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?
• We have to have the same
number and types of atoms on
each side of the reaction!
• We CAN’T lose atoms as we
move from reactant to product!!
How do we Balance Equations?
Let’s start with discussing what the numbers mean!!
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
Tips for Successful Balancing:
 Use correct chemical formulas
 Try balancing atoms that appear only once on each side, first
 Try balancing atoms that appear more than once on a side
until later
 Polyatomic ions that are on each side should be treated as one
species to make them easier.
 An element that appears alone on one side can be balanced
last since it can be any multiple required
Tips to Balancing
How do you know what order to balance in?
Start With:
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 With:
Elements in most
complicated molecules
To balance this equation, use the order: O, H, Pb
Polyatomic Ions and the balanced equation
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-1 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
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?
Should we 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
__