Stoichiometry Unit 8 Lesson 1

Stoichiometry “The study of quantitative relationships between amounts of reactants used and products formed” Stoichiometry is based on the Law of Conservation of Matter Example: 2H2 + O2  2H2O Reads stoichiometrically as “2 moles of hydrogen + 1 mole of oxygen make 2 moles of H2O”

Balanced Equation = Mole Relationship The coefficients in a balanced equation indicate the number of particles (atoms/molecules) of each substance involved. The coefficients also indicate the number of moles of each substance needed or created. N2H4 + 2H2O2  N2 + 4H2O 1 mol N2H4 + 2 mol H2O2 = 1 mol N2 + 4 mol H2O

Mole Relationship = Conversion Factor (using coefficients from balanced equation) N2H4 + 2H2O2  N2 + 4H2O 1 mole N2H4 = 4 moles H2O 2 moles H2O2= 4 moles H2O So 1 mole N2H4 = 2 moles H2O2 1 mole N2H4 = 1 mole N2 2 moles H2O2= 1 mole N2 So 1 mole N2H4 = 2 moles H2O2 (again)

Using the Conversion Factor (coefficients from balanced equation) If 1 mole N2H4 = 2 moles H2O2: How many moles of H2O2 are needed to react completely with 3.5 moles of N2H4? 7.0 moles H2O2

Using the Conversion Factor (coefficients from balanced equation) If 1 mole N2H4 = 4 moles H2O: How many moles of water can be made if we react 3.5 moles of N2H4? 14 moles H2O

Steps for Solving Stoichiometry 1. Write & balance chemical equation 2. Plan conversions using coefficients from balanced equation 3. Write your conversion factors and check units 4. Calculate 5. Adjust for significant figures and include units

Steps for Solving Stoichiometry Example: How many moles of Carbon Dioxide are created from the combustion of 5.0 moles of propane (C3H8)? Step 1: Write & balance equation C3H8 +O2  H2O + CO2 C3H8 +5O2  4H2O + 3CO2 Step 2: Plan conversions 1 mole C3H8 = 3 moles CO2

Steps for Solving Stoichiometry Step 3: Write your equation using coefficients from balanced equation Step 4: Calculate 5.0 X 3 = 15 Step 5: Sig Figs and units 15 moles CO2 3 moles CO2 1 mole C3H8

Mole-Mass Problems 2Na + Cl2  2NaCl Na + Cl2  NaCl Same steps, just need to add a conversion for turning moles into mass Moles A  Moles B Mass B Example: Determine the mass of NaCl produced when 1.25 moles of chlorine gas reacts with sodium metal. Na + Cl2  NaCl 2Na + Cl2  2NaCl

Mass-Mass Problems 4.3 g Al2O3 Adds one more step: Mass A  Moles A Moles B  Mass B Example: In a thermite reaction, powdered aluminum reacts with iron(III) oxide to produce aluminum oxide and molten iron. What mass of aluminum oxide is produced when 2.3 g of aluminum reacts with iron(III) oxide? 2Al + Fe2O3  Al2O3 + 2Fe 4.3 g Al2O3

Volume-Volume Problem What volume of hydrogen gas, along with nitrogen gas, is necessary to produce 16.0 L of ammonia gas (NH3) at STP? N2 + 3H2  2NH3 24.0 L H2

Sample Problem How many moles of bromine (Br2) are produced when fluorine reacts with 347.5 g of potassium bromide? F2 + 2KBr  Br2 + 2KF 1.460 mol Br2

Sample Problem How many liters of oxygen gas are necessary for the combustion of 340. g of ethanol (C2H5OH) gas at 0°C and 1 atm? C2H5OH +3O2  3H2O + 2CO2 496 L O2