Stoichiometry

Ham Sandwich How many sandwiches could you make from 24 slices of bread? How many slices of lettuce would you need? tomato? ham? cheese?

Equations are like recipes. 2C8H18 + 25O2 → 16CO2 + 18H2O Coefficients can be read as ratios of particles or of moles. If 2 mols of C8H18 reacts completely how many moles of CO2 would be produced?

Stoichoimetry = the proportional relationship between 2 or more substances during a chemical reaction Predict amount of product you could make from starting amounts of reactant

The Mole Ratio is the Key In stoichiometry problems, the unit that bridges the gap between one substance and another is the mole. You can use the coefficients in conversion factors called mole ratios

Sample Problem A N2 + 3H2 → 2NH3 3 mols of hydrogen are needed to prepare 2 moles of ammonia. 3 mol H2 = 2 mol NH3 PROBLEM: How many moles of hydrogen are needed to prepare 312 moles of ammonia?

312 mol NH3 x 3 mol H2 = ? mol H2 2 mol NH3 = 468 mol H2 needed

Calculate the amounts requested if 1 Calculate the amounts requested if 1.34 mol H2O2 completely react according to the following equation 2H2O2 → 2H2O + O2 mols of oxygen formed Given: 1.34 mol H2O2 reacts 2 mol H2O2 = 1 mol O2 1.34 mol H2O2 x 1 mol O2 = 0.670 mol O2 produced 2 mol H2O2

Calculate the amounts requested if 1 Calculate the amounts requested if 1.34 mol H2O2 completely react according to the following equation 2H2O2 → 2H2O + O2 b. mols of water formed Given: 1.34 mol H2O2 reacts 2 mol H2O2 = 2 mol H2O 1.34 mol H2O2 x 2 mol H2O = 1.34 mol H2O produced 2 mol H2O2

2. Calculate the amounts requested if 3 2. Calculate the amounts requested if 3.30 mol Fe2O3 completely react according to the following equation Fe2O3 + 2Al → 2Fe + Al2O3 mols of aluminum needed Given: 3.30 mol Fe2O3 reacts 1 mol Fe2O3 = 2 mol Al needed 3.30 mol Fe2O3 x 2 mol Al = 6.60 mol Al needed 1 mol Fe2O3

2. Calculate the amounts requested if 3 2. Calculate the amounts requested if 3.30 mol Fe2O3 completely react according to the following equation Fe2O3 + 2Al → 2Fe + Al2O3 b. mols of iron formed Given: 3.30 mol Fe2O3 reacts 1 mol Fe2O3 = 2 mol Fe formed 3.30 mol Fe2O3 x 2 mol Fe = 6.60 mol Fe formed 1 mol Fe2O3

2. Calculate the amounts requested if 3 2. Calculate the amounts requested if 3.30 mol Fe2O3 completely react according to the following equation Fe2O3 + 2Al → 2Fe + Al2O3 c. mols of aluminum oxide formed Given: 3.30 mol Fe2O3 reacts 1 mol Fe2O3 = 1 mol Al2O3 formed 3.30 mol Fe2O3 x 1 mol Al2O3 = 3.30 mol Al2O3 formed 1 mol Fe2O3

Homework Worksheet “Stoichiometry: Mole-Mole Problems”

Stoichiometry: Mass-Mass Problems You must convert to moles using molar mass of known Then use mole ratio to find moles unknown Convert back to mass using molar mass of unknown

Solving Mass-Mass Problems Molar mass Mass Known Molar Ratio Mol Known Molar Mass Mol Unknown Mass Unknown 1 mol ___ grams Periodic Table Mol unknown Mol known Balanced Chemical Equation ___ grams 1 mol Periodic Table

What mass of NH3 can be made from 1221 g H2 and excess N2 What mass of NH3 can be made from 1221 g H2 and excess N2? N2 + 3H2 → 2 NH3 1221 g H2 X 1 mol H2 x 2 mol NH3 x 17.04g NH3 2.02 g 3 mol H2 1 mol NH3 GRAMS MOLAR MOLAR MOLAR KNOWN MASS RATIO MASS KNOWN UNKNOWN = 6867 g NH3 made

Use the equation below to answer #1-4 Fe2O3 + 2Al → 2Fe + Al2O3 1. How many grams Al needed to completely react with 135 grams Fe2O3 135 g Fe2O3 X 1 mol Fe2O3 x 2 mol Al x 26.98 g Al 159.7 g Fe2O3 1 mol Fe2O3 1 mol Al GRAMS MOLAR MOLAR MOLAR KNOWN MASS RATIO MASS KNOWN UNKNOWN = 45.6 g Al needed

Use the equation below to answer #1-4 Fe2O3 + 2Al → 2Fe + Al2O3 2. How many grams Al2O3 can form when 23.6g Al react with excess Fe2O3? 23.6 g Al X 1 mol Al x 1 mol Al2O3 x 101.96 g Al2O3 26.98 g Al 2 mol Al 1 mol Al2O3 = 44.6 g Al2O3 formed

