NaCl Dissolving in H2O... Entropy at Work? o.O

[DoctorLindenbarg]

Hi guys,

I'm new to this site, so I don't know if people can see this or reply to it, but I've had some questions regarding the dissolution of table salt in water that I'd like some input on:

 

In my high school chem class, we were taught that the reason the ionic compound NaCl dissolves in H2O is because the dipoles on the water molecule pull apart the NaCl. However when I was having a chat with my friend (chem major) and this came up, he said that what we were taught "made no sense" because the dipole-dipole intermolecular force in water is much weaker than the ion-ion intermolecular force in table salt. He then went on to explain that the Na+ and Cl- ions were only able to be pulled apart because of the entropy in the system, which was favorable over the organized structure of the table salt. Could I get a quick explanation on entropy (what it is, how it's measured, how it affects matter)? Does each system have its own entropical (?) value? Is entropy an energy? If not, how can it overcome the intermolecular forces in this example?

 

Thanks a bunch

[Elite Engineer]

To say the salt dissolves via ion interaction is just an abbreviated explanation of thermodynamics. Saying it dissolves via increased entropy is "technically" the correct way, but warrants a lengthy explanation..here goes:

 

The structure of a salt crystal is highly organized, and stable. In contrast, the 2nd law of thermodynamics states that, in a transfer of energy, the entropy of the system will increase, or..."there is a natural tendency of any isolated system to degenerate into a more disordered state". In laymen's terms, entropy is the measure of disorder of a system. This increase in disorder is favored in part because it facilitates energy transfer, and therefore entropy is always increasing in the universe. Think about why gasoline ignites under heat. What's more disordered..a liquid or a gas? A gas is more disordered, and therefore why gasoline (and other volatile substances) ignite when heated, because the process of entropy is favored.

 

Now onto the salt dissolving in water. The crystal is highly organized and stable..which is in direct contrast to the 2nd law. When submerged in water, the crystal structure breaks apart, changing from an organized state, to a more disorganized state..i.e. increase in entropy. Yes entropy can be quantified..but that's a whole other conversation. If your interested, check out "Gibbs Free Energy".

 

~EE

[studiot]

 

Elite Engineer

To say the salt dissolves via ion interaction is just an abbreviated explanation of thermodynamics. Saying it dissolves via increased entropy is "technically" the correct way, but warrants a lengthy explanation..here goes:

 

You are right to say there is more than one way to skin a cat, or more than one way to discuss this.

 

However please be careful not to mix up ideas from different ways.

 

 

The classic second law of Kelvin and Clausius refers to cyclic processes.

This is not a cyclic process.

 

You rightly mention that Chemists introduced the Gibbs Free Energy and chemistry/chemical engineering topics are normally discussed from this viewpoint.

The free energy approach incorporates all forms of energy, heat, work, the entropy-temperature integral etc.

Order is not expressly distinguished, it is included in the calculations.

 

 

If you wish to discuss in statistcal mechanics terms (order and disorder) Caratheodory's formulation is better.

 

https://en.wikipedia.org/wiki/Second_law_of_thermodynamics

 

It is easy to state (it is obvious) that a crystal lattice is more 'ordered' than the solute phase.

Unfortunately most people misunderstand this, so perhaps you would expand on this ??

Edited March 5, 2016 by studiot

[BabcockHall]

Each salt must be decided on a case-by-case basis, and the change in entropy of the solvent cannot be ignored. The entropy change for the dissolution of KCl is a positive number. The entropy change for the dissolution of calcium sulfate is a negative number.

[Elite Engineer]

Each salt must be decided on a case-by-case basis, and the change in entropy of the solvent cannot be ignored. The entropy change for the dissolution of KCl is a positive number. The entropy change for the dissolution of calcium sulfate is a negative number.

But that's when you quantify entropy. In theory, isn't the dissolution of KCl and CaSO4 still an increase in entropy. The difference in the quantified entropy just may be due to a slight difference in the amount of dissolved ions in solution.

[BabcockHall]

For the dissolution of KCl, deltaS = +314. For the dissolution of CaSO₄, deltaS = -569. I don't seem to have a reference handy. But the difference is likely related to the greater charges needing more waters of salvation.