Modeling Chemical Reactions

Modeling Chemical Reactions

Middle School

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Learning Objectives

• Use a model to show how atoms rearrange during a chemical reaction.

• Explain how chemical equations represent reactants, products, and the rearrangement of atoms.

• Explain that mass is conserved because the number and type of atoms are conserved.

• Apply the law of conservation of mass to open and closed systems.

Key Vocabulary

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Atom

The smallest unit of an element that maintains its properties and cannot be broken down.

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Molecule

A group of two or more atoms that are chemically bonded together to form a substance.

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Chemical Equation

A shorthand model using symbols and formulas that describes what happens during a chemical reaction.

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Reactant

A substance that is present at the beginning of a chemical reaction and undergoes a change.

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Product

A new substance that is formed as the result of a chemical reaction taking place.

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Chemical Formula

Represents one molecule of a substance, like H₂O for water, showing the elements present.

Key Vocabulary

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Coefficient

The number placed before a chemical formula that shows how many molecules are in a chemical reaction.

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Conservation of Mass

The law of conservation of mass states that matter cannot be created or destroyed in a reaction.

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Properties

Properties are the unique characteristics of a substance, such as its color, density, or state of matter.

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Open System

An open system is an environment where matter and energy are able to freely enter or escape.

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Closed System

A closed system is a sealed environment where matter cannot enter or leave, but energy can be exchanged.

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What is a Chemical Equation?

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

• It shows the molecules, elements, and their roles in the reaction.

• Reactants are on the left, and products are on the right.

• For example: 4Fe + 3O₂ → 2Fe₂O₃.

Multiple Choice

What is the purpose of a chemical equation?

• To use symbols to describe a chemical reaction

• To list all the elements on the periodic table

• To show how to mix chemicals in a lab

• To measure the temperature of a substance

Multiple Choice

In a chemical equation, what is the role of the substances written on the left side of the arrow?

• They are the reactants, which are the starting materials for the reaction.

• They are the products, which are the new substances formed by the reaction.

• They are the symbols that represent the elements in the reaction.

• They are the catalysts that speed up the reaction.

Multiple Choice

Based on the equation 4Fe + 3O₂ → 2Fe₂O₃, what conclusion can be drawn about the rusting of iron?

• Iron (Fe) and oxygen (O₂) are the reactants that form the product, iron oxide (Fe₂O₃).

• Iron oxide (Fe₂O₃) is the reactant that breaks down into the products, iron (Fe) and oxygen (O₂).

• The reaction produces two reactants and one product.

• The arrow shows that iron and oxygen are equal.

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Reading Equations: Formulas and Coefficients

Chemical Formulas

• A chemical formula shows the elements and number of atoms in a single molecule.

• A subscript shows how many atoms of an element are in that one molecule.

• For example, the '2' in H₂O means there are two atoms of hydrogen.

Coefficients

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

• It shows how many molecules of a substance are involved in the reaction.

• In N₂ + 3H₂ → 2NH₃, the '3' shows three molecules of hydrogen are needed.

Multiple Choice

What does a chemical formula, such as H₂O, represent?

• The elements and number of atoms in a single molecule.

• The number of molecules involved in a chemical reaction.

• The total number of substances in an experiment.

• The speed at which a chemical reaction will occur.

Multiple Choice

What is the key difference between a subscript (like the '2' in H₂) and a coefficient (like the '3' in 3H₂)?

• A subscript shows the number of atoms in one molecule, while a coefficient shows the number of molecules.

• A subscript shows the number of molecules, while a coefficient shows the number of atoms.

• A subscript is a large number in front of a formula, while a coefficient is a small number within a formula.

• A subscript applies to the whole reaction, while a coefficient applies to a single element.

Multiple Choice

In the reaction N₂ + 3H₂ → 2NH₃, how many individual atoms of hydrogen (H) are involved?

• There are 2 hydrogen (H) atoms in total.

• There are 3 hydrogen (H) atoms in total.

