Why are actual yields generally less than theoretical yields?

Question

Limiting Reactant Questions

Answer 1

In an actual reaction, unless circumstances are absolutely perfect (which never occurs), the reactants do not completely react with one another. Theoretical yields are expected yields if every bit of each reactants completely reacts with the other(s).

Answer 2

Actual yields are less than theoretical yields for several reason.  First, no reaction actually ever goes 100% to completion. Secondly, there is experimental error in recovering ALL of the product.  Finally, the chemical reaction is not 100% efficient, which is sort of another way of saying that it doesn't go to completion.