posted by Donna .
The burning of methane in oxygen is a highly exothermic reaction. Yet a mixture of methane and oxygen gas can be kept indefinitely without any apparent change.
Can someone give me an explanation behind this?
Look in your book or on the internet for information about activation energy. The activation energy is a "hump" that the reaction must get over before the reaction can continue. Unless something provides the energy to get over the hump, there is no reaction. For example, one form of carbon is carbon black (soot) [or graphite] which are quite inexpensive but they are the more stable forms of carbon as opposed to diamond. But we don't see people not buying diamond rings even though they are becoming the more common forms (albeit slowly). The reason is that to make the transition the diamond form has an activation energy hump so the reaction occurs but it is so slow that we need not worry about the diamonds disappearing in our lifetime. Another example is burning of wood. You don't see wood in a fireplace suddenly and spontaneously light itself. But if we put a match to the wood it starts to burn, the energy produced keeps the wood burning, it in turn lights another log, etc and we can have a roaring fire in the fireplace. The match provides the activation energy needed to start the first spark of a flame.