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In the Ncert Chemistry Tb, it is given that

$\Delta H$ is negative for exothermic reactions which evolve heat during the reaction and $\Delta H$ is positive for endothermic reactions which absorb heat from the surroundings.

In enthalpy change video lesson which I got access to online, it is given that for $\ce{NH4NO3(s) -> NH4NO3(aq)}$ since the temperature decreases during the reaction, it is positive enthalpy of solution.

How is it positive? I guess both $\Delta H$ and enthalpy change of a solution is almost change. Since the temp decreases here, it is endothermic, so it should be positive right?

Are both of the above processes different from each other and are the sign conventions vice versa for each other?

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    – Philipp
    Commented Sep 8, 2014 at 20:11

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The reaction is endothermic, meaning that it takes energy from the surrounding. The "surrounding" in your example is the solvent, so in the dissolution process, energy is taken from the solvent, thereby decreasing its temperature.

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