1. Chapter 7 Chemical Reactions
7.3
Balancing a Chemical Equation
Copyright © by Pearson Education, Inc.
Publishing as Benjamin Cummings

2. Copyright © 2008 by Pearson Education, Inc.
Publishing as Benjamin Cummings

3. Balancing Chemical Equations
STEP 1 Write the equation with the correct formulas.
NH3(g) + O2(g) NO(g) + H2O(g)
STEP 2 Determine if the equation is balanced.
No, not all atoms are balanced.
STEP 3 Balance with coefficients in front of formulas.
4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)
STEP 4 Check that atoms of each element are equal in
reactants and products.
4 N (4 x 1N) = 4 N (4 x 1N)
12 H (4 x 3H) = 12 H (6 x 2H)
10 O (5 x 2O) = 10 O (4O + 6O)

4. Steps in Balancing an Equation
To balance the following equation,
Fe3O4(s) H2(g) Fe(s) H2O(l)
Work on one element at a time.
Use only coefficients in front of formulas.
Do not change any subscripts.
Fe: Fe3O4(s) + H2(g) Fe(s) H2O(l)
O: Fe3O4(s) + H2(g) Fe(s) H2O(l)
H: Fe3O4(s) + 4H2(g) Fe(s) H2O(l)

5. Learning Check Check the balance of atoms in the following:
Fe3O4(s) H2(g) Fe(s) H2O(l)
A. Number of H atoms in products.
1) 2 2) ) 8
B. Number of O atoms in reactants.
C. Number of Fe atoms in reactants.
1) 1 2) ) 4

6. Solution Fe3O4(s) + 4H2(g) 3Fe(s) + 4H2O(l)
A. Number of H atoms in products.
3) 8 (4H2O)
B. Number of O atoms in reactants.
2) 4 (Fe3O4)
C. Number of Fe atoms in reactants.
2) 3 (Fe3O4)

7. Equation for A Chemical Reaction
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Publishing as Benjamin Cummings

8. Checking a Balanced Equation
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Publishing as Benjamin Cummings
Reactants Products
1 C atom = 1 C atom
4 H atoms = 4 H atoms
4 O atoms = 4 O atoms

9. Learning Check A. __Mg(s) + __N2(g) __Mg3N2(s)
Balance each equation and list the coefficients in the balanced equation going from reactants to products:
A. __Mg(s) + __N2(g) __Mg3N2(s)
1) 1, 3, ) 3, 1, ) 3, 1, 1
B. __Al(s) + __Cl2(g) __AlCl3(s)
1) 3, 3, 2 2) 1, 3, ) 2, 3, 2

10. Solution A. 3) 3, 1, 1 3Mg(s) + 1N2(g) 1Mg3N2(s) B. 3) 2, 3, 2
2Al(s) Cl2(g) AlCl3(s)

11. Equations with Polyatomic Ions
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Publishing as Benjamin Cummings

12. Balancing with Polyatomic Ions
MgCl2(aq) + Na3PO4(aq) NaCl(aq) + Mg3(PO4)2(s)
Balance PO43- as a unit
MgCl2(aq) + 2Na3PO4(aq) NaCl(aq) + Mg3(PO4)2(s)
2PO43- = PO43-
Balance Mg and Cl
3MgCl2(aq) + 2Na3PO4(aq) NaCl(aq) + Mg3(PO4)2(s)
3 Mg = Mg2+
6 Na = Na+
6 Cl = Cl-

13. Learning Check
Balance and list the coefficients from reactants to products:
A. __Fe2O3(s) __C(s) __Fe(s) __CO2(g)
1) 2, 3, 2, ) 2, 3, 4, ) 1, 1, 2, 3
B. __Al(s) + __FeO(s) __Fe(s) __Al2O3(s)
1) 2, 3, 3, ) 2, 1, 1, ) 3, 3, 3, 1
C. __Al(s) + __H2SO4(aq) __Al2(SO4)3(aq) + __H2(g)
1) 3, 2, 1, ) 2, 3, 1, ) 2, 3, 2, 3

14. Solution A. 2) 2, 3, 4, 3 2Fe2O3(s) + 3C(s) 4Fe(s) + 3CO2(g)
B. 1) 2, 3, 3, 1
2Al(s) + 3FeO(s) 3Fe(s) + 1Al2O3(s)
C. 2) 2, 3, 1, 3
2Al(s) + 3H2SO4(aq) Al2(SO4)3(aq) + 3H2(g)