Enthalpy, heat of reaction, and stability

[ThujaOccidentalis]

I am confused because my brain sees some contradictions (or misconceptions).

The more stable a bond is, the less energy it releases when broken, thus the deltaH (enthalpy of reaction) is less negative. However I'm finding some sources that say more stable molecules have more negative enthalpies because it requires more energy to break the bond. So what's the deal here?

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[wizzed101]

some sauces?

I can assure you that activation energy and the overall free energy/enthalpy of a reaction have nothing to do with each other?

 

[BerkReviewTeach]

I am confused because my brain sees some contradictions (or misconceptions).

The more stable a bond is, the less energy it releases when broken, thus the deltaH (enthalpy of reaction) is less negative. However I'm finding some sources that say more stable molecules have more negative enthalpies because it requires more energy to break the bond. So what's the deal here?

First and foremost, you need to put energy into the system to break a bond. But that that is only half of what you need to consider for ∆H. ∆H is equal to energy of bonds broken - energy of bonds formed. So if you break a stable bond to form bonds that are even more stable, you will get a negative ∆H. If you break a stable bond to form less stable bonds, then you get a positive ∆H. You can't just consider the bonds broken when estimating ∆H.

 

[ThujaOccidentalis]

So what I'm getting is a negative deltaH would mean a more stable product is formed, and a positive deltaH means less stable. Is that correct?

If so, I'm still conflicted. There is a question I'm working on that asks to select the molecule that would release the greatest amount of energy if the double bonds were to be broken. The answer is the unsubstituted alkene (the rest are subbed), which has the most negative deltaH given in the passage. And the answer key said it was the most unstable.

 

[aldol16]

So what I'm getting is a negative deltaH would mean a more stable product is formed, and a positive deltaH means less stable. Is that correct?

Yes. If, say, you break a C-Cl bond which has a BDE of ~80 kcal and form a C-H bond in its place, which has a characteristic ~100 kcal, the reaction must have a negative delta H because (as per Hess's law), 80 - 100 = -20 kcal.

If so, I'm still conflicted. There is a question I'm working on that asks to select the molecule that would release the greatest amount of energy if the double bonds were to be broken. The answer is the unsubstituted alkene (the rest of subbed), which has the most negative deltaH given in the passage. And the answer key said it was the most unstable.

The passage answer was incorrect them. Whether something is stable in the most practical use of the term refers to a combination of its kinetic and thermodynamic stability. If there is an extremely negative delta H, it could be most unstable but that would depend on another factor, namely the height of its kinetic barrier. Even if you have an extremely negative delta H, the substrates could be stable if the activation barrier is so high that it's not going to overcome that barrier under ambient conditions. An example is mixing H2 and O2 gas to form water. That delta H is -240 kJ/mol or something like that - extremely exothermic. However, if you ever try mixing those two gases in a balloon, you don't spontaneously get water vapor. You have to provide heat in order for that reaction to go, because H2 and O2 are stable under ambient conditions.

 

[wizzed101]

So what I'm getting is a negative deltaH would mean a more stable product is formed, and a positive deltaH means less stable. Is that correct?

If so, I'm still conflicted. There is a question I'm working on that asks to select the molecule that would release the greatest amount of energy if the double bonds were to be broken. The answer is the unsubstituted alkene (the rest of subbed), which has the most negative deltaH given in the passage. And the answer key said it was the most unstable.

.........
So basically, the question gave you a list of compounds and asked which one will release most heat if you agitate it.

Of course, the most unstable one will. The reaction energy diagram doesn't magically become different if you use the bond broken bond formed thingy to describe the reaction, it still looks the same. You have some reactants with a certain level of energy (stability) and products at another level, the net energy released/absorbed is always the differences between two states regardless of how you get there.

If you have say, a reaction in which the reactants are at -240 kj/mole and the products are at -100kj/mole, 140 kj/mole will be released. It doesn't matter if you have to form 2342123 stable bonds or break 96867 unstable bonds, or have to overcome 999999999999999 kj/mole activation energy. When the reaction is completed, it will release 100 kj/mole.

 

[aldol16]

Of course, the most unstable one will. The reaction energy diagram doesn't magically become different if you use the bond broken bond formed thingy to describe the reaction, it still looks the same. You have some reactants with a certain level of energy (stability) and products at another level, the net energy released/absorbed is always the differences between two states regardless of how you get there.

If you have say, a reaction in which the reactants are at -240 kj/mole and the products are at -100kj/mole, 140 kj/mole will be released. It doesn't matter if you have to form 2342123 stable bonds or break 96867 unstable bonds, or have to overcome 999999999999999 kj/mole activation energy. When the reaction is completed, it will release 100 kj/mole

The problem is that's not what stability is. Would you consider H2 and O2 to be stable? Stability is a combination of kinetic and thermodynamic factors and the book erroneously used "stable" and "unstable" to mean only "thermodynamically stable/unstable." It should always specific. Kinetic stability is not the same as thermodynamic stability and to use "stability" as an all-encompassing term is erroneous.

 

[wizzed101]

I don't have the question so I don't know how the question was framed :/ "Some sources" is pretty unspecific. But from what he provided, I think they mainly concerned with products and reactants energy levels. They asked what would release the most heat IF the reaction came to completion (it might not happen)

 

[ThujaOccidentalis]

Thanks for all the replies. In case you are wondering, the passage is from TBR 2016 ochem.

The GREATEST amount of energy is released by the oxidative cleavage of an alkene that is: A.) unsubstituted, B.) monosubstituted, C.) disubstituted, D.) tri substituted.

TABLE: Enthalpies of hydrogenation
unsubbed, -32.6 kcal/mol
mono, -30.2
cis-di, -28.5
Z-di, -28.3
trans-di, -27.4
tri, -26.7
tetra, -26.4

ANSWER: A, unsubstituted. The hydrogenation of an unsubstituted alkene has the greatest heat of hydrogenation. In general, a less stable reactant yields a greater amount of heat upon reaction.