how do you distinquish between a limiting reactant and the excess reactant in a chemical reaction?

Question

b) how do you distinguish between the theoretical yield and actual yield in stoichiometric calculations and how does the value of the theoretical yield generally compare with the value of the actual yield?

c) why are the actual yields usually less than calculated theoretical yields?

Answer 1

a). The limiting reagent is used completely, the other reagent has some remain unreacted in the procedure.
b). Theoretical yield is what you calculate from the balanced equation. It assumes everything goes 100% according to the procedure. The ACTUAL yield is the amount of material you collect when the experiment is performed. How do they compare? USUALLY the actual yield is lower than the theoretical yield.
c). Three reasons (and I'm sure there are others) could be,
1. we haven't invented perpetual motion; i.e., there is ALWAYS some loss; i.e., there is nothing that is 100%.
2. side reactions where another product is produced which reduces the desired product.
3. Losses due to sloppy lab work (spillage, poorly cleaned lab ware such that contaminants are introduced, bumping of solutions, and that kind of thing.