Thermodynamics: Why does salt lower the melting point of ice?

[hariharan venkatasu]

TL;DR

why salt lowers melting point of ice?

There was a question on "Why salt lower the freezing point of water?"I found the following answer."Thermodynamics teaches that a loss of entropy can be overcome by a gain in so called enthalpy". The loss of entropy by freezing the solution canbe over come at temperature much below 0 degree C because the gain in enthalpy by freezing water rises when temperature goes down."How can there be gain in enthalpy by freezing water instead of loss? I presume that the gain in enthalpy refers to gain by environment.Please confirm.

 

[hariharan venkatasu]

I am sorry that I missed to mention that the question is from internet

 

[mjc123]

The offered explanation doesn't seem to make much sense, not least because it makes no reference to the presence of salt.

The main driving force for lowering the freezing point is the entropy of mixing of the solution. If you consider the process
ice + salt (2 phases) → water + salt (2 phases) → salt solution (1 phase)

If the enthalpy and entropy of melting of ice to pure water is the same in the presence or absence of salt, then the difference is in the thermodynamics of dissolving the salt in the water. This is driven by the positive entropy of mixing. (In the case of a solution of an ionic salt, there is also the enthalpy and entropy of solvation to consider, which complicates things a bit.)

Equilibrium occurs when ΔG = ΔH - TΔS = 0, so T_eq = ΔH/ΔS. Increasing ΔS lowers the equilibrium temperature.

 

[hariharan venkatasu]

Thanks a lot for the prompt reply.But the sentence "...gain in enhalpy by the freezing water rises when temperature goes down.My doubt is how can there be gain in enthalpy by freezing water instead of loss of enthalpy? Could you please clarify this doubt?.Further the last sentence in your above is confusing.You have mentioned "Increasing delta S lowers the equilibrium temperature."Will there be increase in entropy at lower temperature?

Thanks again.

 

[mjc123]

I meant that ΔS is greater for melting a mixture of ice and salt than for melting pure water. Not that ΔS changes with temperature.

Have you got a link to the answer you found? As it stands it makes no sense.

 

[hariharan venkatasu]

Here is the link.The question was "Why does salt lower the freezing point of water? on quora.The answer given was by Dr.Bob Hooft .
My presumption is that the gain in enthalpy referred to in the answer is by the environment.I don't whether I am right.

 

[phinds]

Here is the link.The question was "Why does salt lower the freezing point of water? on quora.The answer given was by Dr.Bob Hooft .
My presumption is that the gain in enthalpy referred to in the answer is by the environment.I don't whether I am right.

Where ?

 

[hariharan venkatasu]

I have indicated the link where I found it .

 

[jim mcnamara]

@hariharan venkatasu there are no links in any of your posts - by link we mean an URL, an internet link to page(s).

 

[phinds]

I have indicated the link where I found it .

When asked for a link on this forum you are supposed to provide the actual link, NOT tell us how to get the link.

 

[DaveC426913]

I searched Quora for the question but there are several - and Quora's format of having multiple (often unrelated) questions and answers all jumbled up on the same page doesn't help matters.

 

[DrDu]

This effect is mostly derived assuming the mixture to be ideal which includes Delta H to be 0. So it is really an effect of the entropy of mixing.

 

[hariharan venkatasu]

quora.com/why-does-salt-lower-the-freezing-point-of-water.This is the link I have found.

 

[mjc123]

Gives an error 308. Going to quora requires a login.

 

[hariharan venkatasu]

Gives an error 308. Going to quora requires a login.

I have placed the relevant text below:

Rob Hooft, PhD in structural chemistry using molecular modeling and X-ray diffraction.
Answered Mar 14, 2011
https://www.quora.com/Why-does-salt-lower-the-freezing-point-of-water#The freezing point depression is explained quite well on the wikipedia page describing the phenomenon (http://en.wikipedia.org/wiki/Fre... ):

The explanation for the freezing point depression is then simply that as solvent molecules leave the liquid and join the solid they leave behind a smaller volume of liquid in which the solute particles can roam. The resulting reduced entropy of the solute particles thus is independent of their properties.

