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An exothermic reaction releases energy to its surroundings.

Video segment. Assistance may be required. Watch the following video showing four different exothermic reactions.

Source: 4 ways to make fire without matches by using chemistry, nurdrage, YouTube

When a chemical bond is formed, energy is released making it an exothermic reaction. The heat released is also called enthalpy (heat content).

In an exothermic reaction, energy can be represented as a product.

X + Y → Z + 150 kJ

It can also be represented as having negative enthalpy.

X + Y → Z + 150 kJ

ΔH = -150kJ

Interactive exercise. Assistance may be required. Sort the following items as endothermic or exothermic.

Exothermic Processes	Endothermic Processes
freezing water solidifying solid salts condensing water vapor forming an anion from an atom in the gas phase annihilation of matter E=mc² splitting of an atom	melting ice cubes melting solid salts evaporating liquid water making an anhydrous salt from a hydrate forming a cation from an atom in the gas phase splitting a gas molecule separating ion pairs cooking an egg baking bread
Exothermic Reactions	Endothermic Reactions
combustion of hydrogen dissolving lithium chloride in water burning of propane decomposition of hydrogen peroxide decomposition of ammonium dichromate	reaction of barium hydroxide octahydrate crystals with dry ammonium chloride dissolving ammonium chloride in water reaction of thionyl chloride (SOCl₂) with cobalt(II) sulfate heptahydrate mixing water and ammonium nitrate mixing water with potassium chloride photosynthesis (chlorophyll is used to react carbon dioxide plus water plus energy to make glucose and oxygen)