Reversible reactions and equilibrium

The idea

In a reversible reaction the products can react to form the reactants again, shown by ⇌. For example, ammonium chloride ⇌ ammonia + hydrogen chloride. If one direction is exothermic, the other is endothermic by the same amount: heating blue hydrated copper sulfate makes white anhydrous copper sulfate (endothermic); adding water reverses it (exothermic). In a closed system, equilibrium is reached when the forward and reverse reactions happen at exactly the same rate.

1. Identify the forward and reverse reactions.
2. If one is exothermic, the other is endothermic.
3. At equilibrium, the rates are equal.

Watch out At equilibrium the reactions have not stopped: they happen at the same rate.