|MadSci Network: Chemistry|
There are several ways. The direct way would be to conduct the reaction (the system) in surroundings that can demonstrate whether the reaction transfers heat energy to the surroundings (exothermic) or the system absorbs heat from the surroundings (endothermic.) A "bomb" calorimeter is a common device used to measure the heat evolved in a combustion reaction. It is used only for exothermic reactions. The evolution of heat is indicated by the rise in temperature of the surroundings (the "bomb" and water surrounding the "bomb.") A styrofoam cup can be used for solution processes. An increase in temperature after the mixing of thwo solutions in such a cup indicates an exothermic reaction and a decrease in temperature indicates an endothermic reaction. If a table of standard enthalpies of formation of substances is available, it can be used to predict wheter a reaction is exothermic or endothermic. First the balanced chemical equation for the reaction must be known. The the sum of the molar standard enthalpies of formation (each multiplied by its coefficient from the balanced equation) of all of the reactants is subtracted from the sum of the molar standard enthalpies of formation (again multiplied by the appropriate coefficients) of all of the reactants. If the resulting value has a negative sign, the reaction is exothermic. If a positive sign the reaction is endothermic. If the molar enthalpies of a series of reactions that can be added to give the desired reaction are known, then the enthalpy of the desired reaction is the sum of the enthalpies of the component reactions. This is referred to as the use of Hess' Law. If a table of Bond Dissociation Energies (BDE) is available, then one can subtract the values for the BDE's for all of the bonds broken (dissociated) from the values for all of the bonds made (opposite sign from the BDE) in the reaction. If the result is negative, exothermic. If positive, endothermic. One should be able to find more informatiom and examples by searching out these topics on the web.
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