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# Counting Atoms & Balancing Equations (65 Favorites)

LESSON PLAN in Molecular Formula, Balancing Equations, Conservation of Mass. Last updated March 25, 2020.

### Summary

In this lesson, students will learn how to count atoms and how to balance chemical equations using videos, simulations and games.

Middle and High School

### Objectives

By the end of this lesson, students should be able to

• Count atoms in a given chemical formula.
• Identify and differentiate between a coefficient and a subscript.
• Read and understand a balanced chemical equation.
• Identify what substances are the reactants and what substances are the products in a chemical reaction.
• Explain the law of conservation of mass.

### Chemistry Topics

This lesson supports students’ understanding of

• Chemical reactions
• Balancing Equations
• Reactants
• Products.
• Conservation of Matter
• Chemical formula
• Coefficients
• Subscripts

### Time

Teacher Preparation: 30 minutes

Lesson: Three 45 minute class meetings

### Materials

• Computer, tablet or Chrome book
• List of balancing chemical equations web sites and You Tube videos (see teacher notes)
• Student worksheet

### Safety

• No specific safety precautions need to be observed for this activity.

### Teacher Notes

• I suggest that students work individually, and at their own pace through this lesson.
• Background:
• Chemical reaction: a process where atoms of the reactant(s) will rearrange themselves to create a new arrangement of atoms, called the product(s).
• Reactant: A substance or substances present at the start of the reaction.
• Product: A resulting substance or substances formed by a chemical reaction.
• Chemical equation: Shorthand form for writing what reactants are used and what products are formed in a chemical reaction; sometimes shows whether energy is produced or absorbed.
• Chemical formula: Combination of chemical symbols and numbers that indicates which elements and how many atoms of each element are present in a molecule.
• Subscripts: The small numbers written to the right of the atoms. For examples, the subscript 2 in H2O means that each molecule of water has two atoms of hydrogen. A subscript outside a parenthesis multiplies all the elements inside the parenthesis. For example, Ba3(PO4)2, indicates there are 3 atoms of Ba, 2 (2 x 1) atoms of P, and 8 (2 x 4) atoms of O.
• Coefficients: The numbers in front of a chemical formula. For example, 2H2O means two molecules of water.
• Law of conservation of mass: The mass of the reactants must equal the mass of the products. All the atoms on the reactant side of and equation are also on the product side. Atoms are not lost, but rearranged.
• Rules for balancing chemical equations:
• Write the correct chemical formulas of reactants and products.
• Count the number of atoms of each type in the reactants and in the products.
• Look at the equation and see which elements are not balanced.
• Balance the elements one at a time by adding coefficients.
• Count the number of atoms to make sure the equation is balanced.
• Finally, all coefficients are converted into the lowest possible whole number.
• Show the students the Fig. 1, to illustrate coefficients and subscripts:
• Never:
• Change a subscript to balance a chemical equation. If you change the formula, you are describing a different chemical reaction: H2O is a different compound than H2O2.
• Put a coefficient in the middle of a chemical formula. 2KCl is alright, K2Cl is not.
To explain to students how to balance chemical equations, show them the following YouTube videos:

After watching videos 1 and 2, the students should answer the attached worksheet. If students show at least 80% proficiency they can skip videos 3 – 6, and move on to the simulations and games. Students who display difficulty understanding the concepts should watch videos 3 and 4, and then answer the attached worksheet. Students who still display difficulty may need to watch videos 5 and 6. The teacher should evaluate these students for understanding.

• The following links are simulations and games for students to understand how to balance chemical equations. The PHET simulations and games have different levels of difficulty. Students should be able to balance the most advanced equations, before they are allowed to play the Rags to Riches and Battleship games:
• Balancing chemical equations simulations and games suggestions:
• Teachers should assign students to use specific simulations and/or games depending on the time available and level.

### Background

• Chemical reaction: a process where atoms of the reactant(s) will rearrange themselves to create a new arrangement of atoms, called the product(s).
• Reactant: A substance or substances present at the start of the reaction.
• Product: A resulting substance or substances formed by a chemical reaction.
• Chemical equation: Shorthand form for writing what reactants are used and what products are formed in a chemical reaction; sometimes shows whether energy is produced or absorbed.
• Chemical formula: Combination of chemical symbols and numbers that indicates which elements and how many atoms of each element are present in a molecule.
• Subscripts: The small numbers written to the right of the atoms. For examples, the subscript 2 in H2O means that each molecule of water has two atoms of hydrogen. A subscript outside a parenthesis multiplies all the elements inside the parenthesis. For example, Ba3(PO4)2, indicates there are 3 atoms of Ba, 2 (2 x 1) atoms of P, and 8 (2 x 4) atoms of O.
• Coefficients: The numbers in front of a chemical formula. For example, 2H2O means two molecules of water.
• Law of conservation of mass: The mass of the reactants must equal the mass of the products. All the atoms on the reactant side of and equation are also on the product side. Atoms are not lost, but rearranged.

Pre-lesson Questions

1. What is a chemical equation? Give an example.
2. What is a coefficient?
3. What is a subscript?

Procedure

Rules for balancing chemical equations:

• Write the correct chemical formulas of reactants and products.
• Count the number of atoms of each type in the reactants and in the products.
• Look at the equation and see which elements are not balanced.
• Balance the elements one at a time by adding coefficients.
• Count the number of atoms to make sure the equation is balanced.
• Finally, all coefficients are converted into the lowest possible whole numbers.
• Never change a subscript to balance a chemical equation. If you change the formula, you are describing a different chemical reaction: H2O is a different compound than H2O2.
• Never put a coefficient in the middle of a chemical formula. 2KCl is correct, but K2Cl is not. Part 1: Videos:

• To learn how to balance chemical equations, watch YouTube videos 1 & 2. Record any important notes in the space provided below.

Video Notes:

*If you have a strong understanding of how to balance an equation, move on and complete the worksheet.

• If you feel like you need additional practice and explanation, continue to watch the following videos:

Video Notes:

Part 2: Simulations & Games:

• Your teacher will direct you to interact with a particular simulation and/or game for this part of the lesson.