Chapter 3. Stoichiometry:
Calculations with Chemical Formulas
and Equations
Thermite
reaction
Demo: minithermite
reaction
CHMVID13
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3.1 Chemical Reactions
What happens during a chemical reaction?
What distinguishes a chemical change from
changes in physical properties?
Draw a picture of a chemical reaction,
showing the changes that occur at a
molecular level.
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Hydrogen Reacts with
Oxygen
Reactants  Products
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Chemical Reactions
Several levels of examining chemical
reactions:
 Macroscopic changes accompanying the
chemical changes
 Changes in microscopic structure
 Rearrangement of atoms
 Representation of reactions with equations:
thermite reaction
2Al(s) + Fe2O3(s)  Al2O3(s) + 2Fe(l)
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Dehydration/Hydration of
CuSO4
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CuSO4.5H2O(s)  CuSO4(s) + 5H2O(g)
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Writing Chemical Equations
Compact representation of chemical reactions
Features: reactants  products
(with physical state noted)
2Al(s) + Fe2O3(s)  Al2O3(s) + 2Fe(l)
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Balancing Equations
 Balancing equations:
 Law of Conservation of Mass
 Conservation of atoms
 Do we need to conserve molecules?
2H2O2(aq)  2H2O(g) + O2(g)
 Adjust coefficients and not subscripts to balance an
equation
 Demo: Elephant’s Toothpaste/Genie in Bottle
 Coefficients are usually integers
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Examples of chemical
reactions
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Fe burning
4Fe(s) + 3O2(g)  2Fe2O3(s)
What is wrong with this drawing?
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Balancing chemical equations
Demo: methanol cannon
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Balance the reaction:
CH3OH(g) + O2(g)  CO2(g) + H2O(g)
Software: Balance.exe
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Guidelines to balancing
equations
Write correct formulas for reactants and
products
Begin balancing with the most complex
formula
Balance polyatomic ions as a single unit
Check each reactant and product to verify
the coefficients (check atom balance)
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Balancing chemical equations
Balance the following equations:

Ca3N2 + 6 H2O  3 Ca(OH)2 + 2NH3
2 H2O + O2
 2 KClO3 2KCl + 3O2
 N2O5 + H2O 2 HNO3
 2 C2H6 + 7 O2 4CO2 + 6 H2O
 2Na + 2H2O 2NaOH + H2
 2 H2O2
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Group Quiz Format
Place TA name and section letters (BA,
or BB, or … , or BH) in upper left
corner of paper.
Place name of group members
participating in the quiz in the upper
right corner. Please use correct
spelling and write or print legibly.
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Group Quiz 3
Balance the following equation:
C8H18 + O2  CO2 + H2O
2C8H18 + 25O2  16CO2 + 18H2O
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