MadSci Network: Chemistry

Re: Is there any way to find that reaction will be endothermic/exothermic?

Date: Mon Jan 11 19:56:39 2010
Posted By: Jerry Franzen, Chemistry Teacher
Area of science: Chemistry
ID: 1263223692.Ch

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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