At the molecular level enthalpy is released/absorbed during a reaction because the energy from the original bonds is released as the bonds are broken and then the new bonds absorb some energy. Depending on whether the reactants or products are more stable determines whether the net result is a positive or negative "release" of energy.
The heat of formation is the enthalpy difference between a molecule and its component elements in their natural state at 298K and 1 atm.
The formation of our molecules of interest simply becomes one of the steps in our fictitious paths now.
Calculate enthalpy changes associated with sensible heat, latent heat, and chemical reaction
Calculate the adiabatic flame temperature of a Bunsen burner. Assume that $CH_4$ starts at 25C, is burned with a stoichiometric amount of oxygen, and is burned completely (that is, it produces $CO_2$ rather than $CO$).