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4 Colligative Properties

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Hydrochemistry
This chapter is in the book Hydrochemistry
4 Colligative Properties4.1 IntroductionColligative properties are solution properties that depend on the amount of dissolvedspecies in the solution but not on their chemical identity. This means that equal amountsof dissolved species always cause the same effects independent of their chemical nature.To express the amount of species present in the solution, the mole fraction, the molality,or the molarity have to be used depending on the type of the colligative property.There are four colligative properties: vapor pressure lowering, boiling point ele-vation, freezing point depression, and osmotic pressure. This means that a solutionshows a decreased vapor pressure, an increased boiling point, and a decreased freez-ing point in comparison to the pure solvent (water in our case). Furthermore, eachsolution causes a concentration-dependent osmotic pressure. In the following sec-tions, at first, the calculation of colligative properties in ideal solutions is considered.After that, the deviations in real systems are discussed separately in Section 4.5.4.2 Vapor Pressure LoweringThe vapor pressure of a liquid is the result of the transfer of molecules from the liquidphase to the gas phase. The transferred molecules cause a certain pressure in the gasphase, the vapor pressure. Since the likelihood of such a transfer increases with in-creasing temperature, the vapor pressure increases with increasing temperature. If asubstance is dissolved in the liquid, a certain number of solvent molecules are re-placed from the surface by the solute species as schematically shown in Figure 4.1.Consequently, at the same temperature, less solvent molecules can be transferred tothe gas phase and the vapor pressure is reduced in comparison to the pure solvent.Furthermore, additional interactions between the solvent and the solute (solvation) re-duce the number of solvent molecules that are able to leave the liquid phase.The vapor pressure of the solvent in the solution,p1,isgivenby:p1=x1p01(4:1)wherep01is the vapor pressure of the pure solvent (water in our case) andx1is the molefraction of the solvent in the solution. Given that the sum of mole fractions equals 1 andassuming that there is only a single nondissociating solute in the solution, we can write:p1=x1p01=ð1−x2Þp01=p01−x2p01(4:2)wherex2is the mole fraction of the solute. Accordingly, the relative lowering of thevapor pressure is given by the mole fraction of the solute:https://doi.org/10.1515/9783110758788-004
© 2023 Walter de Gruyter GmbH, Berlin/Boston

4 Colligative Properties4.1 IntroductionColligative properties are solution properties that depend on the amount of dissolvedspecies in the solution but not on their chemical identity. This means that equal amountsof dissolved species always cause the same effects independent of their chemical nature.To express the amount of species present in the solution, the mole fraction, the molality,or the molarity have to be used depending on the type of the colligative property.There are four colligative properties: vapor pressure lowering, boiling point ele-vation, freezing point depression, and osmotic pressure. This means that a solutionshows a decreased vapor pressure, an increased boiling point, and a decreased freez-ing point in comparison to the pure solvent (water in our case). Furthermore, eachsolution causes a concentration-dependent osmotic pressure. In the following sec-tions, at first, the calculation of colligative properties in ideal solutions is considered.After that, the deviations in real systems are discussed separately in Section 4.5.4.2 Vapor Pressure LoweringThe vapor pressure of a liquid is the result of the transfer of molecules from the liquidphase to the gas phase. The transferred molecules cause a certain pressure in the gasphase, the vapor pressure. Since the likelihood of such a transfer increases with in-creasing temperature, the vapor pressure increases with increasing temperature. If asubstance is dissolved in the liquid, a certain number of solvent molecules are re-placed from the surface by the solute species as schematically shown in Figure 4.1.Consequently, at the same temperature, less solvent molecules can be transferred tothe gas phase and the vapor pressure is reduced in comparison to the pure solvent.Furthermore, additional interactions between the solvent and the solute (solvation) re-duce the number of solvent molecules that are able to leave the liquid phase.The vapor pressure of the solvent in the solution,p1,isgivenby:p1=x1p01(4:1)wherep01is the vapor pressure of the pure solvent (water in our case) andx1is the molefraction of the solvent in the solution. Given that the sum of mole fractions equals 1 andassuming that there is only a single nondissociating solute in the solution, we can write:p1=x1p01=ð1−x2Þp01=p01−x2p01(4:2)wherex2is the mole fraction of the solute. Accordingly, the relative lowering of thevapor pressure is given by the mole fraction of the solute:https://doi.org/10.1515/9783110758788-004
© 2023 Walter de Gruyter GmbH, Berlin/Boston

Chapters in this book

  1. Frontmatter I
  2. Preface V
  3. Contents VII
  4. 1 Introduction 1
  5. 2 Structure and Properties of Water 8
  6. 3 Concentrations and Activities 31
  7. 4 Colligative Properties 56
  8. 5 The Chemical Equilibrium: Some General Aspects 64
  9. 6 Gas–Water Partitioning 74
  10. 7 Acid/Base Equilibria 88
  11. 8 Precipitation/Dissolution Equilibria 130
  12. 9 Calco-Carbonic Equilibrium 141
  13. 10 Redox Equilibria 159
  14. 11 Complex Formation 206
  15. 12 Sorption 229
  16. 13 Solutions to the Problems 256
  17. A. Appendix 311
  18. Nomenclature 327
  19. Bibliography 333
  20. Index 335
Readers are also interested in:
https://doi.org/10.1515/9783110758788-004

Chapters in this book

  1. Frontmatter I
  2. Preface V
  3. Contents VII
  4. 1 Introduction 1
  5. 2 Structure and Properties of Water 8
  6. 3 Concentrations and Activities 31
  7. 4 Colligative Properties 56
  8. 5 The Chemical Equilibrium: Some General Aspects 64
  9. 6 Gas–Water Partitioning 74
  10. 7 Acid/Base Equilibria 88
  11. 8 Precipitation/Dissolution Equilibria 130
  12. 9 Calco-Carbonic Equilibrium 141
  13. 10 Redox Equilibria 159
  14. 11 Complex Formation 206
  15. 12 Sorption 229
  16. 13 Solutions to the Problems 256
  17. A. Appendix 311
  18. Nomenclature 327
  19. Bibliography 333
  20. Index 335
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