Chapter 9 “Stoichiometry” Mr. Mole Chemistry

Stoichiometry is… Greek for “measuring elements” Defined as: calculations of the quantities in chemical reactions, based on a balanced equation. You will need a calculator everyday for this chapter

A balanced equation is a molar ratio In terms of Moles The coefficients tell us how many moles of each substance 2Al2O3  4Al + 3O2 2Na + 2H2O  2NaOH + H2 A balanced equation is a molar ratio

Molar Ratio 2Al2O3  4Al + 3O2 2 mol Al2O3 : 4 mol Al : 3 mol O2 2Na + 2H2O  2NaOH + H2 2 mol Na : 2 mol H2O : 2 mol NaOH : 1 mol H2

Mole to Mole conversions 2Al2O3  4Al + 3O2 each time we use 2 moles of Al2O3 we will also make 3 moles of O2 2 moles Al2O3 3 mole O2 or 3 mole O2 2 moles Al2O3 These are the two possible conversion factors to use in the solution of the problem.

Mole to Mole conversions How many moles of O2 are produced when 3.34 moles of Al2O3 decompose? 2Al2O3 ® 4Al + 3O2 3 mol O2 3.34 mol Al2O3 = 5.01 mol O2 2 mol Al2O3 Conversion factor from balanced equation If you know the amount of ANY chemical in the reaction, you can find the amount of ALL the other chemicals!

Practice: 2C2H2 + 5 O2  4CO2 + 2 H2O If 3.84 moles of C2H2 are burned, how many moles of O2 are needed? How many moles of C2H2 are needed to produce 8.95 mole of H2O? Worksheet here If 2.47 moles of C2H2 are burned, how many moles of CO2 are formed?

Practice: 2C2H2 + 5 O2  4CO2 + 2 H2O (8.95 mol) If 3.84 moles of C2H2 are burned, how many moles of O2 are needed? (9.60 mol) How many moles of C2H2 are needed to produce 8.95 mole of H2O? (8.95 mol) Worksheet here If 2.47 moles of C2H2 are burned, how many moles of CO2 are formed? (4.94 mol)

How do you get good at this?

Steps to Calculate Stoichiometric Problems Correctly balance the equation. Convert the given amount into moles. Set up mole ratios. Use mole ratios to calculate moles of desired chemical. Convert moles back into final unit.

Mol to Mass Calculations You will convert from the mol of the given to the mol of the unknown (using the molar ratio) and then to the mass of the unknown (using the molar mass of the unknown) How many grams of solid zinc are formed when 8.47 mol of aluminum completely reacts? 2Al(s) + 3ZnCl2(aq)  3Zn(s) + 2AlCl3(aq) 831 g Zn

Another Problem CH4(g) + 4Cl2(g)  CCl4(l) + 4HCl(g) If 7.63 mol of HCl are formed in this process, how many grams of methane are required? 30.6g CH4 Worksheet here

Mass to Mol Calculations Mass of known to mol of known (using molar mass), then mol of known to mol of unknown (using molar ratio) 4.63 grams of oxygen gas react completely with solid aluminum to form aluminum oxide. How many mol of aluminum are needed to react? 4Al(s) + 3O2(g)  2Al2O3(s) 0.193 mol Al

Another Problem 503 g of iron (III) oxide react with carbon monoxide to form iron metal and carbon dioxide. How many mol of carbon monoxide are needed for this reaction? Fe2O3 + 3CO  2Fe + 3CO2 Answer: 9.45 mol CO2 Worksheet here

Mass-Mass Problem: 4Al + 3O2  2Al2O3 12.3 g Al2O3 are formed 6.50 grams of aluminum reacts with an excess of oxygen. How many grams of aluminum oxide are formed? 4Al + 3O2  2Al2O3 6.50 g Al 1 mol Al 2 mol Al2O3 101.96 g Al2O3 = ? g Al2O3 26.98 g Al 4 mol Al 1 mol Al2O3 12.3 g Al2O3 are formed (6.50 x 1 x 2 x 101.96) ÷ (26.98 x 4 x 1) =

Another example: 2Fe + 3CuSO4 Fe2(SO4)3 + 3Cu If 10.1 g of Fe are added to a solution of copper (II) sulfate, how many grams of solid copper would form? Iron (III) sulfate is also produced (single replacement). 2Fe + 3CuSO4 Fe2(SO4)3 + 3Cu Answer = 17.2 g Cu

1 crust : 5 oz sauce : 2 cups cheese : 1 pizza Making Pizza The number of pizzas you can make depends on the amount of the ingredients you use. This relationship can be expressed mathematically. 1 crust : 5 oz sauce : 2 cups cheese : 1 pizza We can compare the amount of pizza that can be made from 10 cups of cheese: Since 2 cups cheese : 1 pizza, then, 1 crust + 5 oz tomato sauce + 2 cups cheese  1 pizza

Limiting Reactant, Theoretical Yield Recall our pizza recipe: 1 crust + 5 oz tomato sauce + 2 cups cheese  1 pizza If we have 4 crusts, 10 cups of cheese, and 15 oz tomato sauce. How many pizzas can we make? We have enough crusts to make We have enough cheese to make We have enough tomato sauce to make

Limiting Reactant We have enough crusts for four pizzas, enough cheese for five pizzas, but enough tomato sauce for only three pizzas. We can make only three pizzas. The tomato sauce limits how many pizzas we can make.

