Why do chemical reactions release energy?

[yrjosmiel]

If I have a container filled with oxygen and hydrogen and heat it up, it would react and create water and heat. Why was heat released? What happens in chemical reactions at a microscopic level that makes it release energy in the form of heat?

 

[DrClaude]

If there is heat released, it is because the products have less energy than the reactants. In the case of water, there is more energy in the bonds in 1/2 O₂ and H₂ than there is in the two O-H bonds in water. It is that extra bonding energy that gets converted to heat.

Another to think about it is to consider the inverse reaction. To break water into O₂ and H₂ requires an input of energy: more energy is expanded to break the O-H bonds than is recovered when molecular oxygen and hydrogen form.