The Chemistry of Liquids - Part 4, Colligative Properties

Colligative Properties

The word "colligative" refers to the amount of a solute and its exclusive effect on the properties of a pure solvent.  The solute is non-volatile, meaning that its own vapor pressure is negligible.    There are four effects that a solute can have on a pure liquid: 1) Boiling Point Elevation, 2) Freezing Point Depression, 3) Vapor Pressure Lowering, and 4) Osmotic Pressure.  This post will focus on freezing point depression.

Freezing Point Depression

The freezing temperature of a pure liquid will be lowered when a solid or non-volatile liquid is dissolved in it. For colder climates like Wisconsin, where this blog is written, freezing point depression explains how road salt is able to melt snow and ice.  It goes like this - When rock salt is placed on an icy surface electrostatic attractions between water molecules and salt ions cause a small amount of water to absorb into the surface of the salt crystals.  Because the forces of attraction between water molecules and salt ions are greater than the intermolecular forces in ice, there is a net release of heat energy and the ice begins to melt.  As the ice continues to melt, some of the salt dissolves into the liquid water and the freezing point depression effect begins to occur.  The molten ice, now liquid water, experiences a decrease in freezing point temperature which prevents the water from refreezing.

 

As salt dissolves into water, "gaps" between water molecules form because of the presence of salt ions.  The salt ions interfere with the dipole attractions between water molecules, which causes the liquid to solid phase transition of water to happen at a lower temperature (i.e. the average kinetic energy of water molecules must be lower to counteract the effect of salt ions ).

The Freezing Point Depression Calculation

The change in a solvent's freezing temperature depends on the value of the freezing point depression constant (Kf) and the molality of solute ions present.  The freezing point depression constant changes for different solvents.  Molality (m) is the concentration of solute molecules or ions in terms of moles solute per kilogram of solvent.  The use of molality normalizes the amount of solvent such that the freezing point depression only depends on moles of solute particles for each one kilogram of solvent.  It is important to understand that while Kf  is different for different solvents, the freezing point depression result depends only on solute particle concentration, regardless of the identity of that solute.  The following diagram demonstrates the freezing point depression effect on water for rock salt vs. "super salt".

 

  

The greater colligative effect (freezing pt. depression) of super salt could also be obtained with other 3-ion salts, such as Na2S and BaBr2.

 

That's all for now.  Have a good one!

 

 

A Publication of http://ExcellenceInLearning.biz