Feb 7, 2010
152
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Pre-Medical
Here's a question I got right on TBR, but I think my understanding is still a little shaky despite having read the answer explanation.

upload_2013-12-25_10-25-28.png


So this is a graph of endothermic enthalpy, where the dH of the products is greater than that of the reactants.
upload_2013-12-25_10-26-18.png

The answer explanation says that for an endothermic reaction the dG of the bonds broken are greater than then dG of the bonds formed. To break bonds you need to supply E in the form of H, hence why in this case the reaction is Endothermic because the products are at a higher E than the Reactants.

Lattice E is what holds the salts together and those are the bonds that are being broken,

My question where does the Solvation E come from? the H2O that's breaking the bonds?

And what bonds are being formed here if this salt is dissociating? Is it the individual ions (NH4+ and NO3-) being solvated by H2O mlcls?

In the future, how should I think about this problem? My logic was that if dG of the bonds being broken > dG of the bonds formed, then the products are going to have more dH due to the E released from the broken bonds? Am I thinking of this the right way?
 

jetsfan1234

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Jun 21, 2013
189
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Be careful not to confuse free energy (G) with enthalpy (H). The reaction is endothermic, which means the change in enthalpy (H) must be positive. However, the overall free energy change (Delta G) MUST be negative, otherwise the reaction would not be spontaneous.

Semantics are confusing in this problem. "lattice energy" and "solvation energy" are measures of enthalpy (H).
Lattice energy = energy required to break the ionic bonds of NH4NO3
Solvation energy = energy released when NH4NO3 is solvated by H20 to form a solution.

Basically, the change in H is positive, which means energy (heat) must be added in order to solvate the Nh4N03. The reaction occurs spontaneously because the positive change in entropy (delta S) offsets the positive change in enthalpy, leading to an overall negative DeltaG.

Delta G = Delta H - T(Delta S).