CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g) Why is this reaction classified as exothermic?

Based on the energy requirements or generation, reactions can be classified as either endothermic or exothermic. The reactions which need energy to proceed are known as endothermic reactions. The reactions in which energy is released are known as exothermic reactions. In other words, energy is supplied to endothermic reactions and is obtained from exothermic reactions. 

The given reaction is: 

`CH_4 (g) + 2O_2 (g) -> CO_2 (g) + 2H_2O (g)`

Here, methane reacts with oxygen to generate carbon dioxide and water. 

The binding energy of a molecule is defined as the difference between energy of that molecule and its elements (in their ground state). The binding energy of the various species used in this reaction are:

methane: -75 kJ/mol

oxygen: 0 

carbon dioxide: -394 kJ/mol

water: -242 kJ/mol

Thus, we can see that the formation of products will release more energy than reactants. We can also calculate the energy change when this reaction takes place.

Energy change = binding energy of products - binding energy of reactants

= -394 + 2 x (-242) - (-75) + 2 x 0 = -803 kJ/mol

negative sign indicates that the energy is released. 

Thus, this reaction is exothermic and generates 803 kJ of energy per mole.

Hope this helps.