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8.3 Limiting Reactants Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings 1 A limiting reactant in a chemical reaction is the substance that • Is used up first. • Stops the reaction. • Limits the amount of product that can form. 2 In a table setting, there is 1 plate, 1 fork, 1 knife, and 1 spoon. How many table settings are possible from 5 plates, 6 forks, 4 spoons, and 7 knives? Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings What is the limiting item? 3 limiting reactant: is the reactant that limits the extent of the reaction and determines the amount of the product formed. Excess reactants Reactants that leftover when a reaction stops Four table settings can be made. Initially plates 5 forks 6 spoons 4 knives 7 Use Left over 4 1 4 2 4 0 4 3 The limiting item is the spoon. 5 How many peanut butter sandwiches could be made from 8 slices bread and 1 jar of peanut butter? With 8 slices of bread, only 4 sandwiches could be made. The bread is the limiting item. Copyright © 2008 by Pearson Education, Inc. publishing as Benjamin Cummings 6 When 4.00 mol H2 is mixed with 2.00 mol Cl2,how many moles of HCl can form? H2(g) + Cl(g) → 2HCl (g) 4.00 mol 2.00 mol ??? mol • Calculate the moles of product from each reactant, H2 and Cl2. • The limiting reactant is the one that produces the smaller amount of product. 7 HCl from H 2 4.00 mol H2 x 2 mol HCl = 8.00 mol HCl 1 mol H2 (not possible) HCl from Cl 2 2.00 mol Cl2 x 2 mol HCl = 4.00 mol HCl 1 mol Cl2 (smaller number) The limiting reactant is Cl2 because it is used up first. Thus Cl2 produces the smaller number of moles of HCl. 8 Initially H2 Cl2 4.00 mol 2.00 mol 2HCl 0 mol Reacted/ Formed -2.00 mol -2.00 mol +4.00 mol Left after reaction 2.00 mol Excess 0 mol Limiting 4.00 mol 9 If 4.80 mol Ca mixed with 2.00 mol N2, which is the limiting reactant? 3Ca(s) + N2(g) → Ca3N2(s) Moles of Ca 3 H 2 from Ca 4.80 mol Ca x 1 mol Ca3N2 = 1.60 mol Ca3N2 3 mol Ca (Ca used up) Moles of Ca 3 H 2 from N 2 2.00 mol N2 x 1 mol Ca3N2 = 2.00 mol Ca3N2 1 mol N2 (not possible) All Ca is used up when 1.60 mol Ca3N2 forms. Thus, Ca is the limiting reactant. 10 Calculate the mass of water produced when 8.00 g H2 and 24.0 g O2 react? 2H2(g) + O2(g) 2H2O(l) 11 Calculate the grams of H2 for each reactant. H2: 8.00 g H2 x 1 mol H2 x 2 mol H2O x 18.02 g H2O 2.016 g H2 2 mol H2 1 mol H2O = 71.5 g H2O (not possible) O2: 24.0 g O2 x 1 mol O2 x 2 mol H2O x 18.02 g H2O 32.00 g O2 1 mol O2 = 27.0 g H2O (smaller) 1 mol H2O O2 is the limiting reactant. 12 practice 1- How many moles of CO2 can be produced by the reaction of 5.0 grams of C2H4 and 12.0 grams of O2? 2- How many grams of carbon dioxide could be produced from 2.0 g of C2H4 and 5.0 g of O2? 3- In the reaction between CH4 and O2, if 18.0 g of CO2 are produced, how many grams of water are produced? 4- Cu + 2 AgNO3 → Cu(NO3)2 + 2 Ag. When 50.0 g of copper was reacted with 300.0 g of silver nitrate solution, 149 g of silver was obtained. What is the percent yield of silver obtained?