lattice energy vs energy of solvation

[Dharma]

Can someone help explain this to me (from explanation in TBR cbt3):

"An endothermic reaction means that the bonds broken during the reaction are of higher energy than the bonds formed, which in turn means that the lattice energy (from the bonds broken) is higher than the solvation energy (from the bonds formed). "

I understand that an endothermic reaction means the bonds broken are of a higher energy (hence + deltaH) however I do not understand how lattice energy is associated with broken bonds. I thought that lattice energy is considered to be exothermic.

I know I'm missing something here.

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[Dharma]

I found an explanation by Pi Bond. (I think its for the same exact question).

"the question is testing whether you know which bonds are stronger, the ones broken or formed based on the fact that the reaction is endothermic.

since the water got cooler, the reaction is endothermic. Given that delta H is positive and that we know that our delta H = energy of bonds broken (from lattice) - energy of bonds formed (solvation shell), we can see that the the solvation shell bonds formed must be of lower energy than our lattice energy in order for delta H to be positive."

I'm still still a bit clueless about lattice energy though.

 

[PeterPesto]

Lattice energy is the heat evolved when gaseous ions come together to form an ionic lattice / crystal. It is a negative value, however the negative of lattice energy is a positive value. It represents the strength of the bonds between the ions in the crystal lattice.

Water can help to dissolve ionic compounds due to favorable attractions between water molecules and the charged ions.

In this case, the enthalpy of solution is a positive value, therefore the reaction is endothermic.

That is: -(lattice energy) + solvation energy > 0. It is endothermic because more energy is required to break up the lattice than the energy evolved due to solvation. That doesn't mean it won't dissolve, however as entropy favors dissolution.

 

[Rabolisk]

To add to what PeterPesto said, there is some ambiguity in the definition of terms like lattice energy, solvation energy, and even ionization energy and electron affinity. All of these terms refer to a process, and the sign depends on which direction you consider. If you define lattice energy as the enthalpy of formation of an ionic compound from its constituent elements in their gaseous states, the usual definition, then lattice energy is negative and exothermic. What's important to understand, however, is that the magnitude or the absolute value of the lattice energy represent the strength of the ionic bonds. Also understand that bond formation in all cases releases heat and breaking bonds require heat.

Finally, enthalpy can be considered a form of energy, but is different from free energy, which drives spontaneity. Exothermic/endothermic is different from exergonic/endergonic (although this is just a pet peeve of mine).

 

[Dharma]

Thanks guys, you filled the gaps.

"That is: -(lattice energy) + solvation energy > 0. It is endothermic because more energy is required to break up the lattice than the energy evolved due to solvation. That doesn't mean it won't dissolve, however as entropy favors dissolution."

"What's important to understand, however, is that the magnitude or the absolute value of the lattice energy represent the strength of the ionic bonds. Also understand that bond formation in all cases releases heat and breaking bonds require heat."




I can "see" it better after reading these explanations. Many thank-you's to you's twos!