is often easier to measure with precision, which means measurements using the freezing point depression are more precise.
See also
- Cryoscopic constant
- Boiling-point elevation
- Eutectic point
- Colligative properties
- List of boiling and freezing information of solvents
- Snow removal
- Frigorific mixture
References
- ^ L. Sherwood et al., Animal Physiology - From Genes to Organisms, 2005, Thomson Brooks/Cole, Belmont, CA, ISBN 0-534-55404-0, p. 691-692
- ^ Julius B. Cohen Practical Organic Chemistry 1910 Link to online text
- ^ "DSC Purity Analysis" (PDF). Retrieved 2009-02-05.