Slow Reactions

Given enough time, any reaction in a closed system should reach an equilibrium.  However, enough time may be a very long time.  Some reactions are so slow that they will never appear to reach an equilibrium.  For example, a mixture of natural gas and oxygen

CH₄ (g) + 2 O₂ (g) CO₂ (g) + 2 H₂O (g)

at room temperature will react so slowly that it wouldn't reach an equilibrium in centuries.  Observing this reaction, you would not notice any change.  It would have constant, unchanging properties, not because it is at equilibrium, but because the reaction is so slow at room temperature.

The speed at which reactions occur is the topic studied in chemical kinetics.  Methane and oxygen are unreactive at room temperature because the reaction is very slow, not because the mixture is stable.  The kinetics of the reaction prevent it from reaching an equilibrium.  The addition of a suitable catalyst, or increasing the temperature would increase the reaction rate.

At higher temperatures of course, this reaction is extremely rapid.   In fact, at the temperature of a flame this reaction will proceed so fast in the forward direction that there is almost no reverse reaction at all.  This reaction is normally considered to go to completion, and is not treated as an equilibrium.