Adding salt to ice question

philrainey

When salt is added to a ice water solution it makes the ice melt at a colder temperture than zero degrees C. I'm told because it weakens the bonds that hold the ice together. Is the latent heat asorbed by the melting ice at the new lower temperture the same as if the salt was not added and melted at 0 degrees C? Is there the same amount of joules absorbed from the surroundings of the solution to melt the ice if the surroundings are above 0 degrees C?

Ygggdrasil

The main effect here is on the entropy of the liquid phase. Most phase transitions are due to what are known as order-disorder transitions. In such cases, there is a competition between the two phases. In the solid phase, the molecules are well ordered such that the interactions between molecules are optimized. This gives the system a low potential energy (enthalpy) at the cost of having low entropy (low disorder). In the liquid phase, the molecules have more disorder (higher entropy) at the cost of losing some of the optimal interactions (higher enthalpy).

Remember from thermodynamics that molecular systems would like to minimize their potential energy (like a ball wants to roll down a hill) and increase their entropy (like how it's easy for your room to go from an ordered state to a messy, disordered state but not the other way around). Thus, we have a problem with the solid-liquid transition. Going from solid to liquid raises the entropy (good) but also raises the enthalpy (bad). Conversely, going from liquid to solid lowers the enthalpy (good) at the cost of lowering entropy (bad). So, whether something is solid or liquid depends on whether lowering enthalpy or raising entropy is more important.

For reasons that are too complicated to explain succinctly now, temperature determines the relative importance of entropy and enthalpy. At high temperatures, systems would like to raise their entropy more than they would like to lower their enthalpy, and the opposite is true of lower temperatures.

So, how does salt play into this? Salt is largely excluded from the crystal lattice of ice so it does not significantly affect the enthalpy or entropy of the ice. However, salt dissolves in water, so it affects both the entropy and enthalpy of the liquid phase. The dominant contribution of the salt is to raise the entropy of the liquid phase.

Since the entropy of the liquid phase is larger in the presence of salt, water pays a larger entropic penalty when going from liquid to solid. Therefore at zero degrees Celsius, where the contributions from enthalpy and entropy were previously equal, the balance gets tipped toward the liquid form. This explains why salt can melt ice on roads. To regain the balance between the changes in entropy and enthalpy, one must go to lower temperatures since lower temperatures put less emphasis on the entropic penalty.

Count Iblis

See also here.

Mapes

I believe the amount of energy going into the melting ice from the surroundings would be slightly larger due to the addition of salt, as the dissolution of NaCl in water is slightly endothermic. However, this effect isn't connected to the freezing point depression, which occurs as Ygggdrasil described it. Salts that dissolve exothermally also lead to freezing point depression; it's a colligative effect.

philrainey

why I asked this is because I was thinking if one can get the cooling at a lower temperture(-21) from ice frozen at 0 and put in a salt surray, one perhaps could get colder cooling for less energy (refrigeration energy)input as it takes less energy to cool from zero than minus 21. Perhaps adding a contaminate to another fluild (say CO2 ice in a liquid CO2 surray with contaminate).

philrainey

if one trys to separate the water from salt solution by sucking (evapourating off) the water vapour with a compressor does it take more energy to do so than just evapourating pure water? the entropy of the solution is higher than if it were pure water so would there be a lower entropy difference between the salt solution to water vapour than to pure water to water vapour?( I'm thinking the vapour has a higher entropy than a liquid) Would this mean a higher boiling temperture to offset the smaller entropy gain?Or less heat asorbed in the latent heat of the water turning to vapour?

Mapes

Less, it would seem; if the dissolution of NaCl in water is slightly endothermic, then the reverse reaction would be slightly exothermic.

philrainey

there is two different energies here that we are talking about I think you are referring to the energy asorbed by the water as it boils (asorb less as there is energy released by the exothermic effect heats it a bit) but I was thinking more about the energy to drive the compressor to remove the water vapour and compress it to some fixed condensing temperture.

philrainey

that is I was considering what happens if one has pure ice been dropped into a salt solution and a compressor removing the vapour to keep the solutions concentration the same, and that vapour been condensed and frozen at 0 degrees C before been returned to the solution.