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Chapter 8 Stoichiometry

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**Section 8.1 The Arithmetic of Equations**

OBJECTIVES: Calculate the amount of reactants required, or product formed, in a non chemical process.

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**Section 8.1 The Arithmetic of Equations**

OBJECTIVES: Interpret balanced chemical equations in terms of interacting moles, representative particles, masses, and gas volume at STP.

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Cookies? When baking cookies, a recipe is usually used, telling the exact amount of each ingredient If you need more, you can double or triple the amount Thus, a recipe is much like a balanced equation

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**Stoichiometry Greek for “measuring elements”**

The calculations of quantities in chemical reactions based on a balanced equation. We can interpret balanced chemical equations several ways.

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**1. In terms of Particles Element- made of atoms**

Molecular compound (made of only non- metals) = molecules Ionic Compounds (made of a metal and non-metal parts) = formula units (ions)

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2H2 + O2 ® 2H2O Two molecules of hydrogen and one molecule of oxygen form two molecules of water. 2 Al2O3 ® 4Al + 3O2 2 formula units Al2O3 form 4 atoms Al and 3 molecules O2 2Na + 2H2O ® 2NaOH + H2

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**Look at it differently 2H2 + O2 ® 2H2O**

2 dozen molecules of hydrogen and 1 dozen molecules of oxygen form 2 dozen molecules of water. 2 x (6.02 x 1023) molecules of hydrogen and 1 x (6.02 x 1023) molecules of oxygen form 2 x (6.02 x 1023) molecules of water. 2 moles of hydrogen and 1 mole of oxygen form 2 moles of water.

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**2. In terms of Moles 2 Al2O3 ® 4Al + 3O2 2Na + 2H2O ® 2NaOH + H2**

The coefficients tell us how many moles of each substance

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3. In terms of Mass The Law of Conservation of Mass applies We can check using moles 2H2 + O2 ® 2H2O 2.02 g H2 2 moles H2 = 4.04 g H2 1 mole H2 32.00 g O2 1 mole O2 = 32.00 g O2 1 mole O2 36.04 g H2+O2 36.04 g H2+O2

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**In terms of Mass 2H2 + O2 ® 2H2O 2H2 + O2 ® 2H2O 18.02 g H2O**

2 moles H2O = 1 mole H2O 2H2 + O2 ® 2H2O 36.04 g H2 + O2 = 36.04 g H2O

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**4. In terms of Volume 2H2 + O2 ® 2H2O**

At STP, 1 mol of any gas = 22.4 L (2 x 22.4 L H2) + (1 x 22.4 L O2) ® (2 x 22.4 L H2O) NOTE: mass and atoms are always conserved- however, molecules, formula units, moles, and volumes will not necessarily be conserved!

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Practice: Show that the following equation follows the Law of Conservation of Mass: 2 Al2O3 ® 4Al + 3O2

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**Section 8.2 Chemical Calculations**

OBJECTIVES: Construct mole ratios from balanced chemical equations, and apply these ratios in mole-mole stoichiometric calculations.

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**Section 8.2 Chemical Calculations**

OBJECTIVES: Calculate stoichiometric quantities from balanced chemical equations using units of moles, mass, representative particles, and volumes of gases at STP.

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**Mole to Mole conversions**

2 Al2O3 ® 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 possible conversion factors

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**Mole to Mole conversions**

How many moles of O2 are produced when 3.34 moles of Al2O3 decompose? 2 Al2O3 ® 4Al + 3O2 3 mol O2 3.34 mol Al2O3 = 5.01 mol O2 2 mol Al2O3

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Practice: 2C2H2 + 5 O2 ® 4CO2 + 2 H2O If 3.84 moles of C2H2 are burned, how many moles of O2 are needed? (9.6 mol) How many moles of C2H2 are needed to produce 8.95 mole of H2O? (8.95 mol) If 2.47 moles of C2H2 are burned, how many moles of CO2 are formed? (4.94 mol)

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**How do you get good at this?**

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**Mass-Mass Calculations**

We do not measure moles directly, so what can we do? We can convert grams to moles Use the Periodic Table for mass values Then do the math with the mole ratio Balanced equation gives mole ratio! Then turn the moles back to grams Use Periodic table values

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For example... If 10.1 g of Fe are added to a solution of Copper (II) Sulfate, how much solid copper would form? 2Fe + 3CuSO4 ® Fe2(SO4)3 + 3Cu Answer = 17.2 g Cu

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More practice... How many liters of CO2 at STP will be produced from the complete combustion of 23.2 g C4H10 ? Answer = 35.8 L CO2 What volume of Oxygen would be required? Answer = 58.2 L O2

