M D.
asked • 09/07/18Melting Point of Ice As It Relates to Intermolecular Forces
If adding ions increases the melting point of solids (because the strength of intermolecular forces increase via ion-dipole interactions), then why does adding salt decrease the melting point of ice?
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1 Expert Answer
J.R. S. answered • 09/07/18
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As water freezes the kinetic energy of the molecules decreases, and intermolec forces increase and a lattice of water molecules form which we call ice. During the phase change from liquid to solid, there is an equilibrium between the two phases. In the presence of salt (ions) this equilibrium is distorted and fewer water molcules enter the solid phase and more remain in the liquid phase. This is ultimately seen as a lowering of the freezing point/melting point.
I think you have it backwards when you suggesr that adding ions increases the melting point of solids. The presence of ions or impurities DECREASES the melting point because they interfere with the normal intermolecular forces holding the crystalline lattice together. Thus, less heat is needed to break the lattice and melt the solid and hence melting point is decreased.
M D.
I'm more confused now. In my textbook it states that generally if a liquid has stronger intermolecular attractions it will have a higher boiling point, higher melting point, higher viscosity, higher surface tension but lower vapor pressure. Could this be an error in the text or does it only apply in certain cases? Thank you
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09/07/18
J.R. S.
tutor
All of that is true. The problem comes with adding another substance (what you refer to as ions). They (the ions) do not INCREASE intermolecular attractions between the molecules of liquid, but rather they DECREASE intermolecular attractions. Does this make sense?
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09/07/18
M D.
I think
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09/07/18
M D.
I think I am understanding. So what would be an example of a solid that has a high melting point due to strong intermolecular forces from ion-dipole interactions? If you created a solution of ice water and froze it (rather than sprinkling salt on pure water in the previous case) would this have different physical properties?
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09/07/18
M D.
Sorry, I meant to say a solution of salt water in the previous comment.
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09/07/18
M D.
From my understanding IMF's are ranked in order of strength as follows:
1. Ion-ion
2. Ion-dipole
3. Hydrogen Bonding
4. Dipole-dipole
5. Dipole-induced dipole
6. London Dispersion
So in the case of a salt water solution, the Na+ and Cl- ions have dissociated and form strong interactions with the opposite ends of polar water molecules. This would cause the melting point of a salt water solution to be higher than that of pure water which exhibits hydrogen bonding alone. Am I understanding this correctly?
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09/07/18
J.R. S.
tutor
An ionic solid (crystalline solid) like NaCl will have a high melting point. The forces holding the lattice together are ionic and a strong. In a molecular solid, such as ice, sugar, etc, the forces holding them together are weaker than ionic interactions and so have lower melting points.
A solution of salt water will have the same freezing/melting point as the ice-salt mixture provided the concentration of salt is the same in both cases.
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09/07/18
M D.
So, ion-dipole interactions always lower the melting point of solids? I thought that the force between the dipole of water and the sodium/chloride ions was much stronger than the force of hydrogen bonds between adjacent water molecules so it would take more energy to break them apart.
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09/07/18
J.R. S.
tutor
Yes. The force of attraction between the dipole of water and the ion of Na+ would be stronger than the H-bonding between 2 water molecules. Thats precisely why the salt lowers the freezing point. The water molecules are no longer attracted to EACH OTHER and so less heat is needed to melt the solid. We are NOT concerned with the forces between water and sodium. That’s where more energy would be required.
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09/07/18
M D.
Wow thank you so much. That actually makes a lot more sense than the way my professor explained it.
So we can say that the strength of the IMF matters only when we are comparing 2 or more pure compounds. If we are mixing 2 different compounds (assuming no reaction) then the solution, compared to the solvent, will have a lower melting point and a higher boiling point. Is this correct?
How would this affect the other physical properties? For example, if we create a solution of sugar water I now understand that it will have a lower MP and higher BP than pure water. I’m reasoning that this would still increase viscosity and surface tension and decrease vapor pressure.
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09/07/18
J.R. S.
tutor
Yes, now you have it correct. The solute will affect the properties of the solvent as you described. And the effect is dependent on the number of particles present. For more details you should read about “colligative properties “. But I think you now have it right.
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09/07/18
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M D.
09/07/18