Enthalpy of Formation Sign

[Allie Wilcox]

Could someone please explain (with examples) the conditions of enthalpy of formation that is negative and positive? When I look at a reaction equation (of gasses), can I [censored] the sign of the enthalpy of formation only on the difference in the number of moles of the gas or do I need to consider the bond enthalpies as well? In other words, is it always true that if the reaction results in a reduced number of moles that it will be an exothermic reaction? Thanks!

[hiranya sundar 2D]

I think enthalpies are always negative when the system is releasing heat?

[Tyler Ho 1L]

Enthalpy of formation deals with the change in energy that results from the formation of a compound from its constituent elements. It is denoted delta H and is based primarily on the stability of the reactants vs the products. Firstly, the number of moles on the left and right sides do not determine whether or not delta H is negative or positive. You can have a a lot of reactants and very little products or vice versa, but one will typically always be more stable than the other.

Bond enthalpies are used to calculate the enthalpy of formation.
When delta H is positive, the reaction is endothermic and energy is taken in to form the products. Because the potential energy of the bonds in the products is greater, it is less stable.
When delta H is negative, the reaction is exothermic and energy is released when the products are formed. Because the potential energy of the bonds in the products is less, it is more stable and energy is released.

It was never said that if a reaction results in a reduced number of moles then it will be an exothermic reaction. I do not know where this generalization came about, but it is not about the number of moles. That may shift equilibrium for gaseous molecules, but it definitely does not change the enthalpy of formation. Enthalpy of formation is a fixed value for different compounds in different conditions.

Exothermic reactions are when the products are more stable and the formation of bonds releases more energy than was required to break the bonds of the reactants.
Endothermic reactions are when the products are less stable and the formation of bonds releases less energy than was required to break the bonds of the reactants.

[Nhi Pham 3D]

Even though the formation of bonds is typically associated with a release of energy ("free to form"), the enthalpy of a synthesis reaction isn't always going to be a negative value (meaning the reaction is exothermic and energy is released). Tyler's explanation about how the sign of the enthalpy is dependent on the stability of reactants vs products (and not moles of gas) does a good job at addressing why this is.

To add on to that, an example of a formation/synthesis reaction that has a positive enthalpy is ATP synthesis. Even though ATP is being formed, the reaction is endothermic because the product is less stable than the reactants.

Also, I think you meant to ask about enthalpy in general, and not the enthalpy of formation - which I think is a little different.