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Name: Angela
Status: other
Grade: other
Country: Canada
Date: Winter 2009-2010


Question:
Why do solutions of electrolytes have a greater effect on the boiling point and freezing temperature of solvents than solutions of non electrolytes?



Replies:
The effect of solutes on the freezing point and boiling point of a solvent depends upon the number of "particles" released by the solute into the solvent. This is true in dilute solution whether the solute is an electrolyte or a non-electrolyte. Soluble non-electrolytes typically produce a single molecular species.

Soluble electrolytes typically produce at least two molecular species. So the effect of an electrolyte is approximately double (or more) than of a non-electrolyte. In a text book you will find this discussed under the heading of "colligative properties", which means that the effect just depends upon the number of particles of solute per mol. This is a simplification because when you get down into the nitty gritty things get more complicated, but for your purposes the above suffices.

Vince Calder


Because each mole of an electrolyte contains at least two moles of ions, which dissociate in solution. Non-electrolytes do not dissociate.

This sounded like a homework question. I hope you inform your teacher how you obtained your answer.

Richard Barrans
Department of Physics


Angela,

Electrolytes contribute more than one particle per formula unit when they dissolve. e.g. NaCl dissolves to give two particles per formula unit (Na+ and Cl- ions). Solutions of a non-electrolyte will only give one particle per formula unit. e.g. glucose C6H12O6 dissolves to give one glucose molecule per formula unit, it does not dissociate. This means that solutions of the same concentration of each solute will give different numbers of solute particles in solution (in fact the NaCl will have almost twice as many).

Freezing point depression and boiling point elevation are colligative properties: they depend only on the number of solute particles present as a ratio of the number of solvent particles. With a 1m solution of NaCl you get about 2 moles of particles per kg of solvent, where as a 1m solution of C6H12O6 gives you 1 mole of particles per kg of solvent.

This "effective number of particles per formula unit" (1 for C6H12O6 and about 2 for NaCl) is given as the van't Hoff factor. It is actually experimentally found and doesn't always agree exactly with what we expect. For NaCl it is only 2 for dilute solutions; we think the ions pair up at higher concentrations and reduce the "effective number of particles per formula unit" below the ideal situation.

Interestingly, it does not seem to affect the boiling point elevation and freezing point depression very much if you change what the solute particles are, only the number of particles present seems to matter. This leads us to think that this phenomenon is not based on enthalpy, i.e. the attractions of the solute particles to the solvent particles are not important as the solute is assumed to remain in solution while the pure solvent freezes or boils.

So how do we explain this phenomenon? It is usually done by saying that the mixing of the solute with the solvent increases the entropy of the solution making the freezing process involve a larger negative change in entropy, and the boiling process involve a smaller positive change in entropy. This requires a lower temperature for the freezing to occur and a higher temperature for the boiling to occur as a higher temperature will exacerbate any entropy effects and a lower temperature will lessen any entropy effects. The role of entropy change in phase changes can be explained from the Gibbs equation: DG = DH -TDS, where any spontaneous change will only occur if DG is negative.

Incidentally, this topic is one of those "red flags" in high school: it is VERY easy to have this taught in a misleading way, involving ions "holding onto" water molecules, preventing boiling or freezing, which, although easy to grasp, is not a helpful model and is not backed up by experimental observation. The entropy argument is a bit involved, though!

Best wishes,

Tom Collins


Angela,

In most textbooks, the freezing point depression or boiling point elevation equation is written as:

dT = km; where dT is the change in freezing or boiling point, k is the constant and m is the molality.

A more complete equation is:

dT = ikm; where i is the van't Hoff factor which gives how much ionization is happening in solution.

In dilute solutions, typically, the i can be estimated directly from how many ions can be seen in the molecular formula.

For example: NaCl = 2; MgCl2 = 3; Fe(NO3)3 = 4.

From this you can figure out why electrolytes - on a per mole basis, or of the same molality - can cause higher changes in boiling or freezing points.

Greg (Roberto Gregorius)
Canisius College



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