# Energy in a Chemical Reaction

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?

• You might want to narrow the scope of your query: as written, it is rather broad. – Todd Minehardt Jul 5 '17 at 22:17
• Yes, there's enthalpy of mixing if different compounds are mixed. – Mithoron Jul 5 '17 at 22:52
• 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). – Buttonwood Jul 6 '17 at 11:50