Give an analogy to explain the concept of excess and limiting reactant, and percent yield.  You may not use the cookie analogy or any sandwich analogy.

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Let's look at these concepts using an analogy of making old-fashioned Christmas cards.  You take the cards and put them in envelopes to mail them.  So the "chemical reaction" here would be:

1 card + 1 envelope --> 1 completed Christmas card

So we know that for every one card we need one envelope to make a completed card ready for mailing.  Let's say that we have 10 cards and 12 envelopes sitting in front of us.  Since the molar ratio is 1:1, then the reagent that we have in the smallest amount will be the limiting reagent.  So the limiting reagent will be the cards and the excess reagent will be the envelopes.  When all of the cards are used up, there will still be 2 envelopes left.  So the number of cards (limiting reagent) will determine the total number of completed Christmas cards (product) that will be produced. 

We can also use this same analogy for the concept of percent yield.  The limiting reagent (cards) will determine the theoretical yield of the reaction.  10 cards will produce 10 completed Christmas cards if the reaction goes to 100% completion.  But say I mess up on one card and have to throw it away.  That only gives me 9 completed cards at the end.  We divide the actual yield into the theoretical yield to get the percent yield.  So 9/10 = 90% yield.

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