bond strength,stability and Delta H

[Trisphorin]

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Can someone clarify the relationship between those 3? i am just really confused.

Here is my thought, please correct me if i am wrong. Bond strength is proportional to Delta H since C=C has a higher delta H than C-C bond. and stronger bond are also more stable since it takes higher energy to break them. so from my understanding, all 3 are directly proportional with one another.

but how come in berkeley review, it clearly says in an exothermic reaction, there is a net energy release thus bonds that are formed are stronger than the bonds that are broken. doesn't this indicates that stronger bond correlates with lower delta H since product's delta H is lower in an exothermic reaction? 🙁

sorry if i am being confusing but I would really appreciate it if someone can help me out.

Thanks in advance,

 

[SeminoleVesicle]

Well, energy is required to break a bond, and energy is released when a new bond is formed. If the energy required to break a bond is less than the amount of energy released in forming the new bond, then you have an exothermic rxn.

 

[MT Headed]

While the magnitude of bond strength is proportional to the magnitude of deltaH, the actual relationship is deltaH=-strength.

You probably have a table that says the H-H bond has an enthalpy of 436kJ/mol. If you had two moles of H atoms, spread sufficiently far apart that their electric potential energy was zero, and let them naturally slide together, they would now have a potential energy of -436kJ. Where did that energy go? It was released as 436kJ of heat in an exothermic reaction. The deltaH of this reaction is -436kJ/mol. Likewise, when you said C=C has a higher deltaH than C-C, that is not really correct. C=C has a lower (i.e. more negative) deltaH than C-C.

Humans have difficulties dealing intuitively with negative numbers, so bond enthalpy tables are published as positive numbers, when in reality they ought to be all negative numbers. This annoying bond enthalpy sign convention also explains why

deltaH=(enthalpy of initial bonds)-(enthalpy of final bonds)

when everything else in the physical sciences defines a delta as (final)-(initial).