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Chemical Reactions That Involve Heat

Objectives:

    1.    Describe the difference between exothermic and endothermic reactions.

Key Terms:

    thermochemistry        exothermic reaction        endothermic reaction

Notes:

The study of the changes in the heat of reactions is called thermochemistry.   The heat of reactions is recorded in Joules since heat is a type of energy.  When chemicals react they involve the transfer of energy (heat).  This transfer can result in the heating (exothermy) or the cooling (endothermy) of the product .  

the formation of bonds releases energy
breaking bonds requires energy

(www.gcsevise.com/images/ endo_exo.jpg)

Exothermic Reactions

Exothermic reactions are ones that liberate heat energy.  The excess heat in the reaction is the result of the formation of new bonds in the products.  One of the most obvious types of exothermic reaction is the combustion reaction.

(www.elmhurst.edu/.../chm110/ labs/world3images/lab2a.GIF)

In the above reaction wax and oxygen are reacted producing carbon dioxide, water and 2043kJ/mol reactant.  When you look at the reaction bonds are both being formed and broken.  The amount of heat produced is the difference in the energy required to break the bonds and the energy liberated in the formation of new bonds.

 

Endothermic Reactions

Endothermic reactions are ones that absorb heat energy.  The excess heat absorbed in the reaction is being used to break bonds in the products.  One of the most obvious types of endothermic reaction is the melting of ice.

H2O(s) + energy g  H2O(l)

In the above reaction no bonds are being formed only broken.  As a result the reaction is very endothermic.  

 

 

 

 

 

 

 

 

Last modified: February 05, 2003