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2.  Real reactions, in the lab, are rarely carried out in exact stiochiometric amounts › so... Chemists usually don’t mix reactants together in the exact amounts that chemical equations tell us to › Why not?  Synthetic chemistry is a lot like cooking 2

3.  Limiting Reagent- the reactant that is used up first is the limiting reagent  Excess Reagent- this reactant is present in greater quantities than necessary  By figuring out which reagent is limiting and which is excess we can determine how much product we SHOULD get 3

4.  Okay, think of it like this: › To make one car you need 1 body and 4 tires: 4

5.  We have 2 car bodies and 4 tires?  We can still only make one car! › The second car body is an excess, the tires are limiting how many cars we can make! 5

6.  We have 1 car body and 8 tires?  We can still only make one car because we are now limited by the number of car bodies and have an excess of tires! 6

7.  Think of the two reagents in terms of car bodies and tires › The number of moles tells us how much of each we have to start  Most of the time you are given the amounts of reagents in terms of grams, you need to CONVERT TO MOLES!! GRAM 7

8.  Urea is prepared from the reaction of ammonia with carbon dioxide as follows: 2NH 3(g) +CO 2(g) (NH 2 ) 2 CO (aq) + H 2 O (l)  If 637.2 g of NH 3 are treated with 1142 g of CO 2 which of the two reactants is the limiting reagent? 8

9. 2NH 3(g) +CO 2(g) (NH 2 ) 2 CO (aq) +H 2 O (l)  Convert to moles. 637.2 g1142 g 9 HAVE

10. 2NH 3(g) +CO 2(g) (NH 2 ) 2 CO (aq) + H 2 O (l) 637.2 g1142 g  Pick one (lets arbitrarily pick NH 3 )  Calculate the mols of CO 2 necessary to react with the moles of NH 3 we have › Mols NH 3 mols CO 2 10 =18.7 mols CO 2 NEED

11. Is the amount of CO2 you need more or less than what you have? More CO2 is in LIMITING Less CO2 is in EXCESS Need: 18.7 mols Have : 25.95 mols 11

12.  So, since we have more CO 2 than we actually need to do the reaction, CO 2 is in excess and, hence, NH 3 must be the limiting reagent. 12

13.  1. Convert both reactants to moles.  2. See how many moles of the other reactant your need. › Mole bridge  3. If you have more of a reactant than you need, it is in excess. If you have less than you need, it is limiting. › Ie. Look at which reactant limits the number of moles of product that you can make. 13