Transcript Chemistry You Need to Know

Section 2.7—Balancing
Equations
We need to finish writing those equations we started!
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 Matter
Therefore…
Matter cannot be
created nor
destroyed
during a
chemical or
physical
change
The matter on
the reactants
side and the
matter one the
products side
must be the
same
So we must…
Ensure the
numbers of
each type of
atom are the
same on both
sides of the
equation…by
balancing!
How do we Balance Equations?
Number of
compounds in
the reaction
Coefficients
2 H2 + O2  2 H2O
Subscripts
# of atoms in a
compound
Subscripts balance charges within a compound.
Coefficients balance atoms in an equation
What do Coefficients Really Mean?
CH4 + 2 O2  CO2 + 2 H2O
H
H
C
H
O
O
O
H
O
O
C
O
H
O
O
H
Total:
1C
4H
4O
The equation is balanced.
H
Total:
1C
4H
4O
H
How to Balance Chemical
Equations
How to Balance By Inspection:
1
Make a table of elements
_____ CH4 + _____ O2  _____H2 O + _____ C O2
Reactants
C
H
O
Products
How to Balance By Inspection:
2
Count the number of each element or ion on the
reactants and products side.
Don’t forget to add all the atoms of the same element together—
even if it appears in more than one compound!
_____ CH4 + _____ O2  _____H2 O + _____ C O2
Reactants
Products
C
1
1
H
4
2
O
2
3
How to Balance By Inspection:
3
Add coefficients to balance the numbers
Each time you add a coefficient, update your table with the new
quantities of each atom.
_____ CH4 + _____
2 O2 _____
2 H2 O + _____ C O2
Reactants
Products
C
1
1
H
4
2
4
O
2
3
4
4
How to Balance By Inspection:
4
Place a “1” in any empty coefficient location
Filling each coefficient location lets you and the grader know that you
finished the problem rather than you left some blank because you
weren’t done!
_____
1 CH4 + _____
2 O2 _____
2 H2 O + _____
1 C O2
Reactants
Products
C
1
1
H
4
2
4
O
2
3
4
4
Choosing the Order of Balancing
How do you know what order to balance in?
Start
Elements that appear
only 1 time per side
Save for later
Elements that are
uncombined
Pb + PbO2 + H+  Pb2+ + H2O
Save for later
Elements that appear
more than 1 time per side
Start
Elements in most
complicated molecules
To balance this equation, use the order: O, H, Pb
How is Balancing Affected by Order?
What happens if we balance in the order
determined in the last slide?
O, H, Pb
_____
1 Pb + _____
1 Pb O2 + _____
4 H+ 
_____
2 H2 O + _____
2 Pb2+
Reactants
O
2
H
1
Pb
2
4
Products
1
2
2
4
1
2
What about a different order?
How is it different if we balance in a different order?
H, O, Pb
_____
1 Pb + _____
1 Pb O2
4
+ _____
2 H+ 
_____
2 H2 O + _____
2 Pb2+
Reactants
2
4
2
1
2
2
1
2
H
1
O
Pb
2
4
Products

You’ll still get to the correct answer, but it will take
longer and be more complicated!
Polyatomic Ions
Polyatomic ion – Group of atoms that
together has a net charge
e.g.
Nitrate
Carbonate
NO31CO32-
Balancing with Polyatomic Ions:
1
Make a table of elements
You may leave polyatomic ions together—IF they appear intact on
both sides of the reaction.
OH is a polyatomic ion that is sometimes “hidden” in H2O.
Re-write H2O as HOH to “see” the OH polyatomic ion.
HOH
_____ H3PO4
+
_____ Ca (OH)2  _____ Ca3 (PO4)2
Reactants
H
PO4
Ca
OH
+
Products
_____ H2O
Balancing with Polyatomic Ions:
2
Count the number of each element or ion on the
reactants and products side.
HOH
_____ H3PO4
+
_____ Ca (OH)2  _____ Ca3 (PO4)2
Reactants
+
Products
H
3
1
PO4
1
2
Ca
1
3
OH
2
1
_____ H2O
Balancing with Polyatomic Ions:
3
Add coefficients to balance the numbers
HOH
+
_____
2 H3PO4
_____
3 Ca (OH)2  _____ Ca3 (PO4)2
Reactants
+
Products
H
3
6
1
PO4
1
2
2
Ca
1
3
3
OH
2
6
1
6
6
_____
6 H2O
Balancing with Polyatomic Ions:
5
Place a “1” in any empty coefficient location
HOH
+
_____
2 H3PO4
_____
3 Ca (OH)2  _____
1 Ca3 (PO4)2
Reactants
+
Products
H
3
6
1
PO4
1
2
2
Ca
1
3
3
OH
2
6
1
6
6
_____
6 H2O
Let’s Practice #1
Example:
Balance the
following
equation
__ HCl + __ Ca(OH)2  __ CaCl2 + __ H2O
Let’s Practice #1
Example:
Balance the
following
equation
Did you see the “OH” polyatomic ion &
change H2O to HOH?
2 HCl + __
1 Ca(OH)2  __
1 CaCl2 + __
2 H2O
__
HOH
Let’s Practice #2
Example:
Balance the
following
equation
__ H2 + __ O2  __ H2O
Let’s Practice #2
Example:
Balance the
following
equation
2 H2 + __
1 O2  __
2 H 2O
__
Let’s Practice #3
Example:
Balance the
following
equation
__ Fe + __ O2  ___ Fe2O3
Let’s Practice #3
Example:
Balance the
following
equation
4 Fe + __
3 O2  ___
2 Fe2O3
__