1. Chapter 8 Chemical Reactions
8.2 Balancing a Chemical Equation
Learning Goal Write a balanced chemical equation from the formulas of the reactants and products for a chemical reaction.
© 2014 Pearson Education, Inc.

2. Guide to Balancing a Chemical Reaction

3. Learning Check
Write a balanced chemical equation for the combustion of methane, CH4.

4. Solution
Write a balanced chemical equation for the combustion of methane, CH4.
Step 1 Write an equation using the correct formula of the reactants and products.

5. Write a balanced chemical equation for the combustion of methane, CH4.
Solution
Write a balanced chemical equation for the combustion of methane, CH4.
Step 2 Count the atoms of each element in the reactants and products.
Reactants
Products
1 C atom
1 C atom Balanced
4 H atoms
2 H atoms Not balanced
2 O atoms
3 O atoms Not balanced

6. Solution
Write a balanced chemical equation for the combustion of methane, CH4.
Step 3 Use coefficients to balance each element.
Since H has the most atoms, we balance the H in CH4 first by placing a 2 in front of H2O.

7. Write a balanced chemical equation for the combustion of methane, CH4.
Solution
Write a balanced chemical equation for the combustion of methane, CH4.
Step 3 Use coefficients to balance each element. Recheck other atoms.
Reactants
Products
1 C atom
1 C atom Balanced
4 H atoms
4 H atoms Balanced
2 O atoms
3 O atoms Not balanced

8. Solution
Write a balanced chemical equation for the combustion of methane, CH4.
Step 3 Use coefficients to balance each element.
We can balance O by placing a coefficient in front of O2.

9. Write a balanced chemical equation for the combustion of methane, CH4.
Solution
Write a balanced chemical equation for the combustion of methane, CH4.
Step 4 Check the final equation to make sure it is balanced.
Reactants
Products
1 C atom
1 C atom Balanced
4 H atoms
4 H atoms Balanced
4 O atoms
4 O atoms Balanced

10. Solution
Write a balanced chemical equation for the combustion of methane, CH4.
Step 4 Check the final equation to make sure it is balanced. In a balanced reaction, the coefficients must be the lowest possible whole numbers.

11. Balance the following chemical equation:
Learning Check
Balance the following chemical equation:
Na3PO4(aq) + MgCl2(aq)  Mg3(PO4)2(s) + NaCl(aq)
unbalanced

12. Balance the following chemical equation:
Solution
Balance the following chemical equation:
Na3PO4(aq) + MgCl2(aq)  Mg3(PO4)2(s) + NaCl(aq)
Step 1 Write an equation using the correct formulas of the reactants and products.

13. Balance the following chemical equation:
Solution
Balance the following chemical equation:
Na3PO4(aq) + MgCl2(aq)  Mg3(PO4)2(s) + NaCl(aq)
Step 2 Count the ions of each element in the reactants and products.
We compare the number of atoms in products and reactants to determine if the reaction is unbalanced.

14. Solution
Step 2 Count the ions of each element in the reactants and products.

15. Step 2 Count the ions of each element in the reactants and products.
Solution
Na3PO4(aq) + MgCl2(aq)  Mg3(PO4)2(s) + NaCl(aq)
Step 2 Count the ions of each element in the reactants and products.
Reactants
Products
3 Na+
1 Na Not balanced
1 PO43−
2 PO43− Not balanced
1 Mg2+
3 Mg Not balanced
2 Cl−
1 Cl− Not balanced

16. Balance the following chemical equation:
Solution
Balance the following chemical equation:
2Na3PO4(aq) + 3MgCl2(aq)  Mg3(PO4)2(s) + NaCl(aq)
Step 3 Use coefficients to balance each element.
Reactants
Products
6 Na+
1 Na Not balanced
2 PO43−
2 PO43− Balanced
3 Mg2+
3 Mg Balanced
6 Cl−
1 Cl− Not balanced

17. Balance the following chemical equation:
Solution
Balance the following chemical equation:
2Na3PO4(aq) + 3MgCl2(aq)  Mg3(PO4)2(s) + 6NaCl(aq)
Step 4 Check the final equation and make sure it is balanced. Add a coefficient of 6 to balance Na+, Cl−.
Reactants
Products
6 Na+
6 Na Balanced
2 PO43−
2 PO43− Balanced
3 Mg2+
3 Mg Balanced
6 Cl−
6 Cl− Balanced