[dmcshane8]

Hi i have to write a chem paper for 25% of my overall mark in my class, i was wondering if there is anyone who could tell me what about a salt changes the heat of solution. I'm thinking the more valence electrons a cation has the stronger the bonds will be making them harder to be broken in water,  and when they break they would give off heat making it exothermic

[Corribus]

Generally: charge, size, and solvent characteristics.

[dmcshane8]

charge and size of the cations? or the overall salt, and what are solvent characteristics?

[Corribus]

Charge and size of the dissolved ions.  Solvent characteristics means: is the solvent polar, nonpolar or etc.  Needless to say, heat of solution of calcium ion is going to be different in methanol than it is in water.

[dmcshane8]

anymore suggestions how to predict if they are exo or endothermic.

[Arkcon]

This is a tough one, but Corribus: has given you some good help.  Its hard for me to understand -- the endothermic heat of solution salts must somehow be more ordered (less enthalpy) when dissolved in water.   Which is absolutely backwards from the definition of crystalline solids and solids in solution.  Try to correlate known endo- and exo- thermic salts in solution by the parameters he suggested.  See if you can find a pattern I've missed.  Its your grade, after all, I'm just curious.

[dmcshane8]

I have found a correlation in the the size of the ionic compounds. The bigger they are the more endothermic the solvation seems to become. The only exception is that if you have more than 1 cation because the compound is separated more easily it seems to be exothermic. Another thing i have found is that the more electronegative the compound if the easier it will dissolve making it more exothermic.

I think these are right... I barely have a clue what I am talking about haha

[Corribus]

Another thing i have found is that the more electronegative the compound if the easier it will dissolve making it more exothermic.

You need to be very careful.  Exothermicity is strictly related to enthalpy and is a thermodynamic parameter.  "Easier to dissolve" can mean a lot of things.  It can mean rate, which is related to kinetics.  It can also mean spontaneous (exergonicity - that is, Gibbs Energy change), which like enthalpy is a thermodynamic property, but which is also quite different than enthalpy alone.  The enthalpy change will impact both kinetics and exergonicity, but it is not the only consideration. 

In solvation, particularly aqueous solvation, entropy is very imporant.  Sometimes it is more important than the enthalpic term.  In addition, do not neglect that exothermicity (entropy change) is related to both the formation of intermolecular forces with solvent molecules, but also the breaking of them in the solid lattice. 

It is very complicated and often well-defined trends are hard to find.

[dmcshane8]

Why would having a larger ion make it more difficult to dissolve in water?? ( making the solution colder)

[Corribus]

Take a look here.  This will explain some of the factors involve, and also does a good job of explaining why simple trends like "bigger ion equal more exothermic" don't often work.

http://www.chemguide.co.uk/inorganic/group2/problems.html

Also, a minor error in the second paragraph of my last post accidentally defined the exothermicity as "entropy change".  This should be "enthalpy change".

[dmcshane8]

Thank you this site is exactly what i need !