Colligative Properties of Electrolytes (OpenStax Chemistry 2e)
The colligative properties of a solution depend only on the number, not on the identity, of solute species dissolved. The concentration terms in the equations for various colligative properties (freezing point depression, boiling point elevation, osmotic pressure) pertain to all solute species present in the solution. For the solutions considered thus far in this chapter, the solutes have been nonelectrolytes that dissolve physically without dissociation or any other accompanying process. Each molecule that dissolves yields one dissolved solute molecule. The dissolution of an electroyte, however, is not this simple, as illustrated by the two common examples below:
Considering the first of these examples, and assuming complete dissociation, a 1.0 m aqueous solution of NaCl contains 2.0 mole of ions (1.0 mol Na+ and 1.0 mol Cl−) per each kilogram of water, and its freezing point depression is expected to be
When this solution is actually prepared and its freezing point depression measured, however, a value of 3.4 °C is obtained. Similar discrepancies are observed for other ionic compounds, and the differences between the measured and expected colligative property values typically become more significant as solute concentrations increase. These observations suggest that the ions of sodium chloride (and other strong electrolytes) are not completely dissociated in solution.
To account for this and avoid the errors accompanying the assumption of total dissociation, an experimentally measured parameter named in honor of Nobel Prize-winning German chemist Jacobus Henricus van’t Hoff is used. The van’t Hoff factor (i) is defined as the ratio of solute particles in solution to the number of formula units dissolved:
Values for measured van’t Hoff factors for several solutes, along with predicted values assuming complete dissociation, are shown in Table 1.
Predicated and Measured van’t Hoff Factors for Several 0.050 m Aqueous Solutions
|Formula unit||Classification||Dissolution products||i (predicted)||i (measured)|
|NaCl||Strong electrolyte||Na+, Cl−||2||1.9|
|HCl||Strong electrolyte (acid)||H3O+, Cl−||2||1.9|
|MgSO4||Strong electrolyte||Mg2+, SO42−,||2||1.3|
|MgCl2||Strong electrolyte||Mg2+, 2Cl−||3||2.7|
|FeCl3||Strong electrolyte||Fe3+, 3Cl−||4||3.4|
In 1923, the chemists Peter Debye and Erich Hückel proposed a theory to explain the apparent incomplete ionization of strong electrolytes. They suggested that although interionic attraction in an aqueous solution is very greatly reduced by solvation of the ions and the insulating action of the polar solvent, it is not completely nullified. The residual attractions prevent the ions from behaving as totally independent particles (Figure 1). In some cases, a positive and negative ion may actually touch, giving a solvated unit called an ion pair. Thus, the activity, or the effective concentration, of any particular kind of ion is less than that indicated by the actual concentration. Ions become more and more widely separated the more dilute the solution, and the residual interionic attractions become less and less. Thus, in extremely dilute solutions, the effective concentrations of the ions (their activities) are essentially equal to the actual concentrations. Note that the van’t Hoff factors for the electrolytes in Table 1 are for 0.05 m solutions, at which concentration the value of i for NaCl is 1.9, as opposed to an ideal value of 2.
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
Research Article: Frequency of Electrolyte Derangement after Transurethral Resection of Prostate: Need for Postoperative Electrolyte Monitoring
Date Published: May 18, 2015 Publisher: Hindawi Publishing Corporation Author(s): Wajahat Aziz, M. Hammad Ather. http://doi.org/10.1155/2015/415735 Abstract: Objective. To determine the electrolyte derangement following transurethral resection of prostate (TURP). Methods. All patients undergoing TURP from June 2012 to April 2013 were included. Preoperative electrolytes were performed within a week of procedures. Monopolar TURP using 1.5% … Continue reading
Date Published: November 17, 2015 Publisher: John Wiley and Sons Inc. Author(s): Xin‐Bing Cheng, Rui Zhang, Chen‐Zi Zhao, Fei Wei, Ji‐Guang Zhang, Qiang Zhang. http://doi.org/10.1002/advs.201500213 Abstract: Lithium metal batteries (LMBs) are among the most promising candidates of high‐energy‐density devices for advanced energy storage. However, the growth of dendrites greatly hinders the practical applications of LMBs … Continue reading
Date Published: January 22, 2019 Publisher: John Wiley and Sons Inc. Author(s): Xinhua Liu, Oluwadamilola O. Taiwo, Chengyao Yin, Mengzheng Ouyang, Ridwanur Chowdhury, Baofeng Wang, Huizhi Wang, Billy Wu, Nigel P. Brandon, Qigang Wang, Samuel J. Cooper. http://doi.org/10.1002/advs.201801337 Abstract: Ionogels are a new class of promising materials for use in all‐solid‐state energy storage devices in … Continue reading
Date Published: October 2, 2014 Publisher: BioMed Central Author(s): Eva Z Hesselkilde, Mette E Almind, Jesper Petersen, Mette Flethøj, Kirstine F Præstegaard, Rikke Buhl. http://doi.org/10.1186/s13028-014-0058-y Abstract: Despite increased focus on cardiac arrhythmias in horses, the nature and prevalence is still poorly described. Case reports suggest that arrhythmias occurring secondary to systemic disease are seen more … Continue reading
Research Article: Impact of a new balanced gelatine on electrolytes and pH in the perioperative care
Date Published: April 29, 2019 Publisher: Public Library of Science Author(s): Gernot Marx, Patrick Meybohm, Tobias Schuerholz, Gösta Lotz, Mandy Ledinko, Achim W. Schindler, Rolf Rossaint, Kai Zacharowski, Iratxe Puebla. http://doi.org/10.1371/journal.pone.0213057 Abstract: Balanced fluid replacement solutions can possibly reduce the risks for electrolyte imbalances, for acid-base imbalances, and thus for renal failure. To assess the … Continue reading