Chemical reactions often produce changes in energy.
LEARNING OBJECTIVE [ edit ]
Describe the types of energy changes that can occur in chemical reactions
KEY POINTS [ edit ]
Chemical reactions often involve changes in energy due to the breaking and formation of bonds.
Reactions in which energy is released are exothermic reactions, while those that take
in heat energy are endothermic.
TERMS [ edit ]
enthalpy
In thermodynamics, a measure of the heat content of a chemical or physical system. The change
in enthalpy of a chemical reaction is symbolized as ΔH.
exothermic
A description of a chemical reaction that releases heat energy to its surroundings.
endothermic
A description of a chemical reaction that absorbs heat energy from its surroundings.
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Due to the absorption of energy when chemical bonds are broken, and the release of energy
when chemical bonds are formed, chemical reactions almost always involve a change in
energy between products and reactants. By the Law of Conservation of Energy, however, we
know that the total energy of a system must remain unchanged, and that oftentimes a
chemical reaction will absorb or release energy in the form ofheat, light, or both. The energy
change in a chemical reaction is due to the difference in the amounts of stored chemical
energy between the products and the reactants. This stored chemical energy, or heat content,
of the system is known as its enthalpy.
Exothermic Reactions
Exothermic reactions release heat and light into theirsurroundings. For
example, combustion reactions are usually exothermic. In exothermic reactions, the products
have less enthalpy than the reactants, and as a result, an exothermic reaction is said to have
a negative enthalpy of reaction. This means that the energy required to break the bonds in
the reactants is less than the energy released when new bonds form in the products. Excess
energy from the reaction is released as heat and light.
Chemical reaction
A thermite reaction, which produces molten iron.
Endothermic Reactions
Endothermic reactions, on the other hand, absorb heat and/or light from their surroundings.
For example, decomposition reactions are usually endothermic. In endothermic reactions,
the products have more enthalpy than the reactants. Thus, an endothermic reaction is said to
have a positive enthalpy of reaction. This means that the energy required to break the bonds
in the reactants is more than the energy released when new bonds form in the products; in
other words, the reaction requires energy to proceed.
The decomposition of water into hydrogen and oxygen
When water is heated to over 2000 degrees Celsius, a small fraction will decompose into hydrogen and
oxygen. Significant heat energy is needed for this reaction to proceed, so the reaction is endothermic.