CBT3 #32 Solvation and Lattice Energy

[kevin2400]

Hey all, this seems like a simple question, but I don't quite get the reasoning behind the correct answer--care to elaborate? Is there a strict definition of lattice and solvation energy?

Adding NH4NO3(s) to water lowers its temperature. How can this be explained?
A. The solvation energy is greater than the lattice energy.
B. The solvation energy is less than the lattice energy.
C. NH4NO3(s) dissolves but does not dissociate into water.
D. NH4NO3(s) dissociates but does not dissolve into water.

The answer is: B

The answer explanation: B is the best answer. If the water became cooler after the ammonium nitrate was added, the reaction must have been endothermic and was driven by the increase in entropy associated with dissolving salt into water. 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). The best answer is B.

I would think the answer is A, because if the reaction is endothermic, wouldn't the NH4NO3 solid being dissolved into the solution as endothermic?

Thanks!

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[LoLCareerGoals]

B.

deltaH = Elattice - Esoluvation >0 (since reaction is endo), so then Elattice > Esoluvation.

When NH4NO3 dissolves, being an ionic compound it needs to first overcome the attractive forces between the particles of the lattice. Net energy to do so is Elattice.
After dissociation, solvent particles soluvate the formed ions. That is surround them and stabilize them by keeping them apart from ions that they used to be in a lattice with. The energy associated with overcoming stabilizing forces of the solvent particles is the soluvation energy. Here this energy is "released" since soluvation happens (as opposed being overcome, like when nucleophile is soluvated by a solvent and is prevented from reacting as well because solvent stabilized it).

 

[kevin2400]

"deltaH = Elattice - Esoluvation"

Is this ALWAYS true?

Is that the same as
deltaH = Energy (bonds broken) - Energy (bonds formed)?

I know there ia difference between a "high-energy bond" (e.g. in biochem) vs here (gchem) where the energy corresponds to the bonds broken. Is this more of the latter second case, similar to bond disassociation energy?

 

[dontbeanegaton]

"deltaH = Elattice - Esoluvation"

Is this ALWAYS true?

Is that the same as
deltaH = Energy (bonds broken) - Energy (bonds formed)?

I know there ia difference between a "high-energy bond" (e.g. in biochem) vs here (gchem) where the energy corresponds to the bonds broken. Is this more of the latter second case, similar to bond disassociation energy?

What's the difference? In both cases the higher energy bond is tougher to break and releases more energy when formed. The fact that this is endothermic means that energy put in (i.e. losing lattice) is greater than energy released (i.e. solvation). Thus lattice "bonds" (which are dipole-dipole) are higher energy than solvation (also dipole-dipole).