These guys combine to form water and there is a decrease in enthalpy. And that means that the internal energy of the system decreases. There are less molecules within the system and so there is less Enthropy within the system. Now, different types of reactions exist. Some reactions are in a gas phase when other reactions are in a liquid and solid phases. Now, when you're dealing with reactions in a gas phase, you have to realize that gaseous systems are compressible.