Balancing Chemical Equations

Balancing Chemical Equations

Middle School

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Learning Objectives

• Identify the reactants, products, and yields symbol in a chemical equation.

• Explain the Law of Conservation of Mass and why it is important.

• Describe the difference between coefficients and subscripts in chemical formulas.

• Use steps to balance chemical equations by changing the coefficients.

Key Vocabulary

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Chemical Equation

A description of a chemical change using symbols and formulas to show the reaction.

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Reactant

A starting substance in a chemical reaction, which is written on the left side of the equation.

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Product

A new substance that is formed in a chemical reaction, written on the right side of the equation.

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Coefficient

A number placed before a chemical formula that shows how many molecules there are of a substance.

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Subscript

A number in a formula that shows how many atoms of an element are in a molecule.

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Conservation of Mass

The principle stating that matter is not created or destroyed in a chemical reaction.

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What is a Chemical Equation?

• A chemical equation is a shorthand way to describe a chemical reaction.

• ​Reactants are the starting substances, written on the left side of the arrow.

• Products are new substances formed, written on the right side of the arrow.

• The arrow (→) means 'yields.' For example: 2H₂ + O₂ → 2H₂O.

Multiple Choice

In the chemical equation 2Ag + H₂S → Ag₂S + H₂, which substances are the products?

• 2Ag and H₂S

• Ag₂S and H₂

• Only 2Ag

• The arrow (→)

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Subscripts vs. Coefficients

Subscripts

• A subscript is a small number written after a chemical symbol in a formula.

• It indicates the number of atoms of an element in one molecule.

• For example, the ‘2’ in H₂O means there are two hydrogen atoms.

Coefficients

• A coefficient is a large number written in front of a chemical formula.

• It represents the total number of molecules of that specific compound.

• For example, 3H₂O means there are three separate water molecules in total.

Multiple Choice

In the expression '4CH₄', what does the coefficient '4' tell us?

• There are four atoms of hydrogen.

• There are four atoms of carbon.

• There are four molecules of CH₄.

• There are four elements in total.

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The Law of Conservation of Mass

• Matter is never created or destroyed during a chemical reaction.

• ​The total mass of reactants equals the total mass of products.

• Chemical equations must be balanced to follow this important law.

• A balanced equation has equal atoms on reactant and product sides.

Multiple Choice

Why must chemical equations be balanced?

• To show that new substances are formed.

• To follow the Law of Conservation of Mass.

• To make the equation longer.

• To show the difference between reactants and products.

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Steps to Balance a Chemical Equation

• First, list all elements and count their atoms on each side.

• Next, compare the atom counts to see if the equation is balanced.

• If unbalanced, place coefficients in front of formulas to make atoms equal.

• Remember to never change the subscripts within a chemical formula during balancing.

Multiple Choice

If an equation is unbalanced, what is the correct action to take?

• Change the small subscript numbers to make the atoms equal.

• Add or remove reactants or products from the equation.

• Place large coefficient numbers in front of formulas until atoms are equal.

• Ignore the imbalance as long as the chemicals are correct.

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Common Misconceptions

Misconception	Correction
You can change subscripts to balance an equation.	Never change subscripts. It changes the substance (e.g., H2O vs. H2O2).
Mass is lost when a gas is produced, like burning wood.	Mass is always conserved. The gas produced escapes into the air.
A coefficient only applies to the element it is in front of.	A coefficient multiplies the entire molecule that follows (e.g., 2H2O).

Multiple Choice

How does balancing a chemical equation relate to the Law of Conservation of Mass?

• Balancing equations proves that mass can be created.

• It ensures the mass of reactants equals the mass of products.

• It shows how to change the state of matter of the products.

• Balancing an equation is not related to the conservation of mass.

Multiple Choice

In the balanced equation 4Fe + 3O₂ → 2Fe₂O₃, why are there 6 oxygen atoms on the product side?

• Because there are 2 iron atoms and 3 oxygen atoms.

• Because the coefficient '2' is multiplied by the subscript '3'.

• Because the subscript '3' is added to the coefficient '2'.

• Because there are two different products being formed.

Multiple Choice

What coefficients are needed to correctly balance the chemical equation __N₂ + __H₂ → __NH₃?

• 2, 1, 2

• 1, 1, 1

• 1, 3, 2

• 2, 3, 1

Multiple Choice

Analyze the equation Mg + HCl → MgCl₂ + H₂. Why is it unbalanced and what single step is required to balance it?

• It is unbalanced because there is no Cl₂ on the reactant side; you must change HCl to HCl₂.

• It is unbalanced because there is more Mg on the left; you must add a coefficient of 2 to MgCl₂.

• It is unbalanced with unequal H and Cl atoms; you must add a coefficient of 2 in front of HCl.

• The equation is already balanced and requires no changes.

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Summary

• A chemical equation shows how reactants form products.

• Equations must be balanced, as atoms are not created or destroyed.

• Change coefficients to balance equations, but never change subscripts.

• A balanced equation shows reactant mass equals product mass.

Poll

On a scale of 1-4, how confident are you about balancing chemical equations now?

• 1 (I am very confused)

• 2 (I need more practice)

• 3 (I mostly understand)

• 4 (I feel very confident)