Day 18 Balancing Chemical
Equations
Sci 10
Chemistry
Review
1. What number represents the Coefficient? _____
5H2
2. What number represents the Subscript? _____
3. What element is represented by the letter "H"? _____
4. How many "H's" do you have? _____
Counting Atoms

How many of each type
of atom are present in
the compounds below?






H2SO4
CaCl2
4NaF
2KNO3
3K2SO4
2Mg3(PO4)2
Type of Atom
# of Atoms
Law of Conservation of Mass

Law of conservation of mass: mass of
products = mass of reactants

Video: http://www.youtube.com/watch?v=dExpJAECSL8

the # of atoms of each type in the reactants =
the # of atoms of each type in the products

“reacts
with”
Matter can’t be created or destroyed: atoms can’t be
gained or lost during a chemical reaction
A + B → C + D
Reactants
“produces/ gives”
Products
Balancing Chemical Equations
Skeleton equation: CH4(g) + O2(g)  CO2(g) + H2O(l)
•
The skeleton equation needs coefficients to balance
Subscripts
both sides.
•
show the # of atoms *YOU CANNOT
CHANGE THESE TO BALANCE AN EQUATION
-
Balanced equation: CH4(g) + 2O2(g)  CO2(g) + 2H2O(l)
•
State of matter
Coefficients
indicate the ratio of
compounds in the reaction
-
here, there is twice as much
NO and NO2 than there is O2
-
(aq) = aqueous/dissolved in
water
(s) = solid
( l ) = liquid
(g) = gas
Types of Chemical Equations



Word Equation (words only, no formulas):
Methane + oxygen  carbon dioxide + water
Skeleton Equation (formulas, but not balanced):
CH4(g) + O2(g)  CO2(g) + H2O(l)
Balanced Equation:
Remember:



You can only change the coefficients (the
number in front) to balance
You can not have coefficients that are
fractions
Double check to make sure coefficients can’t
be reduced
Examples
+
Hg
2.
Aluminum reacts with oxygen to form
aluminum oxide.
3.
CaO
+
O2

1.
H 2O

HgO
Ca(OH)2
Strategies for Balancing Equations

Balance chemical equations by following these
steps:







Trial and error will work, but can be very inefficient
Balance compounds first, elements last
Balance one compound at a time
Only add coefficients; NEVER change subscripts!
If H and O appear, attempt to balance them LAST
Polyatomic ions (such as SO42–) can be balanced as a
whole group. They don’t break apart!
Always double-check after you think you are
finished!
More examples
Balance the following:
1.
__ N2 (g) + ___ H2 (g) → ___ NH3 (g)
__ N2 (g) + _3_ H2 (g) → _2_ NH3 (g)
2.
__ Fe (s) + ___ H2SO4 (aq) → ___ H2 (g) + Fe2(SO4)3 (aq)
2_ Fe (s) + _3_ H2SO4 (aq) → _3_ H2 (g) + Fe2(SO4)3 (aq)
Balancing Word Equations
1.
2.
3.
Write the skeleton equation. Remember to
ensure ionic compounds have a neutral
charge (+ and – charges must add to zero)
Then balance
Check coefficients for common factors. If
yes, reduce them.
Try It!

sodium chloride
iron
+
1.
sodium + chlorine
2.
iron (II) oxide
3.
methane + oxygen  carbon dioxide + water
4.
copper (I) oxide


oxygen
copper +
oxygen
Stop & Think!
 Try balancing these:
Fe + Br2  FeBr3
 CaCl2 + Na  NaCl + Ca
 Sn(NO2)4 + K3PO4  KNO2 + Sn3 (PO4)4
 C2H6 + O2  CO2 + H2O

To Do:


Day 18 Practice 1
Text:



p.133 #13, 18
p.137 #8, 9, 11-18
p.134 #11-13