Balancing Equations

Chemistry Tutorial

Balancing Equations


This problem will illustrate how to derive a chemical equation from a word problem and how to balance it.

Sample Problem

Plants use a process called photosynthesis to synthesize energy (glucose). The chemical reaction requires water, carbon dioxide and heat to generate glucose and oxygen. Write the balanced chemical reaction for photosynthesis.


Water= H2O
Carbon dioxide= CO2
Heat= light (photons are not molecules)
Glucose= C6H12O6
Oxygen= O2

H2O + CO2 + heat -->  C6H12O6 + O2

Left side            right side

2 H                  12 H
3 O                  8  O
1 C                  6  C

Now start by balancing the elements:

a. Let’s begin with balancing the carbons

1 C *6     -->        6  C    =    H2O + 6CO2 --> C6H1O6 + O2

b. Now balance the oxygens because the amount of oxygens has changed on the left side of the equation

2 * 6= 12 O   -->    6 O + 2*3= 12 O   =   H2O + 6CO2 --> C6H1O6 + 3O2

c. Lastly, balance the hydrogens

2 * 6= 12 H   -->   12 H  =    6H2O + 6CO2  --> C6H12O6 + 3O2

Now double check

Left side          Right side
12 H               12 H     looks good
18 O               12 O      UH OH, LOOKS LIKE WE NEED TO REVISE
6  C                6 C     looks good

6H2O + 6CO2 -->  C6H12O6  + ?O2

6 O  + 12 O= 18 -->   6 O    +  6 * O2= 18 NOW THE OXYGENS ARE BALANCED. 

d. Let’s rewrite the correct balanced equation:

6H2O + 6CO2  --> C6H12O6 + 6O2 

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