Exothermic reaction

A chemical reaction that releases energy to its surroundings, usually as heat, because its products have lower enthalpy than its reactants.

Enthalpy: A thermodynamic state function equal to internal energy plus pressure times volume. The enthalpy difference between products and reactants determines the reaction’s heat release at constant pressure.
Combustion: A rapid reaction with an oxidizer that releases heat and often light. Combustion is a familiar exothermic process used to release stored chemical energy.
Endothermic reaction: A chemical reaction that absorbs energy from its surroundings, usually as heat. It has the opposite direction of heat transfer from an exothermic reaction.
Methane combustion: The reaction of methane with oxygen to form carbon dioxide and water, releasing heat. It is a standard example of fuel oxidation with negative reaction enthalpy.
Bond energy: The energy required to break a chemical bond, usually reported per mole of bonds. Breaking bonds absorbs energy, while forming stronger product bonds can release more.
Thermite reaction: An aluminum-driven reduction of a metal oxide that produces molten metal and intense heat. Its strongly exothermic chemistry enables welding and metal production in specialized settings.
Spontaneous process: A process that proceeds without continuous external work under specified conditions. Exothermicity alone does not determine spontaneity; entropy and conditions also matter.
Hydrogen combustion: The reaction of hydrogen with oxygen to form water and release energy. It illustrates how product bond formation can release substantial heat.
Reaction enthalpy: The enthalpy change between the products and reactants of a chemical reaction. A negative reaction enthalpy identifies heat release under constant-pressure conditions.
Acid–base neutralization: A reaction in which an acid and base form water and a salt, or related products. Strong acid–base neutralization commonly releases heat as water forms.

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