Theoretical Yield Tomato sauce is the limiting reactant, the reactant that makes the least amount of product. The limiting reactant is also known as the limiting reagent. The maximum number of pizzas we can make depends on this ingredient. In chemical reactions, we call this the theoretical yield. This is the amount of product that can be made in a chemical reaction based on the amount of limiting reactant. The ingredient that makes the least amount of pizza determines how many pizzas you can make (theoretical yield).

Summarizing Limiting Reactant and Yield The limiting reactant (or limiting reagent) is the reactant that is completely consumed in a chemical reaction and limits the amount of product. The reactant in excess is any reactant that occurs in a quantity greater than is required to completely react with the limiting reactant.

In a Chemical Reaction For reactions with multiple reactants, it is likely that one of the reactants will be completely used before the others. When this reactant is used up, the reaction stops and no more product is made. The reactant that limits the amount of product is called the limiting reactant. It is sometimes called the limiting reagent. The limiting reactant gets completely consumed. Reactants not completely consumed are called excess reactants. The amount of product that can be made from the limiting reactant is called the theoretical yield.

“Limiting” Reagent If you are given one dozen loaves of bread, a gallon of mustard, and three pieces of salami, how many salami sandwiches can you make? The limiting reagent is the reactant you run out of first. The excess reagent is the one you have left over. The limiting reagent determines how much product you can make

Limiting Reagents - Combustion

How do you find out which is limited? The chemical that makes the least amount of product is the “limiting reagent”. You can recognize limiting reagent problems because they will give you 2 amounts of chemicals Do two stoichiometry problems, one for each reagent you are given.

Cu is the Limiting Reagent, since it produced less product. If 10.6 g of copper reacts with 3.83 g sulfur, how many grams of the product (copper (I) sulfide) will be formed? 2Cu + S  Cu2S Cu is the Limiting Reagent, since it produced less product. 1 mol Cu 1 mol Cu2S 159.16 g Cu2S 10.6 g Cu 63.55g Cu 2 mol Cu 1 mol Cu2S = 13.3 g Cu2S = 13.3 g Cu2S 1 mol S 1 mol Cu2S 159.16 g Cu2S 3.83 g S 32.06g S 1 mol S 1 mol Cu2S = 19.0 g Cu2S

How much excess reagent will remain? Another example: If 10.3 g of aluminum are reacted with 51.7 g of CuSO4 how much copper (grams) will be produced? 2Al + 3CuSO4 → 3Cu + Al2(SO4)3 the CuSO4 is limited, so Cu = 20.6 g How much excess reagent will remain? Excess = 4.47 grams

The Concept of: A different type of yield than what you see while driving.

More Making Pizzas Assume that while making our pizzas, we burn a pizza, drop one on the floor, or some other uncontrollable events happen so that we only make two pizzas. The actual amount of product made in a chemical reaction is called the actual yield. We can determine the efficiency of making pizzas by calculating the percentage of the maximum number of pizzas we actually make. In chemical reactions, we call this the percent yield.

Summarizing Limiting Reactant and Yield The theoretical yield is the amount of product that can be made in a chemical reaction based on the amount of limiting reactant. The actual yield is the amount of product actually produced by a chemical reaction. The percent yield is calculated as follows:

What is Yield? Yield is the amount of product made in a chemical reaction. There are three types: 1. Actual yield- what you actually get in the lab when the chemicals are mixed 2. Theoretical yield- what the balanced equation tells should be made 3. Percent yield = Actual Theoretical x 100

Example: 6.78 g of copper is produced when 3.92 g of Al are reacted with excess copper (II) sulfate. 2Al + 3 CuSO4 ® Al2(SO4)3 + 3Cu What is the actual yield? What is the theoretical yield? What is the percent yield? = 6.78 g Cu = 13.9 g Cu = 48.8 %

Details on Yield Percent yield tells us how “efficient” a reaction is. Percent yield will not usually be larger than 100 %. Theoretical yield will usually be larger than actual yield. Why? Due to impure reactants; competing side reactions; loss of product in filtering or transferring between containers; measuring