What is an exothermic reaction?
Gerald R. Van Hecke, a Professor of Chemistry at Harvey Mudd College
We can all appreciate that water does not spontaneously boil at room temperature; instead
we must heat it. Because we must add heat, boiling water is a process that chemists call
endothermic. Clearly, if some processes require heat, others must give off heat when they
take place. These are known as exothermic. For purposes of this discussion, processes that
require or give off heat will be limited to changes of state, known as phase changes, and
changes in chemical constitution, or chemical reactions.
Changes of state involve a solid melting, a liquid freezing, a liquid boiling or a gas
condensing. When steam, which is gaseous water, condenses, heat is released. Likewise
when liquid water freezes, heat is given off. In fact heat must be continually removed from
the freezing water or the freezing process will stop. Our experience makes it easy for us to
realize that to boil water or any liquid and thereby convert into a gas, heat is required and
the process is endothermic. It is less intuitive to grasp that when a gas condenses to a liquid,
heat is given off and the process is exothermic.
Perhaps it is easier to explain an exothermic phase change using the following argument.
Liquid water had to have energy put into it to become steam, and that energy is not lost.
Instead, it is retained by the gaseous water molecules. When these molecules condense to
form liquid water again, the energy put into the system must be released. And this stored
energy is let out as exothermic heat. The same argument can be made for the process of
freezing: energy is put into a liquid during melting, so freezing the liquid into a solid again
returns that energy to the surroundings.
Like phase changes, chemical reactions can occur with the application or release of heat.
Those that require heat to occur are described as endothermic, and those that release heat as
exothermic. Although we are generally quite familiar with endothermic phase changes, we
are probably even more familiar with exothermic chemical reactions: Almost everyone has
experienced the warmth of a fireplace or campfire. Burning wood provides heat through the
exothermic chemical reaction of oxygen (O) with cellulose (C6H10O5), the major chemical
component of wood, to produce carbon dioxide (CO), steam (H2O) and heat. The chemical
reaction describing the process is C6H10O5 + 6O2 = 6CO2 + 5H2O + heat.
In today's space age, probably everyone has seen a rocket launch on television or, if lucky,
in person. What powers those rockets are highly exothermic chemical reactions. One rocket
fuel uses a mixture of solid ammonium perchlorate (NH4ClO4) and aluminum metal (Al) to
produce a solid aluminum oxide, hydrochloric acid gas, dinitrogen gas, steam and heat: The
chemical reaction can be described as 6NH4ClO4 + 10Al = 5Al2O3 + 6HCl + 3N2 + 9H2O +
heat.
The great billows of white clouds seen behind launched rockets are really the product gases
dispersing the white aluminum oxide powder. Where is the exothermic heat energy coming
from? The heat comes from the energy stored in the chemical bonds of the reactant
molecules--which is greater than the energy stored in the chemical bonds of product
molecules. In endothermic chemical reactions, the situation is reversed: more chemical
energy is stored in the bonds of the product molecules than in the bonds of the reactant
molecules
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