Heat of dissolution and heat of neutralization

[Metabolics]

1) I was wondering how you can tell if the act of dissolving a substance is exothermic of endothermic? I understand that the energy going into forming the bonds of a solid is often exothermic, so the reverse process (dissolution) is often endothermic. Is there an exception to this, or should I assume that most often dissolution is exothermic?

2) Why is neutralization an exothermic process? Can you explain this to me in terms of stability of atoms, or maybe something besides "memorizing" that the products have less energy than the reactants?

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[MrNeuro]

2) h2O + heat -->H + OH thus the reverse of this which is neutralization is ectothermic this should also answer 1

Pretty sure this is right

 

[pfaction]

1) If you're going to take a solid and dissolve it, it's going to need heat for a phase change (think melting?) so it's going to be endothermic.

2) Exactly what Neuro said.

 

[Morsetlis]

Dissolving a bond takes energy.

Formation of a bond creates free energy.

If the formation of the bond between soluble solutes + solvent (usually water) creates more free energy than is needed to dissolve the bonds of the solid phase of the solutes, there is additional energy given off into the environment -> exothermy.

 

[deleted103644]

1) If you're going to take a solid and dissolve it, it's going to need heat for a phase change (think melting?) so it's going to be endothermic.

Wrong, this varies by solute.

Some heat up, some cool down.

You can't really generalize this stuff without getting into things well beyond the scope of the MCAT.

E.g. not all gases cool down when they expand, which is why a leaking hydrogen tank can be very dangerous.

This can get into some pretty advanced physical chemistry topics, definitely further in depth than I ever went, and I went further than you need for the MCAT.

Also, it's wrong to think of a solute as melting when it goes into solution.

 

[pfaction]

Oh. Er...but is this correct logic for the test?

 

[deleted103644]

Oh. Er...but is this correct logic for the test?

You need a constant for whatever compound is being dissolved. Could be positive or negative.

 

[pfaction]

Damn. Okay, thanks.

 

[Girly X]

I believe endothermic/exothermic will vary depending on the reaction. And even for a specific set of reactants and products it will vary depending on the conditions. So there is no "set" rule other than your net Delta G=Delta H-T Delta S for all reactions involved. Plug in values or think it through for a specified reaction. For the most part if your net Delta S is a positive enough (disorder is increasing) and net Delta H is negative enough, your overall Delta G will be negative (exothermic).