But to first get it started, the covalent bonds between the individual H atoms and the individual O atoms must be broken. This takes energy. Heating a mixture of H2 and O2 with a flame is one way to get the reaction started. If this is done, once the reaction gets started, it proceeds so fast that an explosion can occur. In reactions that that place spontaneously at room temperature such as that between hydrogen and fluorine, the random thermal energy of the molecules is enough to activate a few molecules which then react, liberating the heat needed to activate many others.
Lignite coal has even more.
Reaction Rates 11.9. The interior of a solid is inaccessible. But if broken into a powder, more molecules are exposed to reaction and thus the reaction rate is increased. Which means that both the reactants and the products can coexist. Whichever one proceeds more rapidly will leave more of its products than the reverse reaction.