Limiting Reactants (Reagents)

Grilled Cheese Sandwich Bread + Cheese  ‘Grilled Cheese’ 2 B + 1C  B 2 C 100 bread 30 slices ? sandwiches

Container 1 Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 269

Before and After Reaction 1 All the hydrogen and nitrogen atoms combine. Before the reaction After the reaction Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 269

Container 2 Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 270

Before and After Reaction 2 Before the reactionAfter the reaction Zumdahl, Zumdahl, DeCoste, World of Chemistry  2002, page 270

Real-World Stoichiometry: Limiting Reactants LeMay Jr, Beall, Robblee, Brower, Chemistry Connections to Our Changing World, 1996, page 366 Ideal Stoichiometry Limiting Reactants

Limiting Reactants aluminum + chlorine gas  aluminum chloride Al(s) + Cl 2 (g)  AlCl 3 2 Al(s) + 3 Cl 2 (g)  2 AlCl g 100 g ? g A. 200 gB. 125 gC. 667 gD. ???

Limiting Reactant: Cookies 1 cup butter 1/2 cup white sugar 1 cup packed brown sugar 1 teaspoon vanilla extract 2 eggs 2 1/2 cups all-purpose flour 1 teaspoon baking soda 1 teaspoon salt 2 cups semisweet chocolate chips Makes 3 dozen If we had the specified amount of ingredients listed, could we make 4 dozen cookies? What if we had 6 eggs and twice as much of everything else, could we make 9 dozen cookies? What if we only had one egg, could we make 3 dozen cookies?

Limiting Reactant Most of the time in chemistry there is too much of one reactant than needed to completely use up other reactant. That reactant is said to be in excess (there is too much). The reactant that is completely used limits how much product we get. Once it runs out, the reaction stops. This is called the limiting reactant (LR).

Limiting Reactant To determine the LR calculate the amount of a product (pick one) created from each of the reactants to determine which reactant is the limiting one. (do not copy) Be sure to pick a product! You can’t compare to see which is greater and which is lower unless the product is the same!

The lower amount of the product would be the amount produced. The reactant that makes the least amount of product is the limiting reactant.

Limiting Reactant: Ex 10.0g of Al reacts with 35.0 g of Cl 2 to produce AlCl 3 Which reactant is limiting? Which is in excess? How much product is produced? 2 Al + 3 Cl 2  2 AlCl 3 Start with Al: Now Cl 2 : 10.0 g Al 1 mol Al 2 mol AlCl g AlCl g Al 2 mol Al 1 mol AlCl 3 = 49.4g AlCl g Cl 2 1 mol Cl 2 2 mol AlCl g AlCl g Cl 2 3 mol Cl 2 1 mol AlCl 3 = 43.9g AlCl 3 Limiting Reactant

LR Ex Continued We get 49.4g of AlCl 3 from the given amount of Al, but only 43.9g of AlCl 3 the given amount of Cl 2. Therefore, Cl 2 is the LR. Once the 35.0g of Cl 2 is used up, the reaction comes to a complete stop.

L.R. Practice 15.0 g of K reacts with 15.0 g of I 2 to produce KI. Calculate which is the LR and state how much product is made.

Finding the Amount of Excess By calculating the amount of the excess reactant needed to completely react with the LR, we can subtract that amount from the given amount to find the amount of excess. Can we find the amount of excess K in the previous problem?

Finding Excess Practice 15.0 g of potassium reacts with 15.0 g of iodine. 2 K + I 2  2 KI We found that Iodine is the limiting reactant, and 19.6 g of KI are produced g I 2 1 mol I 2 2 mol K 39.1 g K 254 g I 2 1 mol I 2 1 mol K = 4.62 g K USED! 15.0 g K – 4.62 g K = g K EXCESS Given amount of excess reactant Amount of excess reactant actually used Note that we started with the limiting reactant! Once you determine the LR, you should start with it!

Limiting Reactant: Recap 1.You can recognize a LR problem because there is MORE THAN ONE GIVEN AMOUNT. 2.Convert ALL of the reactants to the SAME product (pick any product you choose.) 3.The lowest answer is the correct answer. 4.The reactant that gave you the lowest answer is the LIMITING REACTANT.

5.The other reactant(s) are in EXCESS. 6.To find the amount of excess, subtract the amount used from the given amount. 7.If you have to find the amount produced of another product, start with the LR. You don’t have to determine which is the LR over and over again!

Read pg Do #23-24 pg 131

Percent Yield Theoretical yield: max mass of a product that is formed in a reaction. Actual yield: mass of product that is actually obtained in a reaction Usually less than theoretical. Why?

Why? Theoretical has assumed that all of L.R. has completely reacted. Many reactions do not go to completion Unexpected competing side reactions limit the formation of products. Some reactants are lost during the separation process Impure reactants Faulty measuring Poor experimental design or technique

Percent Yield = Actual Yield x 100 Theoretical yield

Practice: g of HBrO 3 is reacted with excess HBr. a. What is the theoretical yield of Br 2 ? b. What is the percent yield, if 47.3g is produced? HBrO 3 + 5HBr  3H 2 O + 3 Br 2

Practice alone: 2. When 35g of Ba(NO 3 ) 2 is reacted with excess Na 2 SO 4, 29.8g of BaSO 4 is recovered. Ba(NO 3 ) 2 + Na 2 SO 4  BaSO 4 + 2NaNO 3 a. Calculate the theoretical yield of BaSO 4 b. Calculate the percent yield of BaSO 4

Practice Pg.368 #7, 8, 9