Salt on Ice - how does it melt? 9

[BRT549]

I know about the depression of the melting point of a water/salt solution, but how does salt melt ice if they are both a solid. In other words, if I throw salt down on existing ice, there should be no reaction, eh?

 

[moltenmetal]

There's a chemical potential (i.e. a driving force) for salt to dissolve in water, since there is a higher concentration of sodium and chloride in the salt crystals than in the water. By the same process, salt attracts water to its surface (i.e. it deliquesces). These two processes get the dissolution started, resulting in a film of brine at the ice/salt interface. From there on in, it's a simultaneous heat and mass transfer process looking for an equilibrium.

As the salt dissolves, the resulting brine has a depressed freezing temperature relative to pure water. The "heat of solution" has to come from somewhere- in an insulated container, the resulting mixture temperature drops to compensate for the heat of solution, such that 33 parts of salt added to 100 parts of ice will produce a brine/slush mixture at ~ -21 C. (that's how my grandfather's generation made their ice cream, before mechanical refrigeration made it easier). In an open system, the heat of solution will be withdrawn from the surroundings and the equilibrium will be shifted away from salt/ice and toward brine until one or the other is gone.

Salt is useless below temperatures which will freeze the resulting brine mixtures. The colder you get, the more salt you need- and beyond ~ -20 C it becomes practically pointless to apply salt. At those temperatures, we generally switch to sand as a cheaper and lower environmental impact option to give more traction on ice.

On a busy road, you have hundreds of vehicle tires doing work on the snow, and vigorously admixing the snow/slush with any salt present. It's the heat generated by friction, plus the mobility of the brine/slush to flow and via spray, that accelerates removal of the snow from the road. Hence it gets all over every surface of your car, and also into streams and soils nearby....

 

[rmw]

I was taught that salt dissolving into water was exothermic. Is this right??

Moltenmetal, great post above. I will have to read it several more times to digest it, though.

 

[quark]

No. It was not exothermic. (some of the papers I saw on this topic say that the exothermic reaction is the first misconception people think of)

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[rmw]

Chalk up one more in the category of what the professors told me that I now know to be bogus.

The real hard part for me back then, working my way through college as a full time student (jr) engineer, was sitting in class and biting my tongue when they taught things that I knew to be incorrect because of my industrial experience, albeit limited at that time. At least I was sufficiently intellegent to recognize that I was a participant in an autocracy, even though I live in a democracy. Not in the classroom, however.

Having said that, I acknowlege that the education they gave me is what has brought me to where I am now, and I am grateful for that.

 

[25362]

To rmw, while I don't exactly know what they taught you about heats of solution of chlorides in water at 25[sup]o[/sup]C, let's clarify that of the common chlorides, sodium, potassium, ammonium and silver are endothermic (cool on solution), while lithium, magnesium, calcium and aluminium are exothermic. Talking of sodium compounds, most of them dissolve exothermically, for example: bromide, iodide, hydroxide, carbonate and sulfate; on the other hand, fluoride and nitrate are endothermic.

Salts' dissolution is considered a two-step process. At first, salts ionize through an endothermic process (aka lattice enthalpy), then the ions solvate (hydrate, in water) by an exothermic reaction. Thus the net enthalpy of solution of a salt is the sum of both enthalpies:

NaCl(s)=> Na[sup]+[/sup] + Cl[sup]-[/sup] +787 kJ/mol
Na[sup]+[/sup] + Cl[sup]-[/sup]+ water=> Na[sup]+[/sup](aq) + Cl[sup]-[/sup](aq) -784 kJ/mol
-----------------------------------------------------------------------
NaCl(s)=>Na[sup]+[/sup](aq) + Cl[sup]-[/sup](aq) +3 kJ/mol

Therein lies the explanation of why some salts warm on solution and others cool.

 

[SooCS]

How about when salt molecule touch ice, it depress the ice molecule causing it to give up heat as it lowers it temperature. The heat is then absorbed by the solid ice and caused it to melt. The melt water mixed with the salt solution and is then depress to causing it to give up more heat as it lower it temperature. The process then repeats.

 

[rmw]

Thanks, 25362.

I'm thinking that they did not teach us ME's much about heats of solution, and that it was probably a ME professor that made the statement, not a chemistry prof, and I don't remember taking any Chem E classes at all. Anyway, I have spent my entire professional life wondering if it was in fact, exothermic, so as to provide enough heat to "melt" road ice, then how did it make ice cream in the old freezer. Mystery solved. I can sleep at nights, now. I love this forum.

 

[49078]

I was just wondering this exact thing. I realize that salt added to water lowers the freezing point. I was under the impression that when salt mixed with water it was an exo reaction. I remember a high school chemistry experiment where one chemical caused exo and one endo. I'm wondering now if they are both salts?

I was under the impression as well that calcium chloride is the most common salt. This could be wrong though. I also read that when CaCl2 mixes with water it was exothermic.

Now, assuming that some salts are endo and some are exo which seems to agree with this forum, and possibly my chemistry lab from high school. Am i correct to say that an exo reaction melts the ice forming the salt solution. And that for an endo reaction the salt absorbs energy from the surroundings (the sun, etc) until it dissolves again forming the salt solution?

Both of these result in a salt solution where the freezing point is lowered?

 

[tickle]

49078 - Common (table) salt is sodium chloride. See 25362's post of 18 Feb.

 

[49078]

So road salt is nacl?

 

[BRT549]

Thanks for reviving an old forum topic. Got some great answers, and another good question or two.

Sometimes CaCl2 is used as road salt. I used to work at a magnesium facility that sent a waste stream of HCl to an adjoining plant that made calcium chloride. Seems to me they sold about 60% of it to the Utah highway dept. I think it has less of an environmental impact than sodium chloride, and might also rust metals slower. Likely the most common road salt in an area is based on availability and freight costs.

 

[moltenmetal]

Overwhelmingly the most popular road salt is NaCl, cheap because it's mined in enormous quantities and the run-of-the-mine material is good enough for this use. Other ice melting materials tend to have limited uses (for sidewalks etc.) due to cost.