This document discusses limiting reactants, theoretical yield, actual yield, and percent yield in chemical reactions. It defines limiting reactant as the reactant that limits the amounts of other reactants that can combine and the amount of product that can form. It provides examples of how to use before-during-after charts to determine the limiting and excess reactants. It also demonstrates how to calculate theoretical yield, actual yield, and percent yield, and provides a sample problem calculating percent yield.

1. Limiting reactants and percent yield Chapter 9.3

2. Objectives: Describe a method for determining which of two reactant is a limiting reactant. Calculate the amount in moles or mass in grams of a product, given the amounts in moles or masses in grams of two reactants, one of which is in excess. Distinguish between theoretical yield, actual yield, and percent yield. Calculate percent yield, given the actual yield and quantity of a reactant.

3. Limiting Reactant Defined as: reactant that limits the amounts of the other reactants that can combine and the amount of product that can form in a chemical reaction. Also referred to as limiting reagent Relate to airplane flight Excess reactant : Substance not used up completely in chemical reaction 50 passengers in excess 400 passengers 350 seats Limiting Excess

4. In Chemical Reactions C(s) + O2(g) CO2(g) If you start with: 5 mol 10 mol 5 mol 5 carbon atoms 10 oxygen molecules Used up first!! 5 excess oxygen molecules Limiting reactant

5. Before-During-After charts(B-D-A) C(s) + O2(g) CO2(g) Before (B) 5 mol 10 mol 1 During (D) 5 mol 5 mol 1 5 mol 0 mol After (A) Excess Reactant Limiting Reactant Step 1: Use moles that are present before the reaction starts Step 3: Subtract to determine limiting reactant and excess. Step 2: Use coefficients to determine moles used during reaction If negative number, start over!

6. Sample Problem 1 Silicon dioxide (quartz) is usually quite unreactive but reacts readily with hydrogen fluoride according to the equation below. If 2.0 mol of HF are exposed to 4.5 mol of SiO2, which is the limiting reactant? SiO2(s) + 4HF(g) SiF4(g) + 2H2O(l) mass moles 2.0 mol 4.5 mol B 4.5 mol 2.0 mol 4 1 Limiting Reactant is HF 4.5 mol 18.0 mol D 2.0 mol 0.50 mol 4 1 A 4.0 mol 0 mol -16.o mol 0 mol Start over!!

7. Sample Problem 2 The black oxide of iron, Fe3O4, occurs in nature as the mineral magnetite. This substance can also be made in the laboratory by the reaction between red-hot iron and steam according to the equation below. When 36.0 g of H2O is mixed with 167 g of Fe, which is the limiting reactant? What mass in grams of black iron oxide is produced? What mass in grams of excess reactant remains when the reaction is completed? 3 Fe(s) + 4H2O(l) Fe3O4(s) + 4H2(g) mass 36.0 g 167 g 116 g ÷ 55.85 g/mol ÷ 18.01 g/mol x231.55 g/mol 1 2.99 mol 2.00 mol moles 0.500 mol 4 B 2.99 mol 2.00 mol x55.85 g 1 mol 3 1.49 mol = 83.2 g Fe D 2.00 mol 1.50 mol 4 Excess Fe LR is H2O A 0 mol 1.49 mol Excess Fe

9. Measured amount of product obtained from a reaction.

10. The amount you get in the laboratory

11. Percent Yield:

12. Ratio of the actual yield to the theoretical yield multiplied by 100.Actual Yield Percent Yield = x 100 Theoretical Yield