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I'm pretty sure the sign changes.

But, I want to be sure if it becomes endothermic or exothermic.

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    $\begingroup$ Welcome to chemistry S.E.! Please tell us what you feel, we will correct you if you are wrong :) $\endgroup$ – Freddy Apr 18 '15 at 2:26
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Yes, in a reverse reaction the sign changes. That is because Enthalpy is a state function, one that depends on only the current state of the function. That means the energy required to push a reaction to its products would be directly reversed to push it back to its reactants.

A key use of this knowledge is in Hess's law, and Born Haber Cycle.

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  • $\begingroup$ If a reaction is endothermic for some conditions would be endothermic for all conditions? In other words, could a reaction be endothermic for X conditions and exothermic for Y conditions? Thanks in advance. $\endgroup$ – ado sar Feb 23 at 11:44
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The enthalpy of a reaction can be found by calculating the amount of energy required to break the bonds of the reactants, and then subtracting from this value the amount of energy required to form the bonds of the products. You can find a table of these values here.

The reverse reaction would simply be doing the opposite, meaning that the magnitude would be the same, but the sign would be opposite, as you stated.

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