It is therefore reasonable to take a quantitative look at the relationships between chemical compounds and heat in terms of the modern concept of enthalpy. Enthalpy changes, DH or dH, are quite measurable processes since changes in heat content appear as a sensible flow of heat from a system to its surroundings. Unfortunately, the enthalpy of a system cannot be measured although changes in it can. Some method of establishing the actual heat content or enthalpy H of a system in a particular state is needed. But, since this reaction can be carried out in isolation, the resulting system must be the same system as before but in a different state.
Different states mean different values of the state functions, such as enthalpy.
Now State 1 simply consists of one mole of methane and two moles of oxygen. This is not a great deal of help because we do not know the enthalpy of any compound, since enthalpy changes DH can be measured while enthalpy itself, H, cannot. We don't know the enthalpy of any element, either, for exactly the same reason. Remember that minus sign means that heat is given off in this exothermic reaction. We already knew this, of course, because we keep our homes warm all winter by burning natural gas, whose major component is methane.