Stoichiometry

What is stoichiometry? Quantitative study of reactants and products in a chemical reaction Allows one to calculate how much of a given product in reaction is expected to produce based on how much of the reactants are available. Give the mass OR number of moles of reactants, one can calculate the mass OR number of moles of product.

Mole Practice How many moles are in 36g of H2O? Step 1: Find molar mass of H2O H: 2 x (1.00g) = 2.00 g O: 1 x (16.00g) = 16.00 g Molar Mass = 18.00 g Step 2: Conversion Factor 1 mol H2O = 18.00 g H2O Step 3/4: Set-up problem and solve ___36 g H2O___ 1 mole_____ = 2 mol H2O 18 g H2O

Mole Practice How many grams are in 3 mol of H2O? Step 1: Find molar mass of H2O H: 2 x (1.00g) = 2.00 g O: 1 x (16.00g) = 16.00 g Molar Mass = 18.00 g Step 2: Conversion Factor 1 mol H2O = 18.00 g H2O Step 3/4: Set-up problem and solve ___3 mol H2O___ 18 g H2O_____ = 54 g H2O 1 mol H2O

Molar Ratios Calculations using stoichiometry depend on the molar relationships in the chemical equations; this is why a properly balanced equation is SO important. A properly balanced equation shows the molar ratios of each species present, whether they are reactants or products. USING COEFFICIENTS!

Molar Ratio Example Equation: C3H8 + 5O2 -> 3CO2 + 4H2O Ratios in the equation: 1 mol propane: 5 mol oxygen 1 mol propane: 3 mol carbon dioxide 1 mol propane: 4 mol water 5 mol oxygen: 3 mol carbon dioxide 5 mol oxygen: 4 mol water 3 mol carbon dioxide: 4 mol water Each ratio can be written in reverse order

Moles to Moles Start with the information given, place the number, unit and label in the top left corner of the T-table. Find the appropriate mole ratio from the balanced equation, relate what your given and what you trying to find. Put the ratio correctly in the T-table Check your work: the unit on top left should match the unit diagonally Solve the problem by multiplying by what’s on top and divide by the bottom.

Moles to Mass There is one additional step to find the mass of a substance that is needed or produced given on the number of moles of the substance. Just convert the answer you calculate, from moles into grams using the molar mass of the substance.

Example Heating an ore of antimony, Sb2S3, in the presence of iron gives the pure element Sb and Iron (II) Sulfide. How many moles of Sb2S3 are needed to react with iron to produce 9.40 mol of Sb? What mass of Fe is needed to produce 9.40 mol of Sb?

Mass to Mole This calculation is the first in which you will have to convert to moles of the original substance before using the mole ratio. This step is very important because you MUST be in moles to convert between one compound to another. Convert the given mass into moles Using the appropriate mole ratio turn moles of what your given into moles of what you want.

Mass to Mass Given the mass of one of the substances you can find the mass of reactants need or products produced. 1. Convert the given mass into moles 2. Find the number of moles of the other substance using the proper mole ratio 3. Convert the calculated number of moles into grams

Example Carbon Disulfide is an important industrial solvent. It is prepared by the reaction of coke (carbon) with sulfur dioxide. How many moles of C are needed to react with SO2 to produce 0.762 grams of CS2 How many grams of CO form when 0.762 grams of CS2 are produced?

Predicted yield The predicted yield is determined by the masses used in a reaction and the mole ratios in the balanced equation. This predicted yield is the "ideal". It is not always possible to get this amount of product. Reactions are not always simple.

Percent Yield Percent Yield is a percentage to show the amount of substance that was formed during a chemical reaction based on the actual and predicted yields (mass). Actual yield is obtained during lab or given in the problem.

Percent Yield Example What is the percent yield for a reaction if you predicted the formation of 21. grams of C6H12 and actually recovered only 3.8 grams? 1. Recall definition of percent yield. 2. Substitute the actual and predicted yields. = 18%

Predicted Yield/% Yield Example A reaction between solid sulfur and oxygen produces sulfur dioxide. The reaction started with 384 grams of S6. Assume an unlimited supply of oxygen. What is the predicted yield and the percent yield if only 680 grams of sulfur dioxide are produced?

Woosh Bottle Demo First, write the balanced chemical equation for the combustion reaction of liquid isopropyl alcohol (C3H8O) Using the amount of Isopropyl that we started with calculate the predicted yield of water. After the demo, let weight the amount of water obtained and then calculate the percent yield.

Limiting Reactants This is a similar situation with chemical reactions in which one of the reactants is used up before the others - the reaction stops as soon as one of the reactants is consumed. Because the stoichiometry of the reaction 1 mol of O2 is needed for every 2 moles of H2 2 H2 + 1 O2  2H2O Using stoich, calculate which element will be used up first if you have 10 moles of H2 and 7 moles of O2.

Limiting Reactants Example Suppose you are a chef preparing a breakfast for a group of people, and are planning to cook French toast. You make French toast the way you have always made it: one egg for every three slices of toast. You never waiver from this recipe, because the French toast will turn out to be either too soggy or too dry (arguably, you are anal retentive). There are 8 eggs and 30 slices of bread in the pantry. Thus, you conclude that you will be able to make 24 slices of French toast and not one slice more.