Transcript Balancing Chemical Equations

CHEMICAL
REACTIONS &
BALANCING
EQUATIONS
Chemistry
LEARNING OBJECTIVES
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TLW use the law of conservation of
mass to write and balance chemical
equations (TEKS 8.D
TLW balance equations using 4-step “T”
Chart Process, Drop Down Approach, or
other method
AGENDA
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Law of Conservation of Mass
Cooking with Chemistry – A Recipe for Success
and Making Good Stuff
Chemical Equations & Coefficients
Balancing Act of Reactants and Products –
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The 4-Step “T” Chart Process
Drop Down Approach
Guided Practice
Independent Practice
Nuts & Bolts Lab
CONSERVATION
OF MASS
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Total MASS of PRODUCTS EQUALS
Total MASS of REACTANTS
CONSERVATION OF
MASS
 Mass
of WOOD and
AIR before burning
a fire
EQUALS
Mass of ASH and
GASES after
burning a fire
COOKING WITH
CHEMISTRY
CHEMICAL EQUATIONS ARE
LIKE RECIPES
They tell you QUANTITIES
of INGREDIENTS
(REACTANTS) and
QUANTITIES of PRODUCTS
Trail Mix
Ingredients (Reactants)
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Product
1 c. Mixed Dried Fruit
- 6 c. Yummy and
½ c. Raisins and/or
Healthy Snack
Dried Cranberries
1 c. Unsalted Nuts
peanuts, almonds, or walnuts
1 c. Shelled Seeds
sunflower, pumpkin
½ c. M & Ms
1 c. Cheerios
1 c. Pretzels
Comparing Food Recipes to
Chemical Equations
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How are they similar?
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How are they different?
Discovery Education Video
Time
link
CHEMICAL EQUATIONS
NUMBER of REACTANT
MOLECULES needed to MAKE a
CERTAIN NUMBER of PRODUCT
MOLECULES
2Na + Cl2 ----> 2NaCl
BALANCED
EQUATIONS
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Correct RATIOS of
Molecules are shown as
COEFFICIENTS in
BALANCED EQUATIONS
2H2 + O2 ----> 2H2O
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Coefficients must be
whole numbers
In CHEMICAL REACTIONS,
BONDS BREAK and REFORM
ATOMS are REARRANGED
REACTANTS CHANGE to FORM NEW
PRODUCTS with DIFFERENT
PROPERTIES
CHEMICAL EQUATIONS
 Use
FORMULAS and SYMBOLS
to show REACTION
 CONSERVATION OF ATOMS
 Number of each type of atom
in REACTANTS and
PRODUCTS MUST BALANCE
How to Balance an Equation
A. Rules to remember
1. You can not change the
chemical formula:
NaCl must stay NaCl
2. You must leave subscripts alone:
AlCl3 stays AlCl3
You can NOT change the 3
3. You can only add coefficients
a. coefficients are numbers that
go in front of the chemical
formula
b. 3 FeCl2 + 2 Al
the 3 and 2 are coefficients..
(note - 1 is not written down)
 2 AlCl3 + 3 Fe
Tips
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Balance most complex compounds or
metals first (look at charges)
Balance non-metals next, except O & H
Balance O and H last
Look for polyatomic groupings and
balance those as groups
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The polyatomic group must be on both
sides of equation
Examples: NO3, OH, SO4
Al2(SO4)3 + Ca(OH)2  Al(OH)3 +
CaSO4
Tips
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If you have water (H2O), you can rewrite
it as H and OH separately
You can have an interim step of coefficient
with decimal…. But must multiply through
to get whole numbers as final balancing
act
Chemical Equations
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Remember the Criss-Cross Rule when
working with compounds
Balance the charges of ions so molecule
has zero (neutral) charge
Begin with the positive one first (Fe+3)
Then bring in negative ion (O-2)
Criss-cross the numbers  Fe2O3
 Use
the criss-cross approach of
balancing charges
 Example:
Al2(SO4)3 + 3Ca(OH)2 
2Al(OH)3 + 3CaSO4
Group Practice
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Recognizing How Balanced Equations Look
Recap Rules and Tips
link
Processes to Use
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Some people can see how to balance
equations
Other folks have developed methods such
as:
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4-Step “T” Chart Process
Drop Down Approach
Draw T-chart
4. Recount
Drop Down Approach
N2 + O2  NO2
Re-write to visually show number of
atoms using symbols in each
molecular compound
NN + OO  NOO
NN OO
NOO
OO
NOO
Count rows and you have coefficients
N2 + 2O2  2NO2
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Internet Drop Down Approach Game Classic Chembalancer
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http://funbasedlearning.com/chemistry/chem
Balancer/ques1.htm
Group Practice
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Together let’s work through Guided
Reading and Study Workbook pages 79 &
80
More Group Practice
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P4 + 5O2  ___ P2O5
4Fe + 3O2  ___ Fe2O3
CS2 + 3O2  ___ CO2 + ___ SO2
___N2O5 + __ NO  4N2 + 9O2 + NO3
__ KOH + __ HCl  KCl + H2O
More Fun with Balancing
Equations
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Internet Game
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http://education.jlab.org/elementbalancing/in
dex.html
Individual Practice
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Practice Sets
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Balancing Equations – 2
Balancing Equations and Identifying Reactions
-3
Hard copy of other worksheets