Transcript U3 Balancing Chemical Reaction Equations - Alliance Ouchi


I can compare and contrast different
types of chemical reactions.

I can define and identify a balanced
chemical equation.

I can balance simple chemical
equations.
Parts of a Chemical Reaction
Reactants
Products
Cu (s) + 2 AgNO3 (aq) → Cu(NO3)2 (aq) + 2 Ag (s)
Letters in
parentheses show
the state of matter.
The arrow separates
the reactants and
products.
Coefficients
(numbers) in front of
a chemical formula
show “how many.”
Balancing a Chemical Reaction Equation
Atoms are not created or destroyed during a chemical
reaction.
Scientist know that there must be the same number of
atoms on each side of the arrow.
Is it balanced?
__Cu + __ AgNO3 → __Cu(NO3)2 + __ Ag
Cu =
Cu =
Ag =
Ag =
NO3 =
NO3 =
Balancing a Chemical Reaction Equation
To balance the chemical equation, you must add
coefficients to the different parts of the equation.
Cu (s) + 2 AgNO3 (aq) → Cu(NO3)2 (aq) + 2 Ag (s)
Steps for Balancing Chemical Reaction
Equations
Determine number of atoms for each element for the
reactants and products separately.
Try to add coefficients to the equation to get the same
number of atoms on each side.
Note: Coefficients multiply the atoms in the
compound. Keep the compound formulas the same.
Do not change subscripts.
Hint: Balance anything that is not hydrogen or oxygen
first. Then balance hydrogen. Balance oxygen last.
http://www.youtube.com/watch?v=ODf_sPexS2Q
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dscreen
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Focus Questions:
› How are the two reactions the same? How
are they different? Why?
› If you wanted to do these reactions at
home, what would you need to know?

Matter cannot be created or destroyed in a
chemical reaction

The number of atoms on the reactant side must
equal the atoms of the products
H2 (g) +
H H
O2 (g) 
H2O (l)
O O
H O H
H H
Unbalanced!
H O H
Balanced!

A chemical equation is “balanced” when
there are the same number of atoms of each
element on both sides of the arrow.
NaHCO3 (s) + HC2H3O2 (aq) NaC2H3O2 (aq) +
CO2 (g) + H2O (l)
Is it balanced?
Reactants
Products
Na = 1
Na = 1
H=5
H=5
C=3
C=3
O=5
O=5
Determine the number of atoms of each
element in the reactants and products.
Then, state whether the equation is
balanced.
1. P4 + O2 → P2O5
Not balanced.
2. C3H8 + O2 → CO2 + H2O Not balanced.
3. Ca2Si + Cl2 → CaCl2 + SiCl4 Not balanced.
4. Si + CO2 → SiC + SiO2 Not balanced.
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coefficient
2 H (g) +
2
H= 2
O= 2

4
O2 (g)  2 H2O (l)
H= 2
4
O= 1
2
Only the coefficients can be
changed!
Always balance non-Oxygen’s &
Hydrogen’s first (ex. Chlorine)
 Then balance Hydrogens
 Balance Oxygens Last

P4 + O2 → P2O5
P4 + 5 O2 → 2 P2O5
Ca2Si + Cl2 → CaCl2 + SiCl4
Ca2Si + 4 Cl2 → 2 CaCl2 + SiCl4
Si + CO2 → SiC + SiO2
2 Si + CO2 → SiC + SiO2
C3H8 + O2 → CO2 + H2O
C3H8 + 5 O2 → 3 CO2 + 4 H2O
Synthesis
 Decomposition
 Single-Replacement
 Double-Replacement
 Combustion

Two substances combine to make one
 Synthesis means “to make”

H2 (g)+ O2(g) 
H H
H H
O O
H O O
H
H
H
H2O(l)
SYNTHESIS
Na + Cl2  NaCl
One substance breaks down into 2
 Decomposition means “to break down” or
“decay”

NH3 (l)  N2 (g) + H2 (g)
H
N
H
H
H
N
H
H
Decomposition
NaCl  Na + Cl2
3. SINGLE
REPLACEMENT
AB + C  AC + B
Single Replacement
NaBr + Cl2  NaCl + Br2
Both elements in two
compounds switch places
AB + CD  AD + CB
Double Replacement
NaBr + Pb2S3  Na2S +
PbBr3
K2SO4 + Ba(OH)2 KOH + BaSO4
A hydrocarbon reacts with oxygen gas to
produce carbon dioxide and water vapor
 Combustion means to “burn in oxygen”

CH4 + O2  CO2 + H2O
C2H4 + O2  CO2 + H2O
C6H12O6+ O2  CO2 + H2O