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In my chemistry course, we were discussing types of enthalpy and the enthalpy of hydration came up. My instructor defined this as

heat associated with adding water to an anhydrous salt.

However, I don't intuitively understand this. My instructor just used the example of copper sulfate pentahydrate — you put in heat to remove the water, thus when you put in water to the anhydrous form, you get heat. I do not find this satisfactory.

I was thinking that it could be possible adding water to the anhydrous salt causes bonds to dissociate within the salt, and similar to calcium chloride this generates heat, but this is just speculation.

By what mechanism is heat generated and how should I intuitively understand the enthalpy of hydration?

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    $\begingroup$ Are you familiar with the concept of bond enthalpy? Generally, there is always a change of enthalpy when a bond is created or broken, and this holds for all types of bonds (ionic, covalent, hydrogen, intermolecular ...). Enthalpy of hydration is therefore the change of enthalpy when particles (in your case ions) form bonds with the solvent (water). Usually, it is more instructive to consider the whole process of dissolving, constisting of ... $\endgroup$
    – Domen
    Oct 3, 2022 at 15:00
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    $\begingroup$ ... first breaking the (ionic) bonds (lattice enthalpy), and then forming bonds between ions and water (enthalpy of hydration). Schematically depicted here. $\endgroup$
    – Domen
    Oct 3, 2022 at 15:02

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The distinction between what's considered a chemical bond, an intermolecular attraction, and a coordination/dative bond is largely a manmade distinction. Nature doesn't care one way or another. All are mediated by photons and the physics of the fundamental electromagnetic force.

So, just as it takes energy to break chemical bonds and energy is released when chemical bonds form, it also takes energy to separate particles held together by intermolecular forces or coordination/dative bonds and energy is released when particles get together.

Hydration enthalpy, in the sense of the enthalpy of solvation when water is the solvent, is the enthalpy associated with adding or removing the ion-dipole intermolecular forces between the ions and water molecules. Hydration enthalpy, in the sense of the enthalpy of the creation of hydrates, is the enthalpy associated with creating or destroying the coordination/dative bonds in metal-aquo complexes.

Your instructor seems to be using the second sense of the word. Adding water to anhydrous copper sulfate allows coordination/dative bonds to form between the water molecules and the salt. Energy is released when new bonds form. Energy is required to break those bonds.

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Hydration can have two meanings (IUPAC gold book), a chemical reaction that adds water (usually "across" a double bond), or a process where a particle from a gas phase moves into aqueous phase. Dissolution refers to dissolving a substance (usually a solid) in a solvent.

The enthalpy of dissolution (or of solution) can be artificially broken into two parts. First, you take the solid and remove all intermolecular interactions, effectively turning it into a gas (or maybe plasma for ionic compounds). Second, you take the isolated particles and place them into a solvent.

The enthalpy associated with the first part is called lattice energy (it is endothermic). The enthalpy associated with the second part is called hydration enthalpy (if the solvent is water) or solvation enthalpy (for any solvent). When you add the two, you get the enthalpy of dissolution.

Here is a conceptual diagram from a Libretext book:

enter image description here

Source

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There is a difference between solvation and hydration. It looks to me that you are confusing both concepts. Your question maybe a duplicate of the following question. where a good explanation is provided, specifically for the Copper sulfate example, as it turns out.

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  • $\begingroup$ This is decidedly not the same question. I am asking for an intuitive explanation of hydration enthalpy, not solvation enthalpy. $\endgroup$
    – Max0815
    Oct 4, 2022 at 11:04
  • $\begingroup$ "I was thinking that it could be possible adding water to the anhydrous salt causes bonds to dissociate within the salt, and similar to calcium chloride this generates heat, but this is just speculation." It seems to me that you are referring to solvation here, not hydration. In any case the mentioned question's answer mostly refers to hydration. $\endgroup$
    – Mils
    Oct 4, 2022 at 11:10

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