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**Volume-Volume Calculations**

How many liters of CH4 at STP are required to completely react with 17.5 L of O2 ? CH4 + 2O2 ® CO2 + 2H2O 22.4 L O2 1 mol O2 1 mol CH4 1 mol CH4 22.4 L CH4 1 mol O2 22.4 L CH4 17.5 L O2 22.4 L O2 2 mol O2 1 mol CH4 = 8.75 L CH4

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Avogadro told us: Equal volumes of gas, at the same temperature and pressure contain the same number of particles. Moles are numbers of particles You can treat reactions as if they happen liters at a time, as long as you keep the temperature and pressure the same.

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**Shortcut for Volume-Volume:**

How many liters of H2O at STP are produced by completely burning L of CH4 ? CH4 + 2O2 ® CO2 + 2H2O 2 L H2O 17.5 L CH4 = 35.0 L H2O 1 L CH4 Note: This only works for Volume-Volume problems.

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**Section 8.3 Limiting Reagent & Percent Yield**

OBJECTIVES: Identify and use the limiting reagent in a reaction to calculate the maximum amount of product(s) produced, and the amount of excess reagent.

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**Section 8.3 Limiting Reagent & Percent Yield**

OBJECTIVES: Calculate theoretical yield, actual yield, or percent yield, given appropriate information.

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“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

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**How do you find out? Do two stoichiometry problems.**

The one that makes the least product is the limiting reagent. For example Copper reacts with sulfur to form copper ( I ) sulfide. If 10.6 g of copper reacts with 3.83 g S how much product will be formed?

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**Cu is the Limiting Reagent because it creates less product**

If 10.6 g of copper reacts with 3.83 g S. How many grams of product will be formed? 2Cu + S ® Cu2S Cu is the Limiting Reagent because it creates less product 1 mol Cu 1 mol Cu2S 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 g Cu2S 3.83 g S 32.06g S 1 mol S 1 mol Cu2S = 19.0 g Cu2S

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Another example If 10.1 g of magnesium and 2.87 L of HCl gas are reacted, how many liters of gas will be produced? How many grams of solid? How much excess reagent remains? Limiting Reagent Simulation

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Still another example If 10.3 g of aluminum are reacted with 51.7 g of CuSO4 how much copper will be produced? How much excess reagent will remain?

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%

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**Yield The amount of product made in a chemical reaction.**

There are three types: 1. Actual yield- what you 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

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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?

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**% Yield Problem AY is 6.78 g of Cu**

TY – Use the balanced equation and do a mass to mass calculation 2Al + 3 CuSO4 Al2(SO4)3 + 3Cu 3.92 g x g 3.92 g Al 1 mol Al 3mol Cu 63.55 g Cu 13.9 g Cu 26.98 g Al 2 mol Al 1 mol Cu % Yield = (AY / TY) x 100 = (6.78g / g) x 100 = % yield

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**Details Percent yield tells us how “efficient” a reaction is.**

Percent yield can not be bigger than 100 %.

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The Heat of Reaction At the start of a reaction the reactants have a great heat content. Enthalpy is the amount of heat that a substance has at a given temperature and pressure (see Table 8.1 pg 190) The heat of a reaction is the heat that is released or absorbed during a chemical reaction. Heat of Reaction is represented by The symbol H

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**Heat can be added to make a reaction occur. This is an Endothermic**

Reaction H will be positive. The reaction can be represented as follows: 2 NH N H H = kJ or 2 NH kJ N H2

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**Cold Pack – Endothermic Reaction**

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**When heat is released by a chemical reaction, the reaction is**

said to be Exothermic. H will be negative Heat is most often released by a chemical reaction. The released heat can be represented by the following: Na + Cl NaCl H = kJ or Na + Cl NaCl kJ How can you calculate the heat of a reaction???

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**Glow Stick – Exothermic Reaction**

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**Calculate the heat of reaction ( H), in kilojoules, for the reaction**

Use table 8.1 pg 190 4 CO2(g) O2(g) CO(g) 4 (-393.5) 2 (0) + 4 (-110.5) (-1574) (0) + (-442) H = NRG of Products – NRG of Reactants H = (-442 kJ) – (-1574) H = 1132 kJ (+ indicates an endothermic rxn so) 4 CO2(g) kJ O2(g) CO(g) How much energy is used to make 100g of CO(g)?

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4 CO2(g) kJ O2(g) CO(g) x kJ 100g 100 g CO 1 mol CO 1132 kJ 1010 kJ 4 mol CO 28.01 g CO

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