Part 2: Limiting Reactants & Percentage Yield Stoichiometry Unit Part 2: Limiting Reactants & Percentage Yield

Objectives Identify the limiting reactant for a reaction and use it to calculate theoretical yield Perform calculations involving percentage yield

Reality vs. Theoretical In the previous lesson, we assumed that 100% of the reactants would change into products. However, in the real world, we know that this is not always true The amounts of all reactants, the completeness of the reaction and the product lost in the process, can limit the yield of a reaction

Limiting Reactants Reactants of a reaction are seldom present in ratios equal to the mole ratio in the balanced equation Some reactants are used up first and some are not used completely The substance that controls the quantity of product that can form in a chemical reaction is called the limiting reactant The substance that is not used up completely in a reaction is called the excess reactant

Determining Theoretical Yield from the Limiting Reactant Theoretical yield is the maximum quantity of product that a reaction could possibly make if everything in the reaction works perfectly The theoretical yield of a reaction should always be calculated base on the limiting reactant

Solving Problems with Limiting Reactants and Theoretical Yield Identify the limiting reactant and the theoretical yield in grams of HCl if 75 g PCl3 and 75 g H2O are mixed together PCl3 + 3H2O → H3PO3 + 3HCl Step 1: Gather the information 75 g PCl3 molar mass of PCl3 = 137.32 g/mol 75 g H2O molar mass of H2O = 18.02 g/mol ? g HCl molar mass of HCl = 37.47 g/mol

Solving Problems with Limiting Reactants and Theoretical Yield Step 2: Plan your work by setting up problems that will calculate the mass of HCl you would expect from each reactant Step 3: Calculate

Solving Problems with Limiting Reactants and Theoretical Yield 75 g PCl3 x 1 mol PCl3 x 3 mol HCl x 37.47 g HCl 137.32 g PCl3 1 mol PCl3 1 mol HCl = 61.39 g HCl 75 g H2O x 1 mol H2O x 3 mol HCl x 37.47 g HCl 18.02 g H2O 3 mol H2O 1 mol HCl = 155.95 g HCl The limiting reactant is PCl3 and the theoretical yield is 61.39 g HCl

Limiting Reactants & the Food You Eat In industry, the cheapest reactant is often used as the excess reactant That way the more expensive reactant is completely used up (no waste) Choosing the excess and limiting reactants based on cost is also helpful in areas outside of chemistry

Actual Yield Is the measured amount of a product of a reaction Usually the actual yield is less than the theoretical yield Why? Many reactions do not completely use up the limiting reactant Some of the products turn back into reactants The main product must go through additional steps to purify or separate it from other chemicals Some of the product may be lost in the process There may be side reactions that can use up reactants without making the desired product

Percentage Yield Is the ratio relating the actual yield of a reaction to its theoretical yield It describes the efficiency of the reaction It is calculated by taking the actual yield and dividing by the theoretical yield and then multiplying by a 100 Percentage yield = actual yield x 100 theoretical yield

Calculating Percentage Yield Determine the limiting reactant, the theoretical yield, and the percentage yield if 14.0 g N2 are mixed with 9.0 g H2, and 16.1 g NH3 form. N2 + 3H2 → 2 NH3 Step 1: Gather the information 14.0 g N2 molar mass N2 = 28.02 g/mol 9.0 g H2 molar mass H2 = 2.02 g/mol Actual yield of NH3 = 16.1 g Theoretical yield of NH3 = ? Percentage yield = ?

Calculating Percentage Yield Step 2: Plan your work First, calculate the mass of NH3 you would expect to form from each reactant Then, identify the limiting reactant and theoretical yield Finally, calculate the percentage yield

Calculating Percentage Yield Step 3: Calculate 14.0 g N2 x 1 mol N2 x 2 mol NH3 x 17.04 g NH3 28.02 g N2 1 mol N2 1 mol NH3 = 17.0 g NH3 9.0 g H2 x 1 mol H2 x 2 mol NH3 x 17.04 g NH3 2.02 g H2 3 mol H2 1 mol NH3 = 51 g NH3 Limiting reactant is N2 and the theoretical yield is 17.0 g NH3

Calculating Percentage Yield Step 3: Calculate Percentage yield = actual yield x 100 theoretical yield Percentage yield of NH3 = 16.1 g x 100 = 94.7% 17.0 g Step 4: Verify your result The answer is less than 100%, so the calculation is set up correctly

Determining Actual Yield Actual yield can only be determined experimentally However, a close estimate can be calculated if the percentage yield for a reaction is known

Calculating Actual Yield How many grams of CH3COOC5H11 should form if 4808 g are theoretically possible and the percentage yield for the reaction is 80.5%? Step 1: Gather the information Theoretical yield of CH3COOC5H11 = 4808 g Percentage yield = 80.5% Actual yield = ? Step 2: Plan you work Use the percentage yield and the theoretical yield to calculate the actual yield expected

Calculating Actual Yield Step 3: Calculate 80. 5 % = actual yield x 100 4808 g = 3870.44 g CH3COOC5H11 Step 4: Verify your results The actual yield should be less than the theoretical yield and the units are correct

Quick Lab! Find the limiting reactant for making s’mores with the ingredients on your table. What ingredient(s) was/were the excess reactant(s)?