Answer:
It's well Explained below.
Explanation:
First of Excess product of CaCO_3 would be produced due to the fact that there would not be enough CaCl_2 to react with Na_2•CO_3. The main purpose of having stoichiometric quantities is for us to know the correct amount or near the correct amount of each reactant in order to create a product that will be close to the theoretical amount and thus have a higher percent yield.
This is a double replacement because both Copper and Sodium reacted with each other compounds , hence "double"
So that they will mix in better, or more easily. However you want to say it.