Practice p. 307#1-4 Use the equation below to answer #1-4 Fe2O3 + 2Al → 2Fe + Al2O3 3. How many grams of Fe2O3 react with excess Al to make 475 g Fe? 475 g Fe X 1 mol Fe x 1 mol Fe2O3 x 159.7 g Fe2O3 55.85 g Fe 2 mol Fe 1 mol Fe2O3 = 679 g Fe2O3 react

Use the equation below to answer #1-4 Fe2O3 + 2Al → 2Fe + Al2O3 4. How many grams of Fe will form when 97.6 g Al2O3 form? 97.6 g Al2O3 X 1 mol Al2O3 x 2 mol Fe x 55.85 g Fe 101.96 g Al2O3 1 mol Al2O3 1 mol Fe = 107 g Fe formed

Homework Worksheet “Stoichiometry: Mass-Mass Problems”

9.2 Limiting Reactants and Percentage Yield In this section you will: Identify the limiting reactant for a reaction and use it to calculate theoretical yield. Perform calculations involving percentage yield.

Limiting Reactant To drive a car you need gasoline and oxygen from the air. When the gas runs out, you can’t go any farther even though there is still plenty of oxygen. The gasoline limits the distance you can travel because it runs out first.

Limiting reactant = substance that controls the quantity of product that can form in a chemical reaction; runs out first Excess reactant = substance that is not used up completely in a reaction. Identify the Limiting Reactant from Lab Data Calculate the amount of product that each could form. Whichever reactant would produce the least amount of product is the limiting reactant.

Theoretical yield = maximum amount of product that can be made if everything about the reaction works out perfectly; determined by the limiting reactant. Whenever a problem gives you quantities of 2 or more reactants, you must Determine the limiting reactant (makes less product) Use the limiting reactant quantity to determine the theoretical yield.

Identify the limiting reactant and the theoretical yield of phosphorous acid, H3PO3 if 225g of PCl3 is mixed with 123 g of H2O. PCl3 + 3H2O → H3PO3 + 3HCl Determine how many g of H3PO4 that each reactant could make. Need to calculate molar masses to use as conversion factors.

PCl3 is the limiting reactant because 134 g is less than 187 g. 225g PCl3 x 1mol PCl3 X 1mol H3PO4 x 82.00g H3PO4 137.32g 1 mol PCl3 1 mol = 134 g H3PO4 123g H2O x 1mol PCl3 X 1mol H3PO4 x 82.00g H3PO4 18.02g 3 mol H2O 1 mol = 187 g H3PO4 PCl3 is the limiting reactant because 134 g is less than 187 g. The theoretical yield is 134 grams of H3PO3.

PCl3 + 3H2O → H3PO3 + 3HCl Identify the limiting reactant and the theoretical yield (in grams) of HCl for each pair 3.00 mol PCl3 and 3.00 mol H2O 3.00 mol PCl3 x 3 mol HCl = 9.00 mol HCl 1 mol PCl3 3.00 mol H2Ox 3 mol HCl = 3.00 mol HCl 3 mol H2O H2O is limiting reactant. 3.00 mol HCl x 36.51 g = 109 grams HCl 1 mol theoretical yield (made)

PCl3 + 3H2O → H3PO3 + 3HCl Identify the limiting reactant and the theoretical yield (in grams) of HCl for each pair 75.0 g PCl3 and 75.0 g H2O 75.0g PCl3 x 1 mol PCl3 x 3 mol HCl = 1.65 mol HCl 136.5 g 1 mol PCl3 75.0 g H2O x 1 mol H2O x 3 mol HCl = 4.16 mol HCl 18.02 3 mol H2O PCl3 is limiting reactant. 1.65 mol HCl x 36.51 g = 60.2 grams HCl 1 mol theoretical yield (made)

PCl3 + 3H2O → H3PO3 + 3HCl Identify the limiting reactant and the theoretical yield (in grams) of HCl for each pair 3. 1.00 mol PCl3 and 50.0 g H2O 1.00 mol PCl3 x 3 mol HCl = 3.00 mol HCl 1 mol PCl3 50.0 g H2O x 1 mol H2O x 3 mol HCl = 2.77 mol H3PO3 18.02 3 mol H2O H2O is limiting reactant. 2.77 mol HCl x 36.51 g = 101 grams HCl 1 mol theoretical yield (made)

Limiting Reactants & Industry The most expensive chemicals are chosen as the limiting reactants Less expense reactants can be used in excess to ensure all of the expensive chemicals are completely used up (none wasted).

Actual Yield and Percentage Yield Although equations tell you what should happen, they cannot always tell you what will happen in real life/in the lab. Some reactions do not make all of the product predicted by the theoretical yield. ACTUAL YIELD = the mass of product actually formed, measured in lab, often less than expected (theoretical).

Examples of things that could reduce yield: Incomplete distillation/purification needed to separate product from a mixture. Side reactions that can use up reactants without making desired products

PERCENTAGE YIELD = ratio relating the actual to the theoretical yield; describes how efficient the reaction was. Percentage Yield = actual x 100 theoretical SHOULD ALWAYS BE LESS THAN 100%. If not, you switched the actual & theoretical in the formula!!!