20 trolls and 15 hobbits go to a dance. What is the greatest # of dancing pairs on the dance floor at any one time?

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1 HW - LIMITING REACTANTS Name: Date: Period: Scenario: 20 trolls and 15 hobbits go to a dance. What is the greatest # of dancing pairs on the dance floor at any one time? Question: Who limited the # of teams? Trolls or hobbits? Why The same thing happens in a chemical reaction. If information about BOTH of the reactants is given, it is your job to figure out which reactant was the one which limited the amount of product (we call this the limiting reactant which is always used up) and which reactant was in excess and is not used up. Example: 2.00 grams of NH 3 reacts with 4.00 grams of 0 2. Determine which reactant was the limiting reactant and how much NO was produced. The other product is H 2O. Balanced reaction: 4NH NO + 6H20 To Solve: Step 1 : Use stoichiometry to convert each reactant to the same product. Step 2: Compare the amounts of each product produced. Step 3: The reactant which made the smaller amount of product is the limiting reactant. Step 4: The other one is the excess reactant. The SMALLER amount of product is the actual amount of product produced and the larger amount must be crossed out since it actually never existed NO 4.00 Final answer: 02 was the limiting reactant and 3.00 grams of NO is produced. NH 3 was the excess reactant Guided Practice Limiting Reactant Problems 10

2 1. Predict the mass of aluminum oxide that will form if 20.g of aluminum react with 15.L of oxygen gas at STP. Write a balanced equation for the reaction below and then solve. 2. Fluorine gas is bubbled through a solution of potassium iodide. The products of the reaction are potassium fluoride and iodine. Write a balanced equation for the reaction below and then answer the questions. a. What type of reaction is this? b. 1.26g of fluorine are bubbled through a solution containing 20.15g of potassium iodide. What mass of potassium fluoride should form? c. Identify the limiting reactant: d. Identify the excess reactant: Guided Practice: Limiting Reactant Problems 11

3 1. Given the balanced equation: N2 +3H2 2NH w many grams of NH3 can be produced from the reaction of 28 g ofn2 and 25 g ofh2? 2. Given the balanced equation: Mg + 2HCl MgCl 2 + H 2 What volume of hydrogen at STP is produced from the reaction of 50.0 g of Mg and 75 g of HCl? 3. Given the balanced equation: 3AgN03 + Na3P04 Ag3P04 + 3NaN03 Silver nitrate and sodium phosphate are reacted in equal amounts of 200 g each. How many grams of silver phosphate are produced? 12

4 FINDING THE MASS OF EXCESS REACTANT The next step in the limiting reactant process is determining the amount of excess reactant remaining following the reaction. There is a sample problem on the next page. Follow the steps of the sample problem if you get lost! Try the problem below: 1. What mass of carbon dioxide will be formed if 50.0g of propane gas are burned in 40L of pure oxygen at STP? a. Balanced Reaction: b. Find the expected yield if propane is the limiting reactant: c. Find the expected yield if oxygen is the limiting reactant: d. The limiting reactant is The excess reactant is e. Use the limiting reactant to determine how much of the excess reactant does react. f. Subtract the amount that reacts from the amount you started with to find the mass remaining. SAMPLE PROBLEM: FINDING THE MASS OF EXCESS REACTANT 13

5 What mass of copper(ll) iodide will be formed if35.0g of copper are placed in a solution containing 90.0g of silver iodide? a. Balanced Reaction: b. Find the expected yield if copper is the limiting reactant: c. Find the expected yield if silver iodide is the limiting reactant: d. The limiting reactant is The excess reactant is e. Use the limiting reactant to determine how much of the excess reactant does react. f. Subtract the amount that reacts from the amount you started with to find the mass remaining. 14

6 PERCENT YIELD (How well did you do in the lab?) o If you calculated a 3% error in a lab, then you should have a 97% yield. e If you calculated a 10% error in a lab, then you should have a 90% yield. Percent Yield: a comparison of how much product you produced in the lab and what you theoretically should have made according to your calculations. Percent yield = 100 Ouestion When "X" and "Y" react, you "should" produce 51.0 of "Z" but you only produced 30.5g of "Z". Find the % yield. Guided Practice Percent Yield: 1. Determine the percent yield for the reaction between 3.74 g of Na and enough 02 if 5.34 g of Na20 is recovered. 2. Determine the percent yield for the reaction between 45.9 g of NaBr and excess chlorine gas to produce 12.8 g of NaCI and an unknown quantity of bromine gas. 15

7 3. Determine the percent yield for the reaction between 15.8 g ofnh3 and excess oxygen to produce 21.8 g of NO gas and water. HW : REVEWING MOLE CONVERSIONS 1. How many moles are in 2.35 g ofh20? 2. If there are 9.6 x 10 1s particles of sugar in a solution then how many moles of sugar are there? 3. When there are moles of candy canes how many candy canes are there? 4. How heavy (g) will 6.14 x atoms of gold be? 16