The Wisdom of Gallagher Why are there Interstate Highways in Hawaii? Why are there floatation devices under plane seats instead of parachutes? Why do we drive on parkways and park on driveways? Why do hot dogs come ten to a package and hot dog buns only eight?

Limiting Reactants Percentage Yield

Hot Dogs in the News Takeru Kobayashi of Japan downed 44½ hot dogs in 12 minutes. One hot dog = one hot dog + one bun. WHAT IF… Mr. Kobayashi didn’t do his math correctly. He bought 5 packs of hot dogs (10 per package) and 5 packs of hot dog buns (8 per package). How many hot dogs (according to the official formula) could he have eaten? Source: CNN.com

Hot Dogs in the News Excess 40 possible hot dogs WHAT IF… One hot dog = one hot dog + one bun. WHAT IF… Mr. Kobayashi didn’t do his math correctly. He bought 5 packs of hot dogs (10 per package) and 5 packs of hot dog buns (8 per package). How many hot dogs (according to the official formula) could he have eaten? Excess 5 hot dog packs 10 hot dogs 50 hot dogs x = 1 hot dog pack Limiting Factor 40 possible hot dogs 5 bun packs 8 buns 40 buns x = 1 bun pack Source: CNN.com

Let’s Revisit the Cookies (again)… For 1 batch: In my pantry, I have: 5.5 cups of flour 16 Tbsp of butter lots of everything else 2.25 cups flour 8 Tbsp butter 0.5 cups shortening 0.75 cups sugar 0.75 cups brown sugar 1 tsp salt 1 tsp baking soda 1 tsp vanilla 0.5 cups Egg Beaters 12 oz. Chocolate chips How many batches of cookies can I make?

Let’s Revisit the Cookies (again)… For 1 batch: How many batches of cookies can I make? 5.5 cups 2.25 cups flour 8 Tbsp butter 0.5 cups shortening 0.75 cups sugar 0.75 cups brown sugar 1 tsp salt 1 tsp baking soda 1 tsp vanilla 0.5 cups Egg Beaters 12 oz. Chocolate chips 16 Tbsp EXCESS 1 batch cookies 5.5 c flour x = 2.25 c flour 2.4 batches LIMITING 1 batch cookies 16 Tbsp butter x = 8 Tbsp butter 2.0 batches

Now I Want to Bake a Cake! GONE! SOME FLOUR LEFT OVER… But do I have all the ingredients I need? How much flour do I have left after baking all those cookies? SOME FLOUR LEFT OVER… 5.5 c flour x 2.25 c flour 1 batch cookies = 16 Tbsp butter 1 batch 8 Tbsp butter 2.4 batches of cookies 2.0 batches of cookies GONE! 2.25 cups flour 4.5 cups flour used 2.0 batches x = 1 batch cookies 5.5 cups – 4.5 cups = 1.0 cups left

Limiting Reactants In the laboratory, a reaction is rarely carried out with exactly the required amounts of each reactant. In most cases, one or more of the reactants is present in excess, that is, there is more than the exact amount required to react. Once one of the reactants is used up, no more product can be formed. The substance that is completely used up first in a reaction is the limiting reactant.

The action of a limiting reactant to determine the extent to which a reaction takes place is like producing meals at a cafeteria. When you run out of chicken pieces, no more chicken dinners can be produced, even though there might be a good supply of mashed potatoes and vegetables…..”the other reactants”. Leftover! Not enough!

The limiting reactant (reagent) is the reactant that limits the amounts of the other reactants that can combine and the amount of product that can form in a chemical reaction. The excess reactant is the substance that is not used up completely in a reaction.

Limiting Reactants in Chemistry 5.0 moles of chlorine gas react with 5.0 moles of sodium to produce sodium chloride. Which reagent is the limiting reactant? How much of the excess reactant is left over? Cl2 (g) + 2 Na NaCl EXCESS 2 givens = 2 equations! 2 mol NaCl 5.0 mol Cl2 x = 10. mol NaCl 1 mol Cl2 LIMITING 2 mol NaCl 5.0 mol Na x = 5.0 mol NaCl 5.0 mol Cl2 given 2 mol Na 2.5 mol Cl2 used 1 mol Cl2 2.5 mol Cl2 5.0 mol Na x = 2.5 mol Cl2 left 2 mol Na

Practice Problems 1. 3 CuSO4 + 2 Al Al2(SO4)3 + 3 Cu In this reaction, 20.0 g of CuSO4 reacts with 20.0 g of Al. Which reactant is limiting? How much of the excess reactant is left over? 1 mol CuSO4 3 mol Cu 20.0 g CuSO4 x = 0.125 mol Cu x 159.61 g CuSO4 3 mol CuSO4 20.0 g Al 1 mol Al 3 mol Cu 1.11 mol Cu x x = 26.98 g Al 2 mol Al 1 mol CuSO4 2 mol Al 26.98 g Al 20.0 g CuSO4 x x = x 159.61 g CuSO4 3 mol CuSO4 1 mol Al 2.25 g Al 20.0 g Al – 2.25 g Al = 17.8 g Al EXCESS USED

Practice Problems 2. 2 H2 (g) + O2 (g) 2 H2O In this reaction, 5.0 g of H2 reacts with 5.0 g O2. Which reactant is limiting? How many grams of H2O are produced? 5.0 g H2 x 1 mol H2 2.02 g H2 2 mol H2O 2 mol H2 = 2.5 mol H2O 5.0 g O2 x 32.00 g O2 1 mol O2 2 mol H2O = 0.31 mol H2O 18.02 g H2O 0.31 mol H2O x = 5.6 g H2O 1 mol H2O smallest number of moles

On Perfection “Perfection never exists in reality, but only in our dreams.” - Dr. Rudolf Dreikurs “Perfection is our goal, excellence will be tolerated.” - J. Yahl

Get Real! Johnny took a quiz yesterday. He missed 4 questions and earned 63 points out of 70. Was he perfect? What was his possible score? What was his actual percent score? 70 points = 100 % 90 %

Percentage Yield

Get Real! Mrs. Kennedy ran a reaction in her lab yesterday. She predicted that 183 grams of product would be formed. The reaction only yielded 162 grams of product. Was her reaction perfect? What was the percent yield? 162 grams 100 88.5 % x = 183 grams 162 grams = actual yield 183 grams = theoretical yield

Percentage Yield The theoretical yield is the maximum amount of product that can be produced from a given amount of reactant. In most chemical reactions, the amount of product obtained is less than the theoretical yield. The measured amount of product obtained from a reaction is called the actual yield of the product. The percent yield is the ratio of the actual yield to the theoretical yield, multiplied by 100.