1. The problem statement, all variables and given/known data A stiff and isolated vessel contains oxygen and hydrogen. a spark is made and explosion occurs. temperature and pressure rise high. apply the first law. 2. Relevant equations The first law: ##Q=U_2-U_1+W## 3. The attempt at a solution Is the heat generated in the explosion considered incoming heat? if not, U2=U1 but it's not logical, how can the final state, with it's higher pressure and temperature be equal to the initial? can the internal energy be the same?