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)