It is then disolved in the blood and transported to individual cells which derive the energy they need to survive by oxidizing food molecules that are also carried by the blood. But a more reactive and energetic form exists, O3, called ozone. Ozone is used for purifying water but it is also a major component of smog. The absorbtion of ultraviolet light by ozone in the upper atmosphere is believed to be responsible for protecting the surface of the earth from that potentially harmful form of light radiation. Exothermic reactions liberate energy.
Endothermic reactions absorb energy.
Burning of carbon and hydrogen compounds tends to be very exothermic. Both are present in coal and oil which are major energy sources. Reversing the reaction requires exactly the same amount of energy to be supplied to allow the rection to proceed. The reverse of formation is decomposition . A substance whose decomposition is exothermic or weakly endothermic is usually unstable.