[jplavcan]

Hi everyone, this may be a simple question but I can not seem to find the answer anywhere and don't have access to my professor until next week. My question is, can the heat of formation of a dihydrate be calculated by adding the heat of formation of each of the compounds? I'm specifically asking about CuCl₂·2H₂O. I cannot find the heat of formation of this compound anywhere in any database, so is it as simple as adding the heat of formation of CuCl₂ and the heat of formation of 2H₂O? If this is not how to accomplish what I am looking for, does anyone know how? Thanks!

[sjb]

Hi everyone, this may be a simple question but I can not seem to find the answer anywhere and don't have access to my professor until next week. My question is, can the heat of formation of a dihydrate be calculated by adding the heat of formation of each of the compounds? I'm specifically asking about CuCl₂·2H₂O. I cannot find the heat of formation of this compound anywhere in any database, so is it as simple as adding the heat of formation of CuCl₂ and the heat of formation of 2H₂O? If this is not how to accomplish what I am looking for, does anyone know how? Thanks!

Probably not. Can you find other enthalpies and use Hess' law?

[jplavcan]

I did consider that, but I'm having a severe brain cramp trying to figure out exactly what formation enthalpies to use. Any suggestions?

[AWK]

http://chemistry-reference.com/standard%20thermodynamic%20values.pdf

[jplavcan]

I'm amazed that you found that so quickly, I have looked in every chemical database available to me through school and online and could not find it anywhere. Thank you so so much! 

[jplavcan]

So, while I greatly appreciate the information that was given and it was exactly what I needed, and after thinking about this, I'm still trying to figure out how I would go about using known reactions and their heat of formations to determine the answer, using Hess's Law.

I'm currently learning Hess's Law, and I understand the theory of it and understand the problems given in my chemistry book. The problem is, in the book we are always given two or three known reactions to determine the heat of formation of a compound. It does not actually make me THINK about what reactions to use to determine it. How would I go about determining which known reactions to use to calculate the heat of formation of CuCl₂·2H₂O? I think that the fact that it is a dihydrate is really giving me headache here. Any help would be greatly appreciated! Thanks!

[sjb]

I seem to recall that we did an experiment for calculating the hydration enthalpy of sodium thiosulfate and the corresponding pentahydrate at school. Long time ago though, and I forget the exact details.

From what I recall we dissolved 0.1 mol of anhydrous thiosulfate in 100 ml of water and 0.1 mol of the pentahydrate in 91 ml (so that the total volume of water involved was also 100 ml). My books and notes on this are about 150 miles away, sorry.


If you were only looking at numbers in a table this may not be completely in tune with what you have to do.

[Borek]

Hydration of a cation (which is most common in the hydrated salts) is just another reaction.

[Enthalpy]

[...]is it as simple as adding the heat of formation of CuCl₂ and the heat of formation of 2H₂O?

No, definitely. For instance, when you mix cement and water they get warm.
Nor are you likely to deduce it from other data. That would need some reaction involving the hydrated salt, where all other reactants or products have a known heat of formation, and the heat of reaction is known too. It usually goes the other way: heats of reactions are not tabulated, but heats of formation instead, because from fewer compounds you can deduce more reactions.

More fundamentally, a hydration is a chemical reaction (the dot in the formula shouldn't fool you), and as such, it has a heat of reaction.

[jplavcan]

Hydration of a cation (which is most common in the hydrated salts) is just another reaction.

[...More fundamentally, a hydration is a chemical reaction (the dot in the formula shouldn't fool you), and as such, it has a heat of reaction.

Ok, I do believe that the "dot" is whats fooling me. So, all heat of formation and heat of reactions aside, what exactly is the reaction that takes place when a salt is hydrated? Am I understanding it correctly by saying that a hydrated salt is one with a certain amount of water molecules (In this case 2) surrounded in the structure of the salt itself? What type of bonding is going on here?

And also why is the "dot" used for a hydrated compound and not just written as one compound? I'm assuming that it has to do with the way that the water molecules and the other compound are attracted to each other?

Sorry for the questions about the little nuances here, this hydrated salt has me perplexed, and it is not something that we are even delving into in class, just asking for my own knowledge here in the future. Thanks for all the replies, they have all made me understand this a little more. Only two more days until I can actually ask my chem prof about this. 

[AWK]

Structure of copper chloride dihydrate molecule in crystal. Bond lengths in Angstroms = x100 in picometers), hydrogen atoms omited. Drawn in Mercury from CIF.