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Section 2.7—Balancing Equations
Objectives:•Use the Law of Conservation of Mass to explain the need for balanced equations•Convert word equations to balanced chemical equations
Review
1. How to write chemical formulasa. For ionic compounds:
1) write the symbol and charge for the cation
2) write the symbol and charge for the anion
3) criss-cross the NUMBER only of the charge
to obtain a subscript
4) Reduce the subscripts, if necessary
5) Use parentheses when there is more than
one polyatomic ion needed.
Review
b. For binary covalent compounds:
1) Write the symbols of the first and
second elements
2) Translate any PREFIXES into a
subscript of that element
Example: dinitrogen pentoxide
N2O5
Review
c. For acids without oxygen (the acid will be named with the prefix “hydro”): 1) The cation will be H+
2) The anion will be found in the root of the acid’s name (following the “hydro”) – determine its symbol & charge 3) Criss-cross the numbers of the charges to get the formula
Review d. For acids with oxygen (acid name will end in “ic” or “ous”): 1) The cation is H+
2) If the acid ends in “ic”, look for a polyatomic anion that ends in “ate” 3) If the acid ends in “ous”, look for a polyatomic anion that ends in “ite” 4) Criss-cross the numbers of the charges to get the formula
REMEMBER: Bases are just ionic compounds with the anion “hydroxide”. NH3 (ammonia) is an exception.
Review
e. There are seven elements that are found in nature as diatomic molecules. They must be written that way in a formula.
H2 N2 O2 F2 Cl2 Br2 I2
Review
2. How to write formula equations
a. Locate each name of a compound and
write its formula (using those rules
reviewed in previous slides.)
b. Separate all reactants and products
with a “+” sign
c. Separate reactants ffrom products with
an
Practice
Hydrogen and oxygen react to form water.
Law of Conservation of Matter/Mass
Notice that, in this practice equation, the number of atoms that we start with does not equal the number of atoms we finish with.
2 H atoms + 2 O atoms 2 H atoms + 1 O atom
This violates a scientific law that says matter cannot be destroyed (or created.)
Law of Conservation of Matter/Mass
Law of Conservation of Matter – Matter cannot be created nor destroyed during chemical or physical changes
Also called the Law of Conservation of Mass (since all matter has mass)
How Does the Law Lead to Balancing?
Law of Conservation
of MatterTherefore… So we must…
Matter cannot be created nor destroyed during a chemical or physical change
The matter on the reactants side and the matter on the products side must be the same
Ensure the numbers of each type of atom are the same on both sides of the equation…by balancing!
How do we Balance Equations?
2 H2 + O2 2 H2O
Subscripts
Coefficients
# of atoms in a compound
Number of compounds in the reaction
Subscripts balance charges within a compound.
Coefficients balance atoms in an equation
What do Coefficients Really Mean?
CH4 + 2 O2 CO2 + 2 H2O
Total:1 C4 H4 O
Total:1 C4 H4 O
The equation is balanced.
H
C
H
HH
O O
O O
CO O HO
H
HO
H
H
CC
H
HH
O O
O O
CCO O HO
H
HO
H
How to Balance Chemical Equations
Make a table of elements
_____ __________
How to Balance By Inspection:
1
Reactants Products
CH4 + O2 H2 O CO2+_____
C CH HO O
2 Count the number of each element on the reactants and products side.
_____ __________
How to Balance By Inspection:
Reactants Products
4 H
2 O 3 O
2 H
1 C 1 C
CH4 + O2 H2 O CO2+_____
Don’t forget to add all the atoms of the same element together—even if it appears in more than one compound!
3Each time you add a coefficient, update your table with the new quantities of each atom.
ADD COEFFICIENTS TO BALANCE THE NUMBERS
_____ __________2
How to Balance By Inspection:
Reactants Products
4 H
2 4 O 3 4 O
2 4 H
1 C 1 C
2CH4 + O2 _____ H2 O CO2+
Let’s Practice #1
Example:Balance the
following equation
__ H2 + __ O2 __ H2O
Let’s Practice #2
Example:Balance the
following equation
__ H2 + __ O2 __ H2O2 2
Let’s Practice #2
Example:Balance the
following equation
__ HCl + __ Ca(OH)2 __ CaCl2 + __ H2O
Let’s Practice #1
Example:Balance the
following equation
__ HCl + __ Ca(OH)2 __ CaCl2 + __ H2O2 2
Did you see the “OH” polyatomic ion & change H2O to HOH?
Let’s Practice #3
Example:Write and
balance the following equation
Iron and oxygen combine to produce iron (III) oxide.
Let’s Practice #3
Example:Write and
balance the following equation
__ Fe + __ O2 ___ Fe2O34 3 2