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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. - PowerPoint PPT Presentation
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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 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
H
O
C
CH4 + O2 H2 O CO2+_____
2 Count the number of each element or ion on the reactants and products side.
_____ __________
How to Balance By Inspection:
Reactants Products
H
O
4
2 3
2
C 1 1
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
H
O
4
2 3
2
C 1 1
2
4
44
CH4 + O2 _____ H2 O CO2+
4
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!
Place a “1” in any empty coefficient location
_____ __________2
How to Balance By Inspection:
Reactants Products
H
O
4
2 3
2
C 1 1
2
4
44
1 1CH4 + O2 _____ H2 O CO2+
Choosing the Order of Balancing
Save for laterElements that are uncombined
Save for laterElements that appear more than 1 time per side
StartElements that appear
only 1 time per side
StartElements in most
complicated molecules
How do you know what order to balance in?
Pb + PbO2 + H+ Pb2+ + H2O
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?
Reactants Products
H
Pb
1
2 1
2
O 2 1
4
2
_____ __________1 21 2Pb Pb+ O2 H2 O Pb2++H+_____4+
2
4
O, H, Pb
2
_____
What about a different order?
How is it different if we balance in a different order?
Reactants Products
O
Pb
2
2 1
1
H 1
2
2
_____ __________1 21 2Pb Pb+ O2 H2 O Pb2++H+_____2+
2 4
4
4
You’ll still get to the correct answer, but it will take longer and be more complicated!
H, O, Pb
Polyatomic Ions
Polyatomic ion – Group of atoms that together has a net charge
e.g. Nitrate NO31-
Carbonate CO32-
OH is a polyatomic ion that is sometimes “hidden” in H2O.Re-write H2O as HOH to “see” the OH polyatomic ion.
Make a table of elementsYou may leave polyatomic ions together—IF they appear intact on both sides of the reaction.
__________ __________
Balancing with Polyatomic Ions:
Reactants Products
PO4
Ca
H
Ca (OH)2+H3 Ca3 (PO4)2 H2O+PO4
OH
HOH
1
2 Count the number of each element or ion on the reactants and products side.
__________ __________
Balancing with Polyatomic Ions:
Reactants Products
PO4
Ca
1
1 3
2
H 3 1
Ca (OH)2+H3 Ca3 (PO4)2 H2O+PO4
OH 2 1
HOH
3 Add coefficients to balance the numbers
__________ __________2
Balancing with Polyatomic Ions:
Reactants Products
PO4
Ca
1
1 3
2
H 3 1 6
3
3 6Ca (OH)2+H3 Ca3 (PO4)2 H2O+PO4
OH 2 1
HOH
6
2
6 6
Let’s Practice #1
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 1 1 2
HOH
Did you see the “OH” polyatomic ion & change H2O to HOH?
Let’s Practice #2
Example:Balance the
following equation
__ H2 + __ O2 __ H2O
Let’s Practice #2
Example:Balance the
following equation
__ H2 + __ O2 __ H2O2 1 2
Let’s Practice #3
Example:Balance the
following equation
__ Fe + __ O2 ___ Fe2O3
Let’s Practice #3
Example:Balance the
following equation
__ Fe + __ O2 ___ Fe2O34 3 2
