heat of formation vs. bond dissociation

[Unlearner]

sorry if this is a stupid question, but i'm really confused on the difference between heat of formation equation and heat of reaction bond dissociation equation. how do you know when to use these equations? i think i have been using heat of formation equation (delta H = delta H prod. - delta H react.) for all bond dissociation, heat of reaction and heat of formation questions.

any clarification will be much appreciated!

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[kobe200LATE]

Both equations give you the same result: the total change in enthalpy of the reaction. Just use the equation that they give you the information for.

 

[Unlearner]

to be more specific, how can they be the same equation if the bond dissociation equation: delta H = bonds broken - bonds formed

I thought bonds broken refers to the reactant of the reaction since the bonds need to be broken first in order for them to re-form in the product. so if this is the case, then the equation would be delta H = H reactant - H product. but the heat of formation equation: delta H = H prod - H react.

am i missing an important piece?

 

[kobe200LATE]

Each equation uses different Hreactant and Hproduct values

For the bond dissociation equation:
Delta H = BDE of reactants - BDE of products
The BDE of reactants is positive because in a reaction, you are breaking the bonds of the reactants. The BDE of products is negative because you are forming the bonds of the products, not breaking them (hence the negative).

For the heat of formation equation:
Delta H = Heat of formation of products - Heat of formation of reactants
The heat of formation of products is positive because you are forming the products. The heat of formation of the reactants is negative because you are breaking the bonds of the reactants, not forming them.

Both equations give you the same delta H value for the reaction, but they use different values. One uses the bond dissociation energies and the other uses heat of formation. The bond dissociation energy and the heats of formation are opposites of one another.