Energy is carried by kinetic energy of molecules. Energy flow can be released in the reaction. How much is used and how much is released is interesting > flow. Must distinguish between the system and the surroundings for flow. Can"t take both together you find out that there"s no energy. System: part of the universe we are interested in. Open system: energy can transmitted between system and surrounds in like cooking food, need to add ingredients = Closed system: put lid on bowl, can still feel bowl is hot which means energy can go between system and surroundings but no matter is transmitted. Isolated system: in thermal bottle you dont feel it"s hot cause energy has been isolated. Gas can go to environment total mass will decrease after reaction, Matter goes from one state to the other, energy flow will happen from reactants to these numbers are well defined, together defines the state the matter is product, release energy.