What is a Chemical Equation? - Definition & Examples

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  • 0:00 What's a Chemical Equation?
  • 0:48 Chemical Equation Analogy
  • 1:51 Parts of Chemical Equation
  • 2:52 Chemical Equations = Reactions
  • 3:39 Atoms & Molecules in…
  • 5:46 Lesson Summary
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Lesson Transcript
Instructor: Cynthia Shonberg

Cynthia has taught high school science courses for many years and has a Master of Science degree and a Master of Arts in Teaching.

In this lesson, you will learn how chemical equations are used to represent chemical reactions. You will learn that there are many pieces of information provided in a chemical equation, and you will gain an understanding of how to interpret chemical equations.

What's a Chemical Equation?

A chemical equation is a short-hand way to represent the components of a chemical reaction. There are several pieces of information provided in a chemical equation for those working with the chemical equation or corresponding chemical reaction.

You've worked with equations in mathematics where an equation is used to represent information such as equalities or inequalities. Chemical equations are different from mathematical equations because the two parts of a chemical reaction represent the 'before' and 'after' of a chemical reaction. In mathematical equations, an equals sign separates the two parts of the equation. In chemical equations, equal signs are not used. Instead, an arrow is used to separate the two sides of the equation, and it points in the direction that a chemical reaction will proceed.

Chemical Equation Analogy

Think of a chemical equation as a mini recipe to make a new substance. To make a beautiful cake, you need to add the correct amounts and proportions of eggs, flour, sugar, oil, and flavorings. A chemical equation provides information about the correct proportions of ingredients that are needed to make new substances in chemical reactions.

Here's a recipe for making delicious lemonade:

  • To make a large glass of lemonade, use 1 1/2 cups of water plus ½ cup of lemon juice plus ¼ cup of sugar.

Here's a ''recipe'' for the formation of water from molecular hydrogen and molecular oxygen:

  • 1 mole of oxygen plus 2 moles of hydrogen yields 2 moles of water.

Note that just as a cup is a quantity in a recipe, a mole is a quantity in chemical reactions.

To write the equation using the chemical symbols and components of a chemical equation, we would write:

Equation that yields water

As you can see, both the chemical equation and the recipe for lemonade provide adequate information that shows how to create a new product.

Parts of a Chemical Equation

In general, we represent an equation in this way:

Basic form of a chemical equation

We say that there are left and right sides of an equation and that they are separated by an arrow. The arrow can be a right-pointing arrow, a left-pointing arrow, or a double-headed arrow, depending on the chemical species and equilibrium involved in the chemical reaction. You can read the arrow as ''goes to,'' ''forms'' or ''yields.''

To represent a generic chemical equation, we often use alphabetic letters. We read the equation as ''A plus B yields AB'' (or ''A plus B forms AB'' or ''A plus B goes to AB''). Note that A and B are reactants, and AB is the product (a newly formed substance in a chemical reaction).

There are many different kinds of chemical reactions, and they vary in the number of reactants and products. The diagram shows a schematic of the variety of ways reactants and products can change, or rearrange, in a chemical reaction.


Chemical Equations = Reactions

Examples of reactions

Because a chemical equation represents a chemical reaction, it's important to understand what a chemical reaction is. You observe chemical reactions taking place every day. A chemical reaction occurs when matter is rearranged to form a different or new substance.

Fry an egg - or cook anything really - and a chemical change takes place. The beautiful night sky on the Fourth of July is an amazing display of chemical reactions taking place. Burning a log in a campfire is an example of a combustion chemical reaction. Even that old rusty screw on the floor of your garage is the result of a chemical reaction having taken place.

We say that a chemical change has taken place in a chemical reaction. To help us understand how the matter was rearranged, we use chemical equations to symbolize what chemical changes have taken place.

Atoms & Molecules in an Equation

As you've learned, the main parts of a chemical equation are the reactants and the products. Besides these, however, there are other pieces of information represented in a chemical reaction. Additional important information provided in chemical reactions involves the number of atoms and molecules in a chemical reaction, shown with coefficients and subscripts.

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