Ch 9 section 1
Need books and notes
Background knowledge: what do you already
know? vocab?
Check that you can answer the essential
questions as you study
#1 is signs of a chemical reaction!
Performance Goal:
Writing a Chemical Equation:
◦ Must be balanced and include proper
symbols
What is the difference between:
Word equation
Skeleton equation
Chemical equation
Read and discuss in small groups for 2 minutes
Word- no formulas
Skeleton- formulas but not balanced
Chemical- formulas AND it’s balanced
is a BALANCED equation
reactants, products and how much of
each
(visual page 285)
Which one is the performance goal?
We must start and end with the same
stuff. WHY? P. 288
Besides formulas:
See page 283 vocab and symbols used
Conservation of Mass
In your notes ADD this symbol: ∆
When heating a reaction– place the
symbol ∆ above the arrow
subscripts tell you
◦ When balancing:
◦ You add coefficients in front of the compounds
◦ but you may
not
change the subscripts.
◦ how many atoms of a particular
element are in a compound.
(chapter 7 and 8)
The coefficient tells you
◦ about the quantity of that compound in
a reaction
◦ WHY????
(chapter 9)
Al (s) + O2 (g)
Al2O3 (s)
Use LCM idea: 2 and 3 of oxygen: use total of 6
Al (s) + 3 O2 (g)
4Al (s) + 3 O2 (g)
2 Al2O3 (s)
2 Al2O3 (s)
take the coefficient TIMES subscript so there are 6
O’s on the left and the right
1) Polyatomics
that are on BOTH sides- balance these as a
unit
FYI:
Time saver- instead of doing individual atoms
making up the poly, you do the whole unit
2) Singletons (“solo”acts)
Balance atoms (except H and O) that appear only
ONCE on EACH side
3) Balance other atoms
• 4) Usually save H or O for last (they may
appear in several compounds) and are then
easier if last
• There is no one right way every time
• (your book has a general guide on p. 286-288)
AgNO3 + Cu
Cu(NO3)2 + Ag
Balance polys first
2 AgNO3 + Cu
Cu(NO3)2 + Ag
Then Ag falls out nicely
2 AgNO3 + Cu
Cu(NO3)2 + 2 Ag
◦ No need for N or O if you do the poly as a whole!
Ag NO3
1
1
Cu
1
Cu NO3
1
2
Ag
1
2 AgNO3 + Cu
2
2
1
Cu(NO3)2 + Ag
1
2
1
2 AgNO3 + Cu
2 2
1
Cu(NO3)2 + 2 Ag
1 2
2
Al2O3 + CuCl2
CuO + AlCl3
This type you can start anywhere;
(every element is a “solo act”!)
Al2O3 + CuCl2
CuO + 2AlCl3
Then balance Cl
Al2O3 + 3 CuCl2
CuO + 2 AlCl3
Then Cu
Al2O3 + 3 CuCl2
3CuO + 2 AlCl3
I used Al first for this ex.
O is already done!
PbSO4 + AlF3
Where do you start?
PbF2 + Al2(SO4)3
Do sulfate first
3 PbSO4 + AlF3
PbF2 + Al2(SO4)3
◦ Then Pb (or Al, F, doesn’t matter!)
3 PbSO4 + AlF3
3 PbSO4 + 2 AlF3
Last check Al
3 PbF2 + Al2(SO4)3
3 PbF2 + Al2(SO4)3
There is a category of equations called
COMBUSTION
It is a C and H compound burning in oxygen
(remember the element oxygen is a diatomic!)
PRODUCTS are CO2 and H2O
EX: __ CH4 + __ O2
This is the order of elements to balance
Works EVERY time!
Carbon 1st, using CO2
then H using the water
then finish with O on the left with the diatomic oxygen
___ CO2 + ___ H2O
__ CH4 + __ O2
___ CH4 + __ O2
___ CO2 + ___ H2O
___ CO2 + _2_ H2O
Oxygen: Have 4 oxygen on the right
Carbons are equal, move to H
(2 from CO2 and 2 from H2O)
4 H on left, Need 2 water on right
◦ put a 2 for oxygen on the left
___ CH4 + __ O2
___ CO2 + _2_ H2O
4 H’s on left and now 4 H’s on right
CH4 + 2 O2
CO2 + 2 H2O
EX: ___ C4H10 + __ O2
__ CO2 + __ H2O
4 C’s need 4 CO2’s
___ C4H10 + __ O2
4 CO2 + __ H2O
Tip: Odd # of O on the right?
Put it over 2, (Why? X times 2 = 13, Solving for X)
then double all coefficients.
___ C4H10 + 13/2 O2
10 H’s need 5 waters
___ C4H10 + __ O2
___ C4H10 + __ O2 4 CO2 + 5 H2O
On right side: 8 O + 5 O = 13 O
4 CO2 + 5 H2O
2 C4H10 + 13 O2
4 CO2 + 5 H2O
8 CO2 + 10 H2O
Why double- no half molecules!
EX: __C3H8O3 + __ O2
__C3H8O3 + __ O2
__CO2 + __ H2O
__C3H8O3 + __ O2 3 CO2 + 4 H2O
Right side: 6 O + 4 O = 10 O
3 CO2 + __ H2O
Left: 3 are already in formula,
__C3H8O3 + __ O2
Watch out here
3 CO2 + 4 H2O
Need 10-3 = 7 oxygens needed using the O2
molecule
X times 2 = 7
X = ? (7/2)
__C3H8O3 + 7/2 O2
3 CO2 + 4 H2O
Last DOUBLE all coefficients:
On to the packet of 50 balancing equations
2 C3H8O3 + 7 O2
Do not put 1’s in the blanks
1’s are understood
6 CO2 + 8 H2O
If an equation is already balanced (all 1’s) simply mark
it so- check mark, or write balanced (bal.) or write done
for example
Sodium phosphate + iron (III) oxide
sodium oxide + iron (III) phosphate
Skeleton first:
NEED FORMULAS!!
Sodium phosphate + iron (III) oxide
sodium oxide + iron (III) phosphate
Na3PO4 +
Na2O +
Fe2O3
FePO4
Poly is equal on both sides;
Your choice, either Fe or Na first
Na3PO4 +
Fe2O3
Na2O +
Sodium phosphate + iron (III) oxide
oxide + iron (III) phosphate
FePO4
2 Na3PO4 + Fe2O3
sodium
3 Na2O + 2 FePO4