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Unit 2•Lesson 4 2 min read

Chemical Reactions

10/18

Learning Objectives

Define a chemical reaction and identify its parts (reactants and products).
State the Law of Conservation of Mass.
Explain what a balanced chemical equation represents.
Distinguish between endothermic and exothermic reactions.

The Process of Chemical Change

A chemical change is a process that rearranges the atoms of one or more substances to create one or more new and different substances. This process is called a chemical reaction.

Reactants and Products

Every chemical reaction can be described by a chemical equation. A chemical equation is like a recipe that shows what you start with and what you end up with.

Reactants: The substances you start with in a chemical reaction. They are written on the left side of the equation.
Products: The new substances that are formed in a chemical reaction. They are written on the right side of the equation.

The general form is:

Reactants → Products

Example: The reaction of hydrogen gas with oxygen gas to form water.

`2H₂ + O₂ → 2H₂O`

Reactants: Hydrogen (H₂) and Oxygen (O₂)
Product: Water (H₂O)

The Law of Conservation of Mass

One of the most important laws in chemistry is the Law of Conservation of Mass. It states that:

Matter is not created or destroyed in any chemical reaction.

This means that the total mass of the reactants must be equal to the total mass of the products. Atoms are not lost or gained; they are simply rearranged to form new substances.

This is why we must have a balanced chemical equation. A balanced equation has the same number of atoms of each element on both the reactant side and the product side. The `2H₂ + O₂ → 2H₂O` equation is balanced because there are 4 hydrogen atoms and 2 oxygen atoms on both sides of the arrow.

Energy in Chemical Reactions

Chemical reactions always involve a change in energy. Chemical bonds store chemical potential energy. Breaking bonds requires energy, and forming new bonds releases energy.

Exothermic Reaction: A reaction that releases energy into the surroundings, usually in the form of heat and/or light. In an exothermic reaction, the products have less stored chemical energy than the reactants.
Examples: Burning wood, the explosion of dynamite, a chemical hand-warmer.
The surroundings feel hotter.
Endothermic Reaction: A reaction that absorbs energy from the surroundings. In an endothermic reaction, the products have more stored chemical energy than the reactants.
Examples: Photosynthesis (absorbs light energy), baking a cake (absorbs heat energy), a chemical cold pack.
The surroundings feel colder.

Key Terms

**Chemical Reaction
A process in which one or more substances, the reactants, are converted to one or more different substances, the products.
**Reactant
A substance that enters into and is altered in the course of a chemical reaction.
**Product
A substance that is formed as the result of a chemical reaction.
**Law of Conservation of Mass
A fundamental principle of chemistry stating that matter cannot be created or destroyed in a chemical reaction.
**Balanced Chemical Equation
A chemical equation in which the number of atoms of each element is the same on both the reactant and product sides.
**Exothermic Reaction
A chemical reaction that releases energy, usually in the form of heat.
**Endothermic Reaction
A chemical reaction that absorbs energy from its surroundings.

Check Your Understanding

1

In the chemical reaction `CH₄ + 2O₂ → CO₂ + 2H₂O`, which chemical(s) are the products?

2

The burning of a candle is an example of what type of reaction in terms of energy change?

3

What does the Law of Conservation of Mass tell us about a chemical reaction?

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Chemical Bonds: Ionic and Covalent
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Mixtures and Solutions