Activity Series

In the reaction between aluminum metal and copper (II) chloride:

         Al (s) + CuCl₂ (aq) → AlCl₃ (aq) + Cu (s) + heat

the beaker got hot.  This means the reaction gave off heat, which was lost to the surroundings (the water that the chemicals were dissolved in, the beaker, the air, your hand).  Once the energy was given off, the chemicals didn’t have enough energy to go the other direction.  In other words:

                      Cu (s) + AlCl₃ (aq) → N.R.

(N.R. stands for “No Reaction”.)

For single replacement reactions, there is a list, called the activity series, which lists metals in order, based on how much energy they give off when they combine with a negative ion to form an ionic compound.  A metal that’s higher on the list can replace anything lower on the list (more energy is given off), but a metal that’s lower on the list doesn’t have enough energy to replace one that’s higher up.

Aluminum is higher than copper on the activity series, which means aluminum can replace copper, but copper can’t replace aluminum.