In the extreme situation, completely freezing a mixture of water and salt leaves pure ice and pure salt crystals. The mixture has been unmixed. This unmixing is something compounds do not "like": the universe likes to be uniform (strangely enough, a uniform mixture is maximizing "chaos" in the sense of physics, this is also called "entropy"). Thermodynamics teaches that a loss in entropy can be overcome by a gain in so called "enthalpy". The loss of entropy by freezing the solution can be overcome at temperatures much below 0 degrees Celsius because the gain in enthalpy by freezing water rises when the temperature goes down.

I think this is quite different from the considerations about the boiling point by Judy Levy Pordes.

 

[chemisttree]

I think you are over interpreting what the Dr. is stating. He is merely making a point that for a process to occur, thermodynamics (Gibbs Free Energy relationship) for any given observed process requires any loss of entropy be made up for in the enthalpy term of the GFE.

It should not be interpreted as a truism that enthalpy rises as temperature goes down!

If your question is merely about the gain in enthalpy “by freezing water” then this can be explained by a language barrier in the explanation. Yes, it means enthalpy gained by the environment.

What does “by freezing water” mean? Does it mean the equivalent to “in the process of freezing water” or does it mean the equivalent to “the water phase within a mixture of water and solute?” Obviously it means in the process of freezing water (pure phase) within a system (solution of water and solute). Yes, the enthalpy of that is given to the environment.

 

[Ygggdrasil]

In general, thermodynamic systems prefer to minimize their potential energy (in much the same way a ball rolls downhill to minimize its gravitational potential energy) and maximize their entropy (in much the same way a child's room will gradually become more disorganized over time without expending effort to clean it up).

Often, these two goals will come into conflict as in the case of water freezing. When water freezes, more stable bonds are able to form between water molecules as they crystallize into a solid. Forming these stronger bonds reduces their chemical potential energy and releases heat to the surroundings. Because the chemical potential energy is lowered during freezing, it is an exothermic process that lowers the enthalpy of the system (ΔH < 0). However, because the water molecules can no longer move around freely as is the case in the liquid state, freezing is associated with a loss in entropy (ΔS < 0).

In cases like these, what sets the balance between the propensity of the system to minimize enthalpy versus maximize entropy? Scientists have devised a measure, called free energy, for just such a purpose. In the case of Gibbs free energy (G), this quantity is defined as G = H - TS, and the change in free energy of a system will be ΔG = ΔH - TΔS. As you should be able to tell from the equation, a process will be thermodynamically favorable if the process lowers the free energy of the system (ΔG < 0). The equation also tells us that temperature is the key factor determining whether the system will prefer to minimize enthalpy or maximize entropy. At low temperatures, minimizing enthalpy becomes more important than maximizing entropy and at high temperatures, maximizing entropy becomes more important than minimizing enthalpy.

The freezing point of water occurs when the liquid and solid states of water are at equilibrium, and for a process at equlibrium, ΔG = 0. Therefore, 0 = ΔH - TΔS, which gives an equation for the melting point of water: T = ΔH/ΔS.

What happens when we add salt to the water? Salt will not appreciably affect the strength of the bonds between water molecules in the liquid and solid states, so ΔH is unchanged. Salt also does not get incorporated into the ice, so the entropy of the ice is not changed. However, salt does make the liquid phase more disordered, and raises the entropy of the liquid phase. Since salt water has an even higher entropy than pure water, there is an even greater loss in entropy associated with freezing salt water than pure water.

So, if the magnitude of ΔH is unchanged and the magnitude of ΔS is larger in the presence of salt, the equation T = ΔH/ΔS tells us that ice should melt at a lower temperature in the presence of salt (which is what actually happens).

 

[hariharan venkatasu]

Thank you very much for the excellent long answer It clears my doubts very well.

 

[Wondermine]

Summary:: why salt lowers melting point of ice?

