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Warm Up 3/29-30
 1) What type of chemical reaction is represented in the
energy diagram shown below?
 2) What side of the chemical equation does the energy
term go on? Explain.
Unit 8: Stoichiometry Pt I
The study of mass and mole
relationships in chemical reactions
Objectives
Given a chemical reaction stated in words, write
a balanced chemical equation.
Starting with a balanced chemical equation, and
the number of moles of a reactant or product,
determine the number of moles of any other
reactant or product involved.
Develop the BCA approach to analyzing mole
relationships in chemical reactions.
What does a chemical equation tell you?
 Methane when burned in air forms carbon dioxide and water
 ___CH4 + ___O
2 2  ___CO2 + ___H
2 2O
 What does that mean?
 1 molecule of CH4 combines with 2 molecules of oxygen to
form 1 molecule of CO2 and 2 molecules of H2O
 From our nail lab, we know it also means that:
 1 mole of CH4 combines with 2 moles of oxygen to form 1
mole of CO2 and 2 moles of H2O
 Further, from conservation of mass, it means:
 16g of CH4 combines with 64g of oxygen to form 44g of CO2
and 36g of H2O
 (80 g of reactants and 80 g of product)
Chemical rxns mean chemical change
 Using the BCA approach, we are going to analyze the
reaction during three stages of the reaction:
 How much of each reactant and product is present in the
reaction vessel just before the reaction
 How much change occurs in each substance during the
reaction
 What is present in the reaction vessel immediately after the
reaction is complete.
Example 1:
Lead will react with hydrochloric acid to produce lead(II)
chloride and hydrogen gas. How many moles of hydrochloric
acid are needed to completely react with 4.0 mole of lead?
Equation: __Pb + 2__HCl  __PbCl2 + __H2
Before: 4.0 mol xs mol
Change: -4.0 mol -8.0 mol
After:
0 mol
xs
0 mol
+4.0 mol
4.0 mol
0 mol
+4.0 mol
4.0 mol
8.0 mol of HCl are needed for this reaction
Example 2:
How many moles of hydrogen gas will be produced if 2.5 moles of
calcium hydride react according to the following equation?
2 O  __Ca(OH) + __H
2
Equation: __CaH2 + __H
2
2
2
Before: 2.5 mol
Change: -2.5 mol
After:
0 mol
xs mol
-5.0 mol
xs
0 mol
+2.5 mol
2.5 mol
0 mol
+5.0 mol
5.0 mol
5.0 mol of H2 are produced by this reaction
Example 3:
 How many moles of water will be produced if 0.45 mol of oxygen
reacts according to the following equation?
2 6H6 + 15__O2  __CO
12
6 2O
 Equation: __C
2 + __H
This ones not so easy to do in your head, lets use the mole ratios
to determine amounts
 Notice we used the moles
æ 2mol C6 H 6 ö
0.45mol O2 ç
= 0.060mol C6 H 6
given
÷
è 15mol O ø
2
æ 12mol CO2 ö
çè 15mol O ÷ø = 0.36mol CO2
2
æ 6mol H 2O ö
çè 15mol O ÷ø = 0.18mol H 2O
2
 “Start with the number
you are given”
 Now, plug these values into
our BCA table
Example 3:
How many moles of water will be produced if 0.45 mol of oxygen
reacts according to the following equation?
Equation: 2C6H6 + 15O2  12CO2 + 6H2O
Before: xs
0.45 mol
0 mol
0 mol
Change: -0.060mol-0.45mol +0.36 mol +0.18 mol
After:
xs
0 mol
0.36 mol 0.18mol
So, 0.45 mol O2 with xs benzene produces 0.18 moles of water
Assignment
Read Stoichiometry Handout
HW: Unit 8 WS #1
Due Thursday/Friday
Just Do It