In this explainer, we will learn how to construct equations for chemical reactions and write them using words or chemical symbols.
In order to better understand chemical reactions, it is important that we learn how to write and read chemical equations. A chemical equation indicates the species involved in and produced from a chemical reaction using chemical symbols.
Definition: Chemical Equation
A chemical equation is a written representation of a chemical reaction using chemical symbols.
Let us take a look at some chemical equations:
Each of the equations contains a forward arrow called the reaction arrow, which is often read as “reacts to produce.” We call the species on the left-hand side of the arrow reactants and the species on the right-hand side of the arrow products:
The reactants are the starting materials of a chemical reaction. They are converted into the products. Therefore, the reactants react to produce the products. Notice that in a chemical equation we may have one or more reactants and one or more products. Multiple species on the same side of the reaction arrow are separated by a plus sign . Looking at the chemical equation we can identify and as the reactants and as the product.
Definition: Reactants
Reactants are the starting materials in a chemical reaction. They are written on the left-hand side of the reaction arrow.
Definition: Products
Products are the species formed during a chemical reaction. They are written on the right-hand side of the reaction arrow.
Chemical equations may also include special instructions written above the reaction arrow. Observe the chemical equation
The capital Greek letter delta indicates that heat needs to be applied to the reactants in order for the reaction to occur.
Chemical formulas written above the reaction arrow, as shown below, can represent catalysts added to the reaction:
Example 1: Determining the Products from the Symbol Equation for a Chemical Reaction
The symbol equation for a chemical reaction is shown below:
What are the products of this reaction?
Answer
In a chemical equation, the reactants are written on the left-hand side of the arrow and the products are written on the right-hand side. and appear on the right-hand side of the arrow and are therefore the products.
If more than one of the same species is needed as a reactant or is produced as a product, it is not written separately in the equation. We simply place a number in front of the species. This is called a stoichiometric coefficient, though it is often simply referred to as a coefficient.
Definition: Stoichiometric Coefficient
A stoichiometric coefficient is number appearing in front of a species in a chemical equation to indicate the number of that species present during the reaction.
Take the following equation:
A coefficient of two was placed in front of the and . This means that two atoms of sodium were needed for the reaction and two units of sodium chloride were produced by the reaction.
Sometimes we want to indicate the physical state of each of the reactants and products. This is usually done using italicized letters in parentheses, called state symbols.
Definition: State Symbol
State symