Ch. 10.1 – Chemical Reactions
II. Balancing Equations
read:
(p. 278 – 283)
A. Words to Skeletons First
Read the statement and underline all
reactants and products first.
“Iron and chlorine react to produce
iron(III) chloride.”
Write a word equation, then a skeleton:
iron(s) + chlorine(g) → iron(III) chloride(s)
Fe(s) + Cl2(g) → FeCl3(s)
B. Balancing Steps
1. Write the unbalanced skeleton equation. Check
2.
3.
formulas; look for diatomic elements.
Count atoms on each side.
Never change a formula (don’t change subscripts);
add coefficients to make #s equal.
Coefficient  subscript = # of atoms
4.
Reduce coefficients to lowest
necessary.
possible ratio, if
5. Double check atom balance!!! YOU MUST OBEY
THE LAW.
C. Helpful Tips



Balance one element at a time, working
left to right except for H and O. Save H
for next to last, and O until last.
Update ALL atom counts after adding a
coefficient. NEVER change a formula.
Balance polyatomic ions as single units
if they appear on both sides
 “1 SO4” instead of “1 S” and “4 O”
D. Law of Conservation of Mass
2 Fe(s) + 3 Cl2(g) →
2 FeCl3(s)
D. Law of Conservation of Mass
2 Fe(s) + 3 Cl2(g) →
Fe = 1
X2=2
Cl = 2
X3=6
2 FeCl3(s)
Fe = 1
Cl = 3
X2=2
=6
Atoms are placed in “boxes” to ensure the formula is
not changed; the number of “boxes” can change, but not
the insides of the boxes.
D. Law of Conservation of Mass
Potassium chromate reacts with lead(II)
nitrate to produce potassium nitrate
and solid lead(II) chromate. The
reaction occurs in water.
Write a skeleton equation with correct
formulas.
K1+(CrO4)2- + Pb2+(NO3)1- →
K1+(NO3)1- + Pb2+(CrO4)2K2CrO4(aq) + Pb(NO3)2(aq) → KNO3(aq) + PbCrO4(s)
K2CrO4(aq) + Pb(NO3)2(aq) → 2 KNO3(aq) + PbCrO4(s)
K=2
K=1
CrO4= 1
NO3 = 1
Pb = 1
Pb = 1
NO3 = 2
CrO4 = 1
2
K
1/
x
2
1 CrO4 1
1
Pb
1
2 NO3 1/
2
2
=2
=2
E. Balancing Example
Aluminum and copper(II) chloride react
to form copper and aluminum chloride.
2 Al + 3 CuCl2  3 Cu + 2 AlCl3
2 1
Al
1 2
3 1
Cu
1 3
6 2
Cl
3 6
E. Balancing Example
Solid zinc reacts with hydrogen chloride to
produce zinc chloride and hydrogen gas
1 Zn + 2 HCl →
1 ZnCl2 + 1 H2
1
Zn
1
2
1
Cl
2
2
1
H
2
Your turn…..
1. Sodium phosphate reacts with potassium
hydroxide to produce sodium hydroxide and
potassium phosphate
2. Magnesium fluoride reacts with lithium
carbonate to produce magnesium carbonate and
lithium fluoride
3. Lead (II) hydroxide reacts with hydrogen
chloride to produce dihydrogen monoxide and
lead (II) chloride
The answers…..
1. 1 Na3PO4 + 3 KOH  3 NaOH + 1 K3PO4
2. 1 MgF2 + 1 Li2CO3  1 MgCO3 + 2 LiF
3. 1 Pb(OH)2 + 2 HCl  2 H2O + 1 PbCl2