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Limiting Reagent and Percent Yield

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1 Limiting Reagent and Percent Yield

2 Review In a chemical reaction, how do we relate moles of one compound to moles of another? What are the four steps to go from mass of one reactant or product to the mass of another? MOLE RATIO! 2A + B A2B 1. Write the balanced equation 2. Convert mass of given to moles 3. Use the mole ratio from the balanced equation to find moles of the unknown 4. Convert moles of the unknown to mass of the unknown

3 Chemical Construction
In your group, you have a beaker of hydrogen (H2) and a beaker of carbon You need to make as many methane molecules (CH4) as you can!

4 Activity Analysis Which element limited the number of methane molecules you could make? Which element was present in excess amounts? Did the element present in excess affect the number of methane molecules you could make? Why doesn’t the carbon limit the number of methane molecules you could make, even though they are present in the smallest quantity?

5 Limiting Reactant Reactants are rarely present in amounts that correspond exactly to the mole ratios in the balanced equation Limiting reactant: a reactant that is completely consumed during a chemical reaction and limits the amount of product that can be formed Excess reactant: a reactant that remains after a reaction is over

6 Limiting Reactant 2 mol 1 mol 2 mol L.R. 2 mol 2 mol 4 mol 1 mol L.R.
The limiting reactant forms the smallest amount of product Use stoichiometry to determine which reactant produces the smallest amount of product

7 Practice Problem Lithium nitride reacts with water to form ammonia and lithium hydroxide, according to the following balanced chemical equation: If 4.87 g of lithium nitride reacts with 5.80 g of water, find the limiting reactant.

8 Yield Theoretical yield is the maximum amount of product that can form in a chemical reaction Calculated by assuming that all of the limiting reagent has reacted to form the product In reality: Many reactions do not go to completion Other reactions, called side reactions, may occur Some product may be lost during purification What are some things that might prevent us from reaching the theoretical yield?

9 Yield The actual yield is the amount of a product that is actually obtained from a chemical reaction The actual yield is almost always less than the theoretical yield Actual Yield is an experimentally determined quantity Percent yield: % Yield = actual yield x 100 theoretical yield

10 Crucial Information for Chemists
Consider the synthesis of Diazonamide A Chemical synthesis in pharmaceutical industry or industrial chemistry involves many steps Low percent yield in each step results in a tiny over all yield Wasted reactants translates into loss of money and resources 82% 95% 86% 65% 88% 67% 88% 85% 80% 60% 50% R. R. Knowles, J. Carpenter, S. B. Blakey, A. Kayano, I. K. Mangion, C. J. Sinz, D. W. C. MacMillan, Chem. Sci., 2011, 2,

11 Practice Problem Ammonia can be prepared by reacting nitrogen gas with hydrogen gas: When 7.5 x 101 g of nitrogen reacts with sufficient hydrogen, the theoretical yield of ammonia is 9.10g. If 1.72 g of ammonia is obtained by experiment, what is the percentage yield of this reaction?

12 Homework Practice Problems

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