1. Chemical Reactions CHAPTER 10
10.2 Methods for Balancing Chemical Equations

2. Cellular respiration
What is the number of C, H and O atoms on each side of the reaction arrow?

3. Cellular respiration Reactants Products Carbon (C) Oxygen (O)
Hydrogen (H)

4. 6 8 12 Cellular respiration Reactants Products Carbon (C) Oxygen (O)
Hydrogen (H) 12

5. 6 1 8 3 12 2 Cellular respiration Reactants Products Carbon (C)
Oxygen (O) 8 3
Hydrogen (H) 12 2

6. 6 1 8 3 12 2 Not balanced! Cellular respiration Reactants Products
Carbon (C) 6 1
Oxygen (O) 8 3
Hydrogen (H) 12 2
Not balanced!

7. Cellular respiration (unbalanced equation)
Let’s find a method to balance a chemical equation

8. Write down the unbalanced chemical equation.
C6H12O O CO H2O

9. C6H12O6 + 6O2 6CO2 + 6H2O C appears in 1 compound
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
C6H12O O CO H2O
C appears in 1 compound
H appears in 1 compound
O appears in 2 compounds
C appears in 1 compound
H appears in 1 compound
O appears in 2 compounds

10. C6H12O6 + 6O2 6CO2 + 6H2O C appears in 1 compound
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
C6H12O O CO H2O
Balance
C appears in 1 compound
H appears in 1 compound
O appears in 2 compounds
C appears in 1 compound
H appears in 1 compound
O appears in 2 compounds
Note: We could also balance H first.

11. C6H12O6 + 6O2 6CO2 + 6H2O ? ? C appears in 1 compound 6 C
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first. ?
C6H12O O CO H2O ?
Balance
C appears in 1 compound
6 C
C appears in 1 compound
1 C

12. C6H12O6 + 6O2 6CO2 + 6H2O 1 6 C appears in 1 compound 6 C
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first. 1
C6H12O O CO H2O 6
The number of C atoms is now balanced
Balance
C appears in 1 compound
6 C
C appears in 1 compound
6 C

13. C6H12O6 + 6O2 6CO2 + 6H2O 1 6 ? C appears in 1 compound
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
Continue with the rest of the elements. If a free element is present, it is balanced last. 1
C6H12O O CO H2O 6 ?
H does not appear in CO2, so the coefficient for CO2 is unaffected.
C appears in 1 compound
H appears in 1 compound
O appears in 2 compounds
C appears in 1 compound
H appears in 1 compound
O appears in 2 compounds
Balance
Note: Balance O last; oxygen is a free element in O2.

14. C6H12O6 + 6O2 6CO2 + 6H2O 1 6 ? H appears in 1 compound 12 H
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
Continue with the rest of the elements. If a free element is present, it is balanced last. 1
C6H12O O CO H2O 6 ?
H appears in 1 compound
12 H
H appears in 1 compound
2 H
Balance

15. C6H12O6 + 6O2 6CO2 + 6H2O 1 6 6 H appears in 1 compound 12 H
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
Continue with the rest of the elements. If a free element is present, it is balanced last. 1
C6H12O O CO H2O 6 6
H appears in 1 compound
12 H
H appears in 1 compound
12 H
Balance

16. C6H12O6 + 6O2 6CO2 + 6H2O 1 ? 6 6 C appears in 1 compound
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
Continue with the rest of the elements. If a free element is present, it is balanced last. 1
C6H12O O CO H2O ? 6 6
C appears in 1 compound
H appears in 1 compound
O appears in 2 compounds
C appears in 1 compound
H appears in 1 compound
O appears in 2 compounds
Balance

17. C6H12O6 + 6O2 6CO2 + 6H2O 1 ? 6 6 O appears in 2 compounds
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
Continue with the rest of the elements. If a free element is present, it is balanced last. 1
C6H12O O CO H2O ? 6 6
O appears in 2 compounds
(1 x 6) + (? x 2)
O appears in 2 compounds
(6 x 2) + (6 x 1)
Balance

18. C6H12O6 + 6O2 6CO2 + 6H2O 1 ? 6 6 O appears in 2 compounds
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
Continue with the rest of the elements. If a free element is present, it is balanced last. 1
C6H12O O CO H2O ? 6 6
O appears in 2 compounds
(1 x 6) + (? x 2)
= 6 + (? x 2)
= 18
O appears in 2 compounds
(6 x 2) + (6 x 1) =
= 18
We know that the number of O atoms must be the same on both sides of the reaction arrow.
Balance

19. C6H12O6 + 6O2 6CO2 + 6H2O 1 6 6 6 O appears in 2 compounds
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
Continue with the rest of the elements. If a free element is present, it is balanced last. 1
C6H12O O CO H2O 6 6 6
O appears in 2 compounds
(1 x 6) + (? x 2)
= 6 + (6 x 2)
= 18
O appears in 2 compounds
(6 x 2) + (6 x 1) =
= 18
Balance

20. Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
Continue with the rest of the elements. If a free element is present, it is balanced last.
Check each element to make sure that the equation is balanced. 1
C6H12O O CO H2O 6 6 6
6 C
12 H
18 O
6 C
12 H
18 O

21. C6H12O6 + 6O2 6CO2 + 6H2O C6H12O6 + 6O2 6CO2 + 6H2O 1 6 6 6
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
Continue with the rest of the elements. If a free element is present, it is balanced last.
Check each element to make sure that the equation is balanced.
Make sure the coefficients are the smallest possible whole numbers. 1
C6H12O O CO H2O 6 6 6
C6H12O O CO H2O

22. Another example CH4 + 2O2 CO2 + 2H2O
Write down the unbalanced chemical equation.
CH O CO H2O

23. CH4 + 2O2 CO2 + 2H2O C appears in 1 compound H appears in 1 compound
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
CH O CO H2O
C appears in 1 compound
H appears in 1 compound
O appears in 1 compound
C appears in 1 compound
H appears in 1 compound
O appears in 2 compounds

24. CH4 + 2O2 CO2 + 2H2O C appears in 1 compound H appears in 1 compound
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
CH O CO H2O
Balance
C appears in 1 compound
H appears in 1 compound
O appears in 1 compound
C appears in 1 compound
H appears in 1 compound
O appears in 2 compounds
Note: We could also balance H first.

25. CH4 + 2O2 CO2 + 2H2O ? ? C appears in 1 compound 1 C
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first. ?
CH O CO H2O ?
Balance
C appears in 1 compound
1 C
C appears in 1 compound
1 C

26. CH4 + 2O2 CO2 + 2H2O 1 1 C appears in 1 compound 1 C
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first. 1
CH O CO H2O 1
Balance
C appears in 1 compound
1 C
C appears in 1 compound
1 C
Already balanced

27. CH4 + 2O2 CO2 + 2H2O 1 1 ? C appears in 1 compound
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
Continue with the rest of the elements. If a free element is present, it is balanced last. 1
CH O CO H2O 1 ?
C appears in 1 compound
H appears in 1 compound
O appears in 1 compound
C appears in 1 compound
H appears in 1 compound
O appears in 2 compounds
Balance
Note: Balance O last; oxygen is a free element in O2.

28. CH4 + 2O2 CO2 + 2H2O 1 1 ? H appears in 1 compound 4 H
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
Continue with the rest of the elements. If a free element is present, it is balanced last. 1
CH O CO H2O 1 ?
H appears in 1 compound
4 H
H appears in 1 compound
2 H
Balance

29. CH4 + 2O2 CO2 + 2H2O 1 1 2 H appears in 1 compound 4 H
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
Continue with the rest of the elements. If a free element is present, it is balanced last. 1
CH O CO H2O 1 2
H appears in 1 compound
4 H
H appears in 1 compound
4 H
Balance

30. CH4 + 2O2 CO2 + 2H2O 1 ? 1 2 C appears in 1 compound
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
Continue with the rest of the elements. If a free element is present, it is balanced last. 1
CH O CO H2O ? 1 2
C appears in 1 compound
H appears in 1 compound
O appears in 1 compound
C appears in 1 compound
H appears in 1 compound
O appears in 2 compounds
Balance

31. CH4 + 2O2 CO2 + 2H2O 1 ? 1 2 O appears in 1 compound 2
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
Continue with the rest of the elements. If a free element is present, it is balanced last. 1
CH O CO H2O ? 1 2
O appears in 1 compound 2
O appears in 2 compounds
(1 x 2) + (2 x 1)
Balance

32. CH4 + 2O2 CO2 + 2H2O 1 ? 1 2 O appears in 1 compound 2
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
Continue with the rest of the elements. If a free element is present, it is balanced last. 1
CH O CO H2O ? 1 2
O appears in 1 compound 2
O appears in 2 compounds
(1 x 2) + (2 x 1)
= 2 + 2
= 4
Balance

33. CH4 + 2O2 CO2 + 2H2O 1 2 1 2 O appears in 1 compound 4
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
Continue with the rest of the elements. If a free element is present, it is balanced last. 1
CH O CO H2O 2 1 2
O appears in 1 compound 4
O appears in 2 compounds
(1 x 2) + (2 x 1)
= 2 + 2
= 4
Balance

34. Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
Continue with the rest of the elements. If a free element is present, it is balanced last.
Check each element to make sure that the equation is balanced. 1
CH O CO H2O 2 1 2
1 C
4 H
4 O
1 C
4 H
4 O

35. Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
Continue with the rest of the elements. If a free element is present, it is balanced last.
Check each element to make sure that the equation is balanced.
Make sure the coefficients are the smallest possible whole numbers. 1
CH O CO H2O 2 1 2
CH O CO H2O

36. Strategy to balance a chemical equation:
Write down the unbalanced chemical equation.
Identify the element that occurs in only one compound on both sides, and balance it first.
Continue with the rest of the elements. If a free element is present, it is balanced last.
Check each element to make sure that the equation is balanced.
Make sure the coefficients are the smallest possible whole numbers.