Hi, My mind has been blown up recently when I came across this thread: https://www.physicsforums.com/showthread.php?t=372533. I'd like some explanations about something. Imagine an insulated tank filled with hydrogen and oxygen at 100°C (or greater if possible) and 1 atm. Now a spark starts on. Imagine that all the gas transformed into [tex]H_20[/tex]. The temperature must increase as I've been told, so that the water is under the gas form (despite the fact that at higher pressure water needs to be put at more than 100°C). From the first law of thermodynamics, I know that [tex]\Delta U = Q-W[/tex]. In this case, Q and W are worth 0 J because no heat is exchanged within the surroundings of the system and the volume remains constant. So still according to the first law, [tex]\Delta U=0 J[/tex]. However, in Resnick-Halliday, it is clearly stated that the internal energy of a gas is directly proportional to its temperature and is only temperature dependent. Hence an increase of temperature means an increase of internal energy. Hence according to this book, I'm tempted to say that the system has gained energy. It is in contradiction with the first law. What's going on? Thanks a lot in advance, I really need to know!