Quantities in chemical reactions 7.2 The Limiting Reactant

Introduction - Limiting Reactant In real life chemical reactions rarely have reactants in stoichiometric amounts. Ex - Your BBQ has the following chemical reaction: C3H8(g) + 5 O2(g)  3 CO2(g) + 4H2O(g) What happens when you run out of propane?

Introduction - Limiting Reactant Definitions: Limiting Reactant (LR) This reactant in a chemical reaction yields the least amount of product. This reactant is completely used up (consumed) in the chemical reaction. Excess Reactant (ER) This reactant remains after the reaction is over.

Examples In the following scenarios, which reactants are in excess and which are limiting? 2 car bodies, 7 tires No gasoline in your vehicle but oxygen is still present for combustion 4 hamburgers patties, 2 hamburger buns 6 crackers, 28 chocolate squares, 2 marshmallows Perfect S’more 2 Graham Crackers 4 Chocolate Squares 1 Marshmallow

Examples In the following scenarios, which reactants are in excess and which are limiting? 2 car bodies, 7 tires No gasoline in your vehicle but oxygen is still present for combustion 4 hamburgers patties, 2 hamburger buns 6 crackers, 28 chocolate squares, 2 marshmallows Perfect S’more 2 Graham Crackers 4 Chocolate Squares 1 Marshmallow

Practice 1. If 4.80 g of calcium are mixed with 2.00 g of nitrogen, which is the limiting reactant, Ca or N2? 3Ca(s) + N2(g)  Ca3N2(s)

mass Ca moles Ca mole ratio moles Ca3N2 3Ca(s) + N2(g)  Ca3N2(s) mass Ca moles Ca mole ratio moles Ca3N2 4.80 g Ca produces 0.0400 mol Ca3N2

mass N2 moles N2 moles Ca3N2 mole ratio 3Ca(s) + N2(g)  Ca3N2(s) mass N2 moles N2 mole ratio moles Ca3N2 2.00 g N2 produces 0.0714 mol Ca3N2

Which is the limiting reactant, Ca or N2? The limiting reactant is Ca 0.0400 mol Ca3N2 < 0.0714 mol Ca3N2

Practice: 2. We have a sample of 2.00 g of ammonia and 4.00 g of oxygen. The products of the reaction are water and nitrogen monoxide. a) What mass of nitrogen monoxide will be produced? b) What amount of excess reactant remains after the reaction is complete? 4 NH3(g) + 5 O2(g)  6 H2O(g) + 4 NO(g) 

The limiting reactant is Step-2 Determine the limiting reactant by calculating how much NO is produced by the amount of each reactant. Note: The reactant that yields a smaller amount of NO is the limiting reactant (LR). 2.00 g NH3 produces 0.117 mol NO The limiting reactant is O2 4.00 g O2 produces 0.100 mol NO

Step-3 Determine the mass of NO is produced by the limiting reactant by converting the moles of NO into mass. 4.00 g O2 produces 3.00g NO

Step-4 Determine the mass of excess reactant (NH3) that remains by: i) calculating how many moles of NH3 react. ii) calculating how many moles of NH3 remain unreacted. iii) convert the moles of NH3 into mass. 0.29 g NH3 remain 0.100 mol NH3 react 0.017 mol NH3 remain unreacted

SUMMARY Strategy for Solving Limiting Reactant Problems: Calculate how much product can be produced from each of the reactants. Compare the molar amounts of the product. The substance that produces the least amount of the product is the limiting reactant. -This reactant governs how much product is formed. Mass of product can be determined by multiplying moles by molar mass.

Questions

- Complete prediction (show your calculations) Textbook p. 254 #23 - 26 p. 257-258 #27 - 30 Investigation 7A - Complete prediction (show your calculations) - Copy observation tables (2) - Read procedure NOTE: TEST UNIT 3 (Ch 5-7) in 5 school days (see suggested textbook review)