Enthalpy & Stability: Products and Reactants

[justadream]

Can someone please confirm whether my understanding of these concepts is correct?

So in a reaction:

Higher deltaH (less negative for exothermic reactions or more positive for endothermic) means that the reactant(s) is more stable (because reactants release less energy) = higher bond energy of reactants

So the products of these higher deltaH reactions are Less Stable because less energy was released (in exothermic) or more energy had to be put in (in endothermic) = lower bond energy of products

Conversely,

Lower deltaH (more negative for exo and closer to 0 for end) means that reactant(s) are less stable (because the reactants release more energy) = lower bond energy of reactants

The products of these lower deltaH reactions are More Stable because more energy was released (in exothermic) or less energy was put in (in endothermic) = higher bond energy of products


Am I understanding these concepts correctly?

Thanks!

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[BerkReviewTeach]

You're close, but seem to be mixing two different concepts together.

Endothermic: More heat energy must be added than what is released, so overall heat energy enters the system. The bonds broken in the reactants are stronger (more stable) than the bonds formed in the products. This means that the reaction is uphill in terms of heat energy.

Exothermic: More heat energy is released than what is added, so overall heat energy exits the system. The bonds broken in the reactants are weaker (less stable) than the bonds formed in the products. This means that the reaction is downhill in terms of heat energy.

I think the other concept you have incorporated in your post is comparing one reaction to another reaction. The following is a general guide, where I've taken the liberty of assuming that entropy is insignificant. Stable vs. unstable tells us about favorability and not heat of reaction, but if entropy is trivial, then the free energy change (deltaG) and enthalpy (deltaH) can be interchanged.

unstable reactants ---> highly stable products = very favorable and highly exothermic
semi-stable reactants ---> stable products = favorable and exothermic
unstable reactants ---> semi-stable products = favorable and exothermic

semi-stable reactants ---> unstable products = unfavorable and endothermic
stable reactants ---> semi-stable products = unfavorable and endothermic
highly stable reactants ---> unstable products = very unfavorable and highly endothermic

Hope this helps!