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Section 6: Enthalpy
Figure 7-9. Thermite Welding
Thermite welding in progress. Liquid iron produced by the thermite reaction will flow into the mold around the rail gap.
© Wikimedia Commons. Author: Skatebiker.
Many chemical reactions are useful because of the energy transformations that accompany them. All chemical reactions exchange energy with the surroundings by releasing or absorbing heat. Reactions that produce or consume gases also exchange energy as pressure-volume work.
The amount of heat exchanged with the surroundings is an important property of a chemical reaction. The amount of heat absorbed or released during a reaction under constant pressure is called "enthalpy" and is represented by the variable H. When the pressure of the system doesn't change, the change in enthalpy of a system equals the heat exchanged with the surroundings:
ΔH = q
If a chemical reaction releases heat, it is exothermic. If a chemical reaction absorbs heat, it is endothermic. Many useful reactions are exothermic; we value them because we can harness the heat they release. Burning fuels such as coal and gasoline is an exothermic reaction; we use the heat produced by these reactions to power machinery and generate electricity. Digestion—the process by which our bodies break down nutrients from food—is an exothermic reaction that powers our movements and metabolism.
One of the most dramatic exothermic reactions is the thermite reaction, in which aluminum powder is mixed with a metal oxide, usually iron. Aluminum forms stronger bonds with oxygen than the metal does, so the aluminum reduces the oxide to metal:
Fe2O3(s) + 2Al(s) → Al2O3(s) + Fe(l)
The reaction releases so much heat that the iron produced is a liquid. This makes the thermite reaction useful in welding applications, such as fusing steel railroad tracks together. (Figure 7-9)
Sometimes, it is not desirable or useful for chemical reactions to release heat. The exothermic curing of concrete sometimes poses engineering challenges. For example, when workers poured the enormous blocks of concrete in the Hoover Dam, they embedded cooling pipes in the slabs. Without cold water flowing through these pipes, engineers estimated it would have taken 125 years for the dam to cool. A thorough knowledge of the enthalpy in the curing process was crucial to the project's success.
Because endothermic processes pull heat away from the surroundings, they cool things. Melting ice is endothermic, which is why ice packs reduce swelling from sports injuries. Chemical cold packs, which need no refrigeration, achieve the same thing with an endothermic chemical reaction. One of the common reactions inside cold packs is the dissolving of ammonium chloride:
NH4Cl(s) → NH4+(aq) + Cl-(aq)
The end result is the same: The endothermic chemical reaction cools the surroundings.