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Limiting Reagents

Limiting Reagents. Chemistry 11. Why Do We Care?. 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?

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Limiting Reagents

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  1. Limiting Reagents Chemistry 11

  2. Why Do We Care? • 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

  3. So What Is A Limiting Reagent? • 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

  4. “I Don’t Follow....” • Okay, think of it like this: • To make one car you need 1 body and 4 tires:

  5. Now, What If... • 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!

  6. Or, What If... • 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!

  7. Lets Apply This To Chemistry • 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!! HINT GRAM

  8. Okay, Lets Try It • Urea is prepared from the reaction of ammonia with carbon dioxide as follows: 2NH3(g) +CO2(g) (NH2)2CO(aq) + H2O(l) • If 637.2 g of NH3 are treated with 1142 g of CO2 which of the two reactants is the limiting reagent?

  9. Step 1- Convert One Reactant to Moles 2NH3(g)+CO2(g) (NH2)2CO(aq) +H2O(l) • Convert to moles. 637.2 g 1142 g HAVE HAVE

  10. Step 2- Calculate How Many Grams of Second Reactant Required 2NH3(g)+CO2(g) (NH2)2CO(aq) + H2O(l) 637.2 g 1142 g • Pick one (lets arbitrarily pick NH3) • Calculate the mols of CO2 necessary to react with the moles of NH3 we have • Mols NH3mols CO2 NEED =18.7 mols CO2

  11. Step 3- Compare Need: 18.7 mols Have : 25.95 mols

  12. Conclusion • So, since we have more CO2 than we actually need to do the reaction, CO2 is in excess and, hence, NH3 must be the limiting reagent.

  13. General Steps: • 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.

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