# Why will internal energy decrease in this reaction?

1 mol zinc reacts with 1 mol sulfuric acid in a bomb calorimeter. I know since volume is constant work done will be zero .. but why will the internal energy decrease?? I know that the reaction is not endothermic or exothermic so there is no heat change involved, and as E = w + Q, the internal energy should also have been unaffected. But in an mcq book, the answer clearly states that the internal energy is decreasing.

Internal energy loosely can be seen as the measure of energy which are stored in the substance. Internal energy is equal to $$\Delta E = q + w ~~~~~~~~(where ~~w = -p\Delta V)$$ Since we know that volume is constant, w = 0. Also, now we need to determine the sign of q. Where you got stuck is saying that the reaction is neither endothermic or exothermic. Any reaction between a strong acid with a reactive metal will be exothermic. This means that heat energy is released from the system into the surroundings (water). That is why the water gets hot and forms the foundation of how the bomb calorimeter works.
Therefore, since we know that heat is transferring from the system to the surroundings, q must be negative. Therefore, $\Delta E$ will be negative, meaning that the internal energy decreased.
• $\Delta E$ is the change in internal energy. May 5, 2021 at 10:43