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In a chemical reaction, is energy always either lost or gained by the reactants? As much as I am concerned, changes in energy can occur during the absorption of heat or the emission of light or heat (these are the most often occurring). Is every chemical reaction accompanied by these conditions? Aren't there any chemical reactions without these conditions? Usually when we observe common chemical reactions (sugar that is melting in the water), we don't see any emissions or absorptions. Are these changes so trifling that we can't feel them (while they actually exist), or there are reactions with no emissions or absorptions?

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  • $\begingroup$ You might want to narrow the scope of your query: as written, it is rather broad. $\endgroup$ – Todd Minehardt Jul 5 '17 at 22:17
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    $\begingroup$ Yes, there's enthalpy of mixing if different compounds are mixed. $\endgroup$ – Mithoron Jul 5 '17 at 22:52
  • $\begingroup$ Your example of "sugar that is melting in the water" likely is an error as your observation relates to dissolution of sugar, rather than to its melting. In the instance of Sucrose (en.wikipedia.org/wiki/Sucrose), there is only a decomposition temperature known, $\pu{186 ^\circ{}C}$, which is much above the temperature of serving a hot tea. For monitoring even small changes in enthalpy in the course of a reaction, there are (reaction) calorimeters (e.g. DOI: 10.1021/ie070050d, or in biochem, e.g. doi: 10.1074/jbc.M806473200). $\endgroup$ – Buttonwood Jul 6 '17 at 11:50
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your question is good yet lacks the specifics. there are different so called "energy fields" in which changes occur (entropy, potential ,gibes and so forth). the thermodynamic field is endless and can be spanned with any variables (using maxwells equations and Legendre transformation) there is a sort of an axiom that states that all matter will try to find a way to a lower state of energy. but that said its not to scientific.

now for the question: every reaction creates some amounts of electromagnetic waves (as a price for electron "movement") some can be seen with the naked eye as they emit waves in the Uv-vis spectrum some are just infrared radiations of inner states of electron decay to a lower energetic state via vibrational relaxation.

ideally no reaction will occur in the ideal zero as there is no energy to "pay" for any kind of a reaction.

for further read: quantitive chemical analysis by harris chapter 17 fundamentals of spectrophotometry.

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  • $\begingroup$ There are plenty of reactions, e.g., ammonia inversion, where the reactants and products have the same energy. $\endgroup$ – Zhe Sep 4 '17 at 18:03

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