• There are 5 hydrogen (H) atoms in total.

• There are 6 hydrogen (H) atoms in total.

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How Atoms Rearrange in Reactions

• In a chemical reaction, atoms are rearranged to create new substances.

• Atoms are not created or destroyed, just reconnected in a new way.

• This gives products new properties, like how H₂ and O₂ form water.

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Multiple Choice

What happens to atoms during a chemical reaction?

• They are rearranged to create new substances.

• They are completely destroyed and disappear.

• They are turned into energy.

• They are mixed together but do not change.

Multiple Choice

Which statement describes what it means for atoms to be 'reconnected in a new way'?

• The total number and type of atoms do not change.

• New atoms are formed to create the new substance.

• Some atoms are lost during the reconnection process.

• The properties of the atoms themselves are changed.

Multiple Choice

Hydrogen and oxygen are gases, but when they react to form water, the product is a liquid. What is the best explanation for this change in properties?

• Because the arrangement of hydrogen and oxygen atoms is different in water.

• Because forming a new substance requires creating new atoms.

• Because water is a liquid, and liquids always have different properties.

• Because the original atoms are destroyed to make the new substance.

Law of Conservation of Mass

• Matter is not created or destroyed during a chemical reaction.

• In a reaction, atoms are only rearranged, not lost or gained.

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

Multiple Choice

Which statement best describes the Law of Conservation of Mass?

• Matter is not created or destroyed during a chemical reaction.

• The mass of products is always greater than the reactants.

• Some atoms are lost during all chemical reactions.

• Energy is converted into matter in a chemical reaction.

Multiple Choice

Why does the total mass of reactants equal the total mass of products in a chemical reaction?

• The atoms of the reactants are rearranged to form the products.

• Some of the reactant atoms are converted into energy.

• New atoms are created that are equal in mass to the reactants.

• The products are always in a different state of matter.

Multiple Choice

If 12 grams of carbon react completely with 20 grams of oxygen in a sealed container, what can be concluded about the mass of the products formed?

• The total mass of the products will be 32 grams.

• The total mass of the products will be less than 32 grams.

• The total mass of the products will be more than 32 grams.

• The mass of the products cannot be determined.

Systems: Open vs. Closed

• In an open system, matter can freely enter or escape.

• A closed system is sealed, so matter cannot get in or out.

• Closed systems show that mass is always conserved in a reaction.

Multiple Choice

What is the key difference between an open system and a closed system?

• Whether matter can enter or leave the system

• The temperature of the system

• The type of energy in the system

• The color of the substances in the system

Multiple Choice

If you put a lid on a pot of boiling soup, what kind of system have you created and why?

• A closed system, because the lid prevents matter from escaping.

• An open system, because the soup is hot.

• A closed system, because the soup will cool down.

• An open system, because a reaction is happening inside.

Multiple Choice

An ice cube is placed in a sealed, closed container and allowed to melt. What will happen to the total mass of the container's contents?

• The total mass inside the container will stay exactly the same.

• The total mass will decrease as the ice melts.

• The total mass will increase as the water gets warmer.

• The mass of the water will disappear entirely.

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Common Misconceptions

Misconception	Correction
New atoms are created in a reaction.	Atoms are only rearranged to form new molecules, not created or destroyed.
A product’s properties are an average of the reactants’ properties.	Products are new substances with their own unique properties.
When something burns and turns to ash, mass is lost.	Mass is not lost; it is converted into gases like CO
Balancing an equation means making the number of molecules equal.	Balancing requires an equal number of atoms of each element on both sides.

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Summary

• Chemical equations model reactions, showing how reactants form products.

• Atoms are rearranged in reactions, giving products new properties.

• Atoms are conserved, so reactant mass equals product mass in a closed system.

• Coefficients in equations ensure that all atoms are conserved.

Poll

On a scale of 1-4, how confident are you that you can model how atoms are conserved in a chemical reaction?

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• 2

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• 4