Download Mead Chemistry Lap 11: Stoichiometry Chapter 12 12.1 Balanced

Mead Chemistry
Lap 11: Stoichiometry
Chapter 12
12.1 Balanced Chemical Equations
A. Balanced equations
• Used to find how much reactant is needed
• Used to predict how much product will be made
• You can use amount of one substance to find the amounts of the other substances
• Quantity usually in moles or grams
• Stoichiometry: calculations of quantities involved in a chemical reaction
▫ Use balanced equation like a recipe
B. Interpreting Chemical Equations
• Can use balanced equation to find different quantities: atoms, molecules, moles, grams
• Consider: N2 +3H2 2 NH3
• Number of atoms
▫ Can count the number of atoms on both sides of an equation
▫ 2 atoms of N, 6 atoms of H
• Number of molecules
▫ Use coefficients to find # of molecules
▫ 1 molecule N2 reacts with 3 molecules H2 to form 2 molecules of NH3
• Number of moles
▫ Use coefficients to find # of moles
▫ __ mole of N2 reacts with __ moles of H2 to form __ moles of NH3
▫ Ratio of 1 N2: 3 H2 :2 NH3
• Mass in grams
▫ All reactions must follow law of conservation of mass
▫ Atoms and mass will always follow conservation
▫ Use mole ratios and molar mass of each element to find the amount of grams of
each reactant and product
• Example:
▫ 1 mole of N2
 2 x 14.0 g = 28.0 g
 28.0 g x 1 mole = 28. 0g
▫ 3 moles of H2
 2 x 1.0 g = 2.0 g
 2.0 g x 3 moles = 6.0 g
▫ 2 moles NH3
 14.0 g + (3 x 1.0) = 17.0g
 17.0 g x 2 moles = 34.0 g
▫ Add up reactants and products should be the same
 28.0 g + 6.0 g = 34.0 g
•
Example: 2H2S + 3O2  2SO2 + 2H2O
Find the number of molecules, moles and grams of the reactants and the products
Number molecules
Number moles
▫ H2S
▫
O2
▫
SO2
▫
H2O
To find the mass of each: Multiply the molar mass by the # moles
▫ H2S
▫
O2
▫
SO2
▫
H2O
12.2 Chemical Calculations
A. Mole Ratio
• Relate moles of reactants to moles of product
• Example: N2 +3H2 2 NH3
• 1 mole of N2: 3 moles H2: 2 moles NH3
• In calculations we will only use 2 at a time
• Can be reactant A: reactant B
•
Or reactant A: product
•
Or reactant B: product
•
Sample: Write the mole ratios for this equation
2H2S + 3O2  2SO2 + 2H2O
B. Mole to mole calculations
• We use mole ratios to find the number of moles of a reactant or product
• You must be given the # moles of one substance
• Steps
1. Find the given and unknown
2. Write a mole ratio including given and unknown
 The unknown should be on top
 The given should be on the bottom
3. Set up your conversion
 1st set of parenthesis: Write your unknown over 1
 2nd set of parenthesis: Write your mole ratio
 Cancel and calculate
•
Sample #1: How many moles of NH3 are produced from 0.60 moles of N2?
N2 +3H2 2 NH3
▫ Step 1: Determine the given and unknown
 Given ____________
Unknown_________
▫ Step 2: Find the mole ratio
▫
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Step 3: Set up conversion
Sample #2: How many moles of O2 are needed to react with 2.4 moles of Al?
4 Al + 3 O2  2 Al2O3
▫ Step 1: Determine the given and unknown
 Given ____________
Unknown_________
▫ Step 2: Find the mole ratio
▫
Step 3: Set up conversion
C. Mass to mass calculations
• We must convert grams to moles
• Then use mole ratio to find new moles
• Finally convert moles to grams
• Need to use molar mass and mole ratios
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Steps
1.
2.
3.
4.
Find given and unknown in grams (mass)
Convert given grams to given moles using molar mass of given
Use mole ratio to convert moles of given to moles of unknown
Convert moles of unknown to grams of unknown using molar mass of unknown
Sample #1: How many grams of NH3 is produced from 5.4 grams of H2?
N2 +3H2 2 NH3
Steps
1. Find the given and unknown in grams
Given ___________
Unknown_________
2. Convert grams of given to moles of given
3. Use mole ratio to convert moles of given to moles of unknown
4. Convert moles of unknown to grams of unknown
•
Sample #2: How many grams of Aluminum are needed to make 3.5 grams of Al2O3?
4 Al + 3 O2  2 Al2O3
Steps
1. Find the given and unknown in grams
Given ___________
Unknown_________
2. Convert grams of given to moles of given
3. Use mole ratio to convert moles of given to moles of unknown
4. Convert moles of unknown to grams of unknown
12.3 Percent Yield
• Actual reactions do not always produce the amount predicted
• Percent yield measures the efficiency of a reaction carried out in the lab
• Percent yield is often less than 100%
▫ Due to impure ingredients
▫ Side reactions occurring
▫ Loss during filtration or separation
▫ Faulty techniques
▫ Calculate percent yield:

Percent yield =
•
•
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▫ Actual yield: value from the experiment or lab
▫ Predicted yield: (also called theoretical yield) calculated in a mass-mass problem
Problems will give you a given mass and the actual yield
You will need to find the predicted yield using mass to mass
Then find the percent yield
actual yield X 100%
predicted yield
•
Sample: Find the percent yield if 24.8 grams of CaCO3 is heated and makes 13.1 grams
of CaO?
CaCO3  CaO + CO2
•
Step 1: Find given and actual yield of unknown
▫ Given = ________
Unknown = _____
▫ Actual yield unknown= __________
•
Step 2: Calculate the predicted value of unknown
▫ Complete a mass-mass problem
•
Step 3: Calculate percent yield