Balancing Chemical
Equations
Part II
Balancing Chemical Reactions
Follow these steps
1. Check for diatomic gases
2. Balance formulas of compounds
3. Conservation of atoms
4. Add coefficients if needed
5. Conservation of Mass
A. Check For Diatomic Gases
•
•
Diatomic Gases – molecules containing only two atoms.
You must include the subscript “2” when the symbol is written by
itself.
•
Seven Diatomic Gases
nitrogen, N2
oxygen, O2
fluorine, F2
chlorine, Cl2
bromine, Br2
iodine,
I2
hydrogen, H2
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Diatomic Gases
Example
H2(g) + O2(g) →
H2O(g)
Hydrogen and oxygen are diatomic gases
when written ALONE.
You must include the subscript “2”.
Note: The equation isn’t balanced.
Balancing A Decomposition
Equation
A.
Check for diatomic gases
H2O(ℓ)
H + O
→
Diatomic gases: hydrogen & oxygen.
Add a subscript “2” to each
H2O(ℓ)
→
H2(g) + O2(g)
B. Check The Formulas of
Compounds
• The compound in the equation is water.
• The formula is balanced.
H2O(ℓ)
→
2H2(g) + O2(g)
C. Same Elements on Both
Sides
H2O(ℓ)
→
H2(g) + O2(g)
Reactants
Products
hydrogen
oxygen
hydrogen
oxygen
D. Conservation of Atoms
H2O(ℓ)
2–H
1–O
→
2H2(g) + O2(g)
2-H
2–O
Atoms aren’t conserved. Equation isn’t balanced.
E. Balanced Equation
2H2O(ℓ)
→
2H2(g) + O2(g)
The coefficients “2” are placed in front of water,
and hydrogen.
4–H
2–O
The equation is balanced.
4–H
2–O
F. Law of Conservation of Mass
Mass of Reactants = Mass of Products
2H2O(ℓ)
2(18 g)
36 g
Equation is balanced.
2H2(g) + O2(g)
→
=
4g
=
36 g
+
32 g
Balancing A Synthesis Equation
• Equation: Na + Cl →
A. Diatomic Gas:
NaCl
Cl is a diatomic gas.
Na isn’t a diatomic gas.
Na is a metal.
Na + Cl2 → NaCl
Formula of compound balanced? Yes. Na+Cl-
Balancing A Synthesis
Equation
B. Put a “2” in front of the Na and a “2” in front of
NaCl. Check number of each element again.
2Na + Cl2 →
Na = 2
Cl = 2
The equation is balanced.
2NaCl
Na = 2
Cl = 2
Balancing A Synthesis Equation
C. Check number of each element on both
sides.
Na + Cl2 → NaCl
Na = 1
Na = 1
Cl = 2
Cl = 1
Not Balanced.
Balancing A Synthesis Equation
D. Conservation of Mass
2Na + Cl2 →
2NaCl
(2 x 23)g + 71g = 2(23 + 35.5)g
117 g
= 117 g
Balancing A Single
Replacement Equation
C. Same elements on both sides
NaI + F2(g) → NaF + I2(g)
sodium
iodine
fluorine
sodium
iodine
fluorine
Balancing A Single
Replacement Equation
A.
Check for diatomic gases
NaI + F → NaF + I
F and I are diatomic gases.
Add “2” as a subscript on both.
NaI + F2(g) → NaF + I2(g)
Balancing A Single
Replacement Equation
B. Balance the formulas of compounds
+ + -
Na I + F2(g) → Na F + I2(g)
The compounds, NaI and NaF are balanced.
Balancing A Single
Replacement Equation
D.
Same number of atoms or ions on both sides
NaI + F2(g) → NaF + I2(g)
1 – Na
1–I
2–F
Equation is incorrect.
1 – Na
2–I
1–F
Balancing A Single
Replacement Equation
“2” is needed in front of NaI and NaF.
2NaI + F2(g) → 2NaF + I2(g)
2 – Na
2–I
2–F
Equation is correct.
2 – Na
2–I
2–F
Balancing A Single
Replacement Equation
E. Same mass on both sides
2NaI + F2(g) → 2NaF + I2(g)
2(23 + 127)+ 2(19) = 2(23 + 19) + 2(127)
338 g = 338 g
Balancing A Double
Replacement Equation
A. Check for diatomic gases.
Fe S(s) + H Cl(aq) → H S(g) + Fe Cl(aq)
No diatomic gases. Chlorine isn’t ALONE.
B. Check The Formulas of
Compounds
All formulas must be balanced.
Incorrect:
+
2+ 2+ 22+ Fe S(s) + H Cl(aq) → H S(g) + Fe Cl(aq)
Correct:
2+ 2+ 2+ 22+ 2Fe S(s) + H Cl(aq) → H2 S(g) + Fe Cl2(aq)
C. Same Elements on Both
Sides
FeS(s) +
HCl(aq) → H2S(g) FeCl2(aq)
iron
sulfur
hydrogen
chlorine
iron
sulfur
hydrogen
chlorine
D. Conservation of Atoms
The number of ions or atoms each element must be the same on both
sides
Fe S(s) + H Cl(aq) → H2 S(g) + Fe Cl2(aq)
Reactants
Fe =1
S=1
H=1
Cl = 1
This equation is incorrect.
Products
Fe = 1
S=1
H=2
Cl = 2
D. What’s Needed?
• We need 2 hydrogen and 2 chlorine.
• You can’t write in subscripts. This is incorrect. H2Cl2.
This formula isn’t balanced. Hydrogen and chlorine are
written with the subscript “2” when they are alone in an
equation.
• You must write the 2 in front of HCl as a coefficient.
• You have 2-H and 2-Cl.
E. Balanced Equation
Balanced Formula
Fe S(s) + 2HCl(aq) →
Reactants
Fe =1
S=1
H=2
Cl = 2
This equation is correct.
H2S(g) + FeCl2(aq)
Products
Fe = 1
S=1
H=2
Cl = 2
F. Law of Conservation of
Mass
Calculate the total molar mass on each side.
They must be equal.
Fe S(s) + 2HCl(aq)
(56+32) + (2 + 71)
88g
+ 73g
161g
→ H2S(g) + FeCl2(aq)
= (2 +32) + (56 + 71)
= 34g
+ 127g
=
161g