Difference Between Colligative Properties of Electrolytes and Nonelectrolytes

Key Difference – Colligative Properties of Electrolytes vs Nonelectrolytes
 

Colligative properties are physical properties of a solution that depends on the amount of a solute but not on the nature of solute. This means similar amounts of completely different solutes can alter these physical properties in similar quantities. Hence, the colligative properties depend on the ratio of the solute amount and solvent amount. The three major colligative properties are vapor pressure lowering, boiling point elevation and freezing point depression. For a given solute-solvent mass ratio, all colligative properties are inversely proportional to solute molar mass. Electrolytes are substances that can form solutions that are able to conduct electricity through this solution. Such solutions are known as electrolytic solutions. Nonelectrolytes are substances that are not capable of forming electrolytic solutions. Both these types (electrolytes and nonelectrolytes) have colligative properties. The key difference between colligative properties of electrolytes and nonelectrolytes is that the effect of electrolytes on colligative properties is very high compared to that of the nonelectrolytes.

CONTENTS

1. Overview and Key Difference
2. What are Colligative Properties of Electrolytes
3. What are Colligative Properties of Nonelectrolytes
4. Side by Side Comparison – Colligative Properties of Electrolytes vs Nonelectrolytes in Tabular Form
5. Summary

What are Colligative Properties of Electrolytes?

Colligative properties of electrolytes are the physical properties of electrolytic solutions that depend on the amount of solutes regardless the nature of solutes. The solutes present in electrolytic solutions are atoms, molecules or ions having either lost or gained electrons to become electrically conductive.

When an electrolyte is dissolved in a solvent such as water, the electrolyte separates into ions (or any other conductive species). Therefore, dissolving one mole of electrolyte always yields two or more moles of conductive species. Hence, the colligative properties of the electrolytes are considerably changing when an electrolyte is dissolved in a solvent.

For example, the general equation used in describing freezing point and boiling point changes is as follow,

ΔT_b = K_bm and ΔT_f = K_fm

ΔT_b is boiling point elevation, and ΔT_f is freezing point depression. K_b and K_f are boiling point elevation constant and freezing point depression constant respectively. m is the molarity of the solution. For electrolytic solutions, the above equations are modified as follows,

ΔT_b = iK_bm and ΔT_f = iK_fm

“i” is an ion multiplier known as Van’t Hoff factor. This factor is equal to the number of moles of ions given by an electrolyte. Therefore, Van’t Hoff factor can be determined by finding the number of ions released by an electrolyte when it is dissolved in a solvent. For example, the value of Van’t Hoff factor for NaCl is 2 and in CaCl₂, it is 3.

Figure 01: A graph showing the Chemical Potential against Temperature describing Freezing Point Depression and Boiling Point Elevation

However, the values given for these colligative properties are different from the theoretically predicted values. That is because there can be solute and solvent interactions that reduce the effect of ions on those properties.

Above equations are further modified to be used for weak electrolytes. The weak electrolytes partially dissociate into ions, hence some of the ions do not affect the colligative properties. The degree of dissociation (α) of a weak electrolyte can be calculated as follows,

α    =  {(i-1)/(n-1)} x 100

Here, n is the maximum number of ions formed per molecule of the weak electrolyte.

What are Colligative Properties of Nonelectrolytes?

Colligative properties of nonelectrolytes are the physical properties of non-electrolytic solutions that depend on the amount of solutes regardless the nature of solutes. Non-electrolytes are substances that do not create conductive solutions when dissolved in a solvent. For example, sugar is a nonelectrolyte because when sugar is dissolved in water, it exists in molecular form (does not dissociate into ions). These sugar molecules are incapable of conducting electric currents through the solution.

The number of solutes present in a non-electrolytic solution is less compared to an electrolytic solution. Therefore, the effect of nonelectrolytes on colligative properties is also very low. For example, the degree of vapor pressure lowering by adding NaCl is higher compared to the addition of sugar to a similar solution.

What is the Difference Between Colligative Properties of Electrolytes and Nonelectrolytes?

Summary – Colligative Properties of Electrolytes vs Nonelectrolytes

Colligative properties are physical properties of solutions that do not depend on the nature of a solute but the amount of solutes. The difference between colligative properties of electrolytes and nonelectrolytes is that the effect of electrolytes on colligative properties is very high compared to nonelectrolytes.

Reference:

1.“5.9: Colligative Properties of Electrolyte Solutions.” Chemistry LibreTexts, Libretexts, 21 July 2016. Available here   
2.“Colligative Properties.” Wikipedia, Wikimedia Foundation, 10 Mar. 2018. Available here
3.Britannica, The Editors of Encyclopaedia. “Electrolyte.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 7 June 2017. Available here   

Image Courtesy:

1.’Freezing point depression and boiling point elevation’By Tomas er – Own work, (CC BY-SA 3.0) via Commons Wikimedia  

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