## Balancing Chemical Equations

### Using Models to Simplify Balancing Equation

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.

#### Next Generation Science Standards

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.This activity is appropriate for high school students and addresses the following Next Generation Science Standards for grades 9–12:

Physical Science or Chemistry DCI

PS1.B: Chemical Reactions

#### What Students Need

Working in pairs, students will need the following:

- 60 Gumdrops (12 gum drops of 5 different colors)
- Small Plastic Bags
- Toothpicks

#### What Teachers Need to Do

- 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.
- Divide your class into pairs and distribute the materials.
- 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.

#### A Model for Balancing Equations: Class Example

Element Symbol < and Gumdrop Color | Reactant Side (final number) | Product Side (final number) |

N Color: ___________ | ||

H Color: ___________ |

_{2}+ __H

_{2}→ __NH

_{3}

#### Student Balancing Equation Practice

- 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.
- Lay models out and group them into reactants and products. It may be helpful to crease a sheet of notebook paper in half and label the left side “reactants” and the right side “products.”
- Once the molecules are built and the reaction is laid out, to balance the reactions you must add complete molecules—not individual gumdrops. 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. Once you build a molecular model, the defined subscripts are unchangeable. Only the coefficients may change in balancing the chemical equation.
- 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.
- If those numbers are unequal, 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. Place the coefficients in the equation and record the final total of each type of atom on the product and reactant side.

__Fe + __HCl → __H_{2} + __FeCl_{3}

__CH_{4} + __O_{2} → __CO_{2} + __H_{2}O

__K + __H_{2}O → __KOH + __H_{2}

__HCl + __NaOH → __NaCl + __H_{2}O

__FeS + __HCl → __H_{2}S + __FeCl_{2}

__C_{2}H_{4} + __O_{2} → __CO_{2} + __H_{2}O

#### Balanced Chemical Equations

Class example: N_{2} + 3H_{2} → 2NH_{3}

2Fe + 6HCl → 3H_{2} + 2FeCl_{3}

CH_{4} + 2O_{2} → CO_{2} + 2H_{2}O

2K + 2H_{2}O → 2KOH + H_{2}

HCl + NaOH → NaCl + H_{2}O

FeS + 2HCl → H_{2}S + FeCl_{2}

C_{2}H_{4} + 3O_{2} → 2CO_{2} + 2H_{2}O

This straightforward activity should clear up any confusion that some students may still have between coefficients and subscripts in chemical equations. For additional student practice in balancing equations and identifying reaction types see the links below.

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