There was a question on "Why salt lower the freezing point of water?"I found the following answer."Thermodynamics teaches that a loss of entropy can be overcome by a gain in so called enthalpy". The loss of entropy by freezing the solution canbe over come at temperature much below 0 degree C because the gain in enthalpy by freezing water rises when temperature goes down."How can there be gain in enthalpy by freezing water instead of loss? I presume that the gain in enthalpy refers to gain by environment.Please confirm.

I would say that the answer is in the fact that salt disassociates into ions in the water. There are hydrogen bonds formed with Na ion. Water atoms without the salt form a lattice which is ice at 0C. The weak attraction of the ions disrupt that water lattice potential structure and requires a lower temperature to form crystal lattices .
The why would be because salt dissociates into ions in the solvent which form weak hydrogen bonds thereby interfering with the nature of pure H2O.

 

[snorkack]

I would say that the answer is in the fact that salt disassociates into ions in the water. There are hydrogen bonds formed with Na ion. Water atoms without the salt form a lattice which is ice at 0C. The weak attraction of the ions disrupt that water lattice potential structure and requires a lower temperature to form crystal lattices .
The why would be because salt dissociates into ions in the solvent which form weak hydrogen bonds thereby interfering with the nature of pure H2O.

I don´ t think that´ s the reason. The attraction between salt ions and water molecules can be and is strong.
And if you freeze salt solution, you actually do NOT get pure salt back!
The attraction between salt ions and water molecules is strong enough that what you get instead is a crystal hydrate NaCl⋅2H₂O.

The actual reason why salt lowers freezing point of water is that few solutes are isomorphic with and soluble in ice. The crystal hydrate NaCl⋅2H₂O is not isomorphic with fresh ice - what does not form is salt ions randomly dispersed in a perfect ice crystal. Freezing forces salt with some water to unmix from a solid with a different integer composition (in the case, fresh ice). And in order to force such unmixing in face of thermal movement tending to mix molecules, the temperature must be lowered towards the side of freezing.

The only solutes which I am aware of that are isomorphic with and soluble in ice are isotopically substituted waters - and they don´ t lower freezing point of water. Any others you know?

 

[Wondermine]

I don´ t think that´ s the reason. The attraction between salt ions and water molecules can be and is strong.
And if you freeze salt solution, you actually do NOT get pure salt back!
The attraction between salt ions and water molecules is strong enough that what you get instead is a crystal hydrate NaCl⋅2H₂O.

The actual reason why salt lowers freezing point of water is that few solutes are isomorphic with and soluble in ice. The crystal hydrate NaCl⋅2H₂O is not isomorphic with fresh ice - what does not form is salt ions randomly dispersed in a perfect ice crystal. Freezing forces salt with some water to unmix from a solid with a different integer composition (in the case, fresh ice). And in order to force such unmixing in face of thermal movement tending to mix molecules, the temperature must be lowered towards the side of freezing.

The only solutes which I am aware of that are isomorphic with and soluble in ice are isotopically substituted waters - and they don´ t lower freezing point of water. Any others you know?

So what you describe confirms what I suggested does it not? That the hydrogen bonding between salt(ions) and the water molecules lowers the freezing temperature. The final form of the solid is another topic is it not?

 

[snorkack]

So what you describe confirms what I suggested does it not? That the hydrogen bonding between salt(ions) and the water molecules lowers the freezing temperature. The final form of the solid is another topic is it not?

No. The final form of the solid is vital.
Namely whether there is one final form or several.

Light water and heavy water form hydrogen bonds between each other. Heavy water is so similar to light water that heavy water molecules can enter into a perfect ice crystal and replace light water molecules at random positions and in arbitrary amounts. Freezing does not force full unmixing of light and heavy water. (There is some unmixing because freezing is preferential between isotopes). Since light and heavy water are freely mixed in solid as well as liquid, heavy water does not lower freezing point of water. The freezing point is intermediate between that of light and heavy water (thus higher).

Salt and water form hydrogen bonds, but salt hydrate is not isomorphic with ice. Salt, unlike heavy water, cannot enter a perfect ice crystal in random positions and arbitrary amounts. Freezing forces full unmixing - fresh ice freezes till the brine saturates relative to salt hydrate. This kind of requirement to unmix is what lowers freezing point.