Is the reaction between methane and oxygen endothermic or exothermic? explain why we burn fossil fuels, such as natural gas.

The thing that governs whether a reaction is exothermic is the energy given out / used up to break / form the bonds in the reaction. 

When two substances react, the bonds in those substances first break up, releasing energy, before re-forming in a different way, taking in energy. The nature of the bonds that are broken up and reformed determines whether more energy is given out (exothermic) or taken in (endothermic)

Ethene molecular fórmula= c2h4molecular weight = 29.046g/molhope it : )