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Published byJohanna Bransom Modified over 2 years ago

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Limiting Reagents When you just cant get enough!

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2 eggs + 1 cup flour 3 cookies If I have a dozen eggs, how many cookies can I make? NONE – I have no flour! If I have 6 cups of flour, how many cookies can I make? NONE – I have no eggs!

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10 cups of flour + 12 eggs 2 eggs + 1 cup flour = 3 cookies A. 18 B. 30 C. 6 D. 12 E. 10

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2 eggs + 1 cup flour 3 cookies If I have a dozen eggs, and 10 cups of flour, how many cookies can I make? 18 – then Im out of eggs. If I have 6 eggs and 2 cups of flour, how many cookies can I make? Six – then Im out of flour.

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CONGRATULATIONS! You now understand Limiting Reagents!

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Emphasis on limiting The key to limiting reagents is the limiting. In most reactions, there is neither an infinite supply of reactants nor a perfect balance between the finite supply. In most reactions, you run out of 1 reactant before any of the others and the reaction stops.

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2 H 2 + O 2 2 H 2 O 8.35 g of hydrogen and g of oxygen are mixed and allowed to react. Which is the limiting reagent? What is the theoretical yield of water? Grams is good, MOLES! MOLES! MOLES! IS BETTER!

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2 H 2 + O 2 2 H 2 O 8.35 g H 2 * 1 mol H 2 = 4.14 mol H g H g O 2 * 1 mol O 2 = mol O g O mol H 2 * 2 mol H 2 O = 4.14 mol H 2 O 2 mol H mol O 2 * 2 mol H 2 O = mol H 2 O 1 mol O 2

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O 2 is the Limiting Reagent It can make the least amount of product. After you make mol H 2 O, it doesnt matter how much hydrogen you have. You are DONE, because you are out of O 2 !

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Theoretical Yield mol H 2 O * g H 2 O = 17.2 g H 2 O 1 mol H 2 O Notice, this is another case of The Dance! g mol mol g Only we do The Dance twice and take the smallest quantity!

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Clicker Question N 2 + H 2 NH 3 Ammonia can be made from the gas phase reaction of nitrogen and hydrogen g of nitrogen and 2.47 g of hydrogen were mixed and reacted for 72 hours at 850 °C. How many grams of ammonia would you expect to produce? A g B g C g D g E g

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Clicker Question N 2 + H 2 NH 3 Ammonia can be made from the gas phase reaction of nitrogen and hydrogen g of nitrogen and 2.47 g of hydrogen were mixed and reacted for 72 hours at 850 °C. How many grams of ammonia would you expect to produce? A g (conservation of mass doesn t apply) B g (wrong limiting reagent) C g (unbalanced equation) D g (correct) E g (unbalanced equation and wrong limiting reagent)

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N 2 + H 2 NH 3 Ammonia can be made from the gas phase reaction of nitrogen and hydrogen g of nitrogen and g of hydrogen were mixed and reacted for 72 hours at 850 °C. How many grams of ammonia would you expect to produce? If g NH 3 were collected, what is the % yield of the reaction?

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N H 2 2 NH g N 2 * 1 mol N 2 * 2 mol NH 3 = 2.57 mol NH g N 2 1 mol N g H 2 * 1 mol H 2 * 2 mol NH 3 = 4.14 mol NH g H 2 3 mol H 2 N 2 is the limiting reagent 2.57 mol NH 3 * g NH 3 = g NH 3 1 mol NH 3

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Yield! Theoretical Yield = amount you expect to get Actual Yield = amount you actually get % yield = actual yield x 100 theoretical yield

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Yield! g NH 3 actual x 100 = % yield g NH 3 expected UNITS! UNITS! UNITS! % yield = g actual 100 g expected

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N H 2 2 NH 3 Ammonia can be made from the gas phase reaction of nitrogen and hydrogen g of nitrogen and 8.00 g of hydrogen were mixed and reacted for 72 hours at 850 °C. How many grams of ammonia would you expect to produce if the reaction proceeds with a 76% yield?

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N H 2 2 NH g N 2 * 1 mol N 2 * 2 mol NH 3 = mol NH g N 2 1 mol N g H 2 * 1 mol H 2 * 2 mol NH 3 = 2.65 mol NH g H 2 3 mol H 2 N 2 is the limiting reagent mol NH 3 * g NH 3 = g NH 3 theoretical 1 mol NH 3

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Actual Yield? g NH 3 expected 76% yield Just another UNITS! UNITS! UNITS! problem: g NH 3 expected * 76 g NH 3 actual = g NH 3 actual 100 g NH 3 expected

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N H 2 2 NH 3 Ammonia can be made from the gas phase reaction of nitrogen and hydrogen. If this reaction is known to proceed with 85% yield, how much nitrogen and hydrogen would you need to start with to get g of ammonia?

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10.00 g NH 3 actual* 100 g theoretical = g NH 3 85 g actual g NH 3 * 1 mol NH 3 * 3 mol H 2 * g H 2 = g H g NH 3 2 mol NH 3 1 mol H g H 2 * 1 mol H 2 * 1 mol N 2 * g N 2 = g N g H 2 3 mol H 2 1 mol N 2 Notice g H g N 2 = g NH 3 Of course, since I only get g NH 3, what happened to the other g of stuff?

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A. It is still N 2 and H 2, it didnt react. B. It is hydrazine, N 2 H 2. C. Oxygen from the air created H 2 O and N 2 O. D. None of the above.

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