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Balancing Chemical Equations

By Shuana Jordan
Product Developer

Molecular model

To balance chemical equations, students must know how to count atoms, understand the difference between coefficients and subscripts, and identify reactants and products. Balancing equations reinforces understanding of the law of conservation of mass as students keep in mind that the same number and types of atoms must be on the reactant and product sides of the chemical equation.

In this activity, students build models of compounds to help them visualize the reactants and products in chemical reactions and determine the coefficients in chemical equations. This activity is appropriate for a class of high school students working in pairs.

National Science Education Standards

This activity is appropriate for high school students and addresses the following National Science Education Standards for grades 9–12:

  • Physical Science: Structure and Properties of Matter; Chemical Reactions

Materials (per pair of students)

  • 60 Gumdrops (5 different colors)
  • Small Plastic Bags
  • Toothpicks

Preparation and procedure

  1. For each pair of students, place gumdrops (12 of each color) in a small plastic bag. Each student pair also needs the toothpicks. You may either hand those out or just make several boxes available during the activity.
  2. Divide your class into pairs and distribute the materials.
  3. Give an overview of the activity—explaining that the pairs are to construct molecular models using the gumdrops and toothpicks. They are to model chemical reactions, assigning a color of gumdrop to a specific element. Still using their models, they balance each reaction. Then they record their final data before moving to the next reaction.
    __N2 + __H2 → __NH3
    Element Symbol< and Gumdrop Color Reactant Side (final number) Product Side (final number)
    N Color: ______________    
    H Color: ______________    

    Figure 1 Example data table.

  4. Following are some simple reactions that you can assign to get the pairs started. You may pass out paper copies or project the reactions on your whiteboard.

    __N2 + __H2 → __NH3

    __Fe + __HCl → __H2 + __FeCl3

    __CH4 + __O2 → __CO2 + __H2O

    __K + H2O → __KOH + __H2

    __HCl + NaOH → __NaCl + __H2O

    __FeS + HCl → __H2S + FeCl2

    __C2H4 + __O2 → __CO2 + __H2O
  5. Have students build models of the reactants and products, using the gumdrops and toothpicks for each equation. The gumdrops represent the atoms, and the toothpicks, the bonds. For the purpose of this balancing exercise, it is not important that students model correct bond angles; numbers and types of atoms are the important things.
  6. Have students lay their models out and group them so that they know which models represent the reactants and which represent the products. It may be helpful to have students crease a sheet of notebook paper in half and label the left side “reactants” and the right side “products.”
  7. Once the molecules are built and the reaction is laid out, let students know that in order to balance the reactions they must add complete molecules—not individual gumdrops. Reinforce the difference between a coefficient and a subscript. The coefficient is the number in front of the chemical formula in a chemical equation, indicating the number of molecules. (Absence of a coefficient is understood to indicate 1 molecule.) A subscript indicates how many atoms of an element are in each molecule of a compound. Students should understand that once they build a molecular model, the defined subscripts are unchangeable. Only the coefficients may change in balancing the chemical equation.
  8. Have students count the number of atoms of each element present on the reactant side and compare it with the number of atoms of that element on the product side.
  9. If those numbers are unequal, students must build additional molecules until the numbers match. Then, the number of models of each compound on each side provides the coefficients needed to balance the equation. Have students place the coefficients in the equation and record the final total of each type of atom on the product and reactant side.
Here are the coefficients that balance the equations above:

N2 + 3H2 → 2NH3

2Fe + 6HCl → 3H2 + 2FeCl3

CH4 + 2O2 → CO2 + 2H2O

2K + 2H2O → 2KOH + H2

HCl + NaOH → NaCl + H2O

FeS + 2HCl → H2S + FeCl2

C2H4 + 3O2 → 2CO2 + 2H2O

Conclusion and extension

This straightforward activity should clear up any confusion that some students may still have between coefficients and subscripts in chemical equations.

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