1. Chapter 5 Chemical Reactions and Quantities
5.2
Chemical Equations

2. Chemical Equations
A chemical equation gives the chemical formulas of the reactants on the left of the arrow and the products on the right.
Reactants Product
O2 (g)
CO2 (g)
C(s)

3. Symbols Used in Equations
Symbols used in chemical
equations show
the states of the reactants.
the states of the products.
the reaction conditions.
TABLE 5.2

4. Chemical Equations are Balanced
In a balanced
chemical reaction
atoms are not gained or lost.
the number of reactant atoms is equal to the number of product atoms.

5. A Balanced Chemical Equation
In a balanced chemical equation,
there must be the same number of each type of atom on the reactant side and on the product side of a balanced equation.
numbers called coefficients are used in front of one or more formulas.
Al S  Al2S Not Balanced
2Al S  Al2S3 Balanced
2 Al = Al
3 S = S

6. Learning Check State the number of atoms of each element on the
reactant side and the product side for each of the
following balanced equations.
A. P4(s) Br2(l)  4 PBr3(g)
B. 2Al(s) Fe2O3(s)  2Fe(s) Al2O3(s)

7. Solution A. P4(s) + 6Br2(l)  4 PBr3(g) 4 P 4 P 12 Br 12 Br
B. 2Al(s) Fe2O3(s)  2Fe(s) Al2O3(s)
2 Al 2 Al
2 Fe Fe
3 O 3 O

8. Learning Check Determine if each equation is balanced or not.
A. Na(s) N2(g)  Na3N(s)
B. C2H4(g) H2O(l)  C2H5OH(l)

9. Solution Determine if each equation is balanced or not.
A. Na(s) N2(g)  Na3N(s)
No. 2 N on reactant side, 1 N on product side.
1 Na on reactant side, 3 Na on product side.
B. C2H4(g) H2O(l)  C2H5OH(l)
Yes. 2 C = 2 C
6 H = 6 H
1 O = 1 O

10. Guide to Balancing a Chemical Equation
Copyright © by Pearson Education, Inc.
Publishing as Benjamin Cummings

11. 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)  3Fe(s) H2O(l)
O: Fe3O4(s) + H2(g)  3Fe(s) H2O(l)
H: Fe3O4(s) + 4H2(g)  3Fe(s) H2O(l)

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

13. Equation for A Chemical Reaction

14. Checking a Balanced Equation
Reactants Products
1 C atom = 1 C atom
4 H atoms = 4 H atoms
4 O atoms = 4 O atoms

15. Learning Check Check the balance of atoms in the following.
Fe3O4(s) H2(g)  3Fe(s) H2O(l)
A. Number of H atoms in products.
1) 2 2) 4 3) 8
B. Number of O atoms in reactants.
C. Number of Fe atoms in reactants.
1) 1 2) 3 3) 4

16. 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)

17. Learning Check
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

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

19. Equations with Polyatomic Ions

20. 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)
2 PO = PO43-
Balance Mg and Cl
3MgCl2(aq) + 2Na3PO4(aq)  6NaCl(aq) + Mg3(PO4)2(s)
3 Mg = Mg2+
6 Na = Na+
6 Cl = Cl-

21. 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

22. 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)  1Al2(SO4)3(aq) + 3H2(g)