4.24     understand the concept of dynamic equilibrium

Some reactions are not just reversible, but go forwards and backwards at the same time. Take the equation for the formation of ammonia: N2(g) + 3H2(g) ⇌ 2NH3(g)

At the same time as some of the nitrogen and hydrogen molecules combine to make ammonia, ammonia molecules that have already formed decompose to make nitrogen and hydrogen again.

When the forward and reverse reactions occur at the same rate, there is no overall visible change, even though things are constantly changing on the molecular level.  This is called dynamic equilibrium. It is like someone running up an escalator at the same speed it is coming down - constant movement is seen close up, but no change is observed overall.