We do this by defining the enthalpy of the Standard State of the Elements, which is only about 100 definitions and should cover all possible matter since all matter is made of elements. The symbol q p is the heat of the reaction carried out at constant pressure. As you recall, the enthalpy of a substance is a measure of how much internal energy it has, or its' ability to produce heat when reacted. Substances with a positive Heat of Formation are less stable energetically than the elements from which they are formed. When a reaction proceeds from products with large enthalpy to products of low enthalpy, the reaction releases Heat, so reactants with high Enthalpy are good fuels or explosives.
Hydrocarbon fuels burn in oxygen to produce carbon dioxide and water and release heat.
Remember, Enthalpy is an Extensive property, so you must know the quantity of a substance to determine its energy content. Remember Enthalpy is an extensive property, so the reaction stoichiometry is very important. Wouldn't it be nice if you could make a diamond from burning methane in oxygen to make carbon and water. We can use Thermochemistry to determine the molar heat of reaction. We can relate all the reactants an products back to the standard state of the elements from which they are formed.