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In my book it’s written that “A rise in temperature is given a positive sign. So the value of H is negative for an exothermic reaction. A fall in temperature is given a negative sign. So the value of H is positive for an endothermic reaction.”

What I don’t understand is that why is sign of exothermic negative doesn’t exothermic rise temperature of sorrounding? And why is endothermic taken as positive doesn’t it take in energy and cool down the surrounding?

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  • $\begingroup$ Is the rising or falling of the T that is not required here. This is often see at introductory level in spire of being unnecessary. See answer $\endgroup$ – Alchimista Nov 24 '18 at 15:44
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The reaction is said to be exothermic or endothermic according to the sign of the Gibbs free energy. To obtain Gibbs free energy you subtract the final energy of products (or more accurately the enthalpy) from that of reactants. So, if the final energy of the system is more than the initial one the result is negative and this energy difference manifests it self in the system as heat and temperature increases. The opposite is true for endothermic reactions.

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  • $\begingroup$ you're welcome my friend. $\endgroup$ – M.ghorab Nov 24 '18 at 13:48

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