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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.

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    $\begingroup$ Please see the site homework policy. You should amend your question to include the work you have done so far towards an answer. Otherwise, the question might get closed. $\endgroup$ – Todd Minehardt Sep 11 '15 at 12:43
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    $\begingroup$ Hint: Are you sure the reaction is not exothermic? $\endgroup$ – Todd Minehardt Sep 11 '15 at 13:02
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    $\begingroup$ If the reaction is exothermic and if you are not keeping the temperature constant then the internal energy of the chamber will increase. That is the how bomb calorimeter works. However it is the enthalpy of the molecules that will reduce because for them the entropy is not held constant. This is a bit tricky. $\endgroup$ – WYSIWYG Sep 11 '15 at 13:37
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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.

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  • $\begingroup$ I am glad that I was able to help. If you do find my answer useful, could you please accept it. $\endgroup$ – Nanoputian Sep 11 '15 at 20:16
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If the reaction is exothermic, and the system you choose includes both the insulated chamber and its mixture of reactants and products, then the change in internal energy of this system will be zero. However, if the system you choose includes only the mixture of reactants and products, then heat will be transferred from the mixture to the chamber, and the internal energy of your mixture will decrease.

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When we add zinc with $\ce{H2SO4}$ in a bomb calorimeter, we observe that there will be constant temp and no heat is evolved as a result, we can say it as an isothermal process, therefore there will be no change in internal energy and $\Delta U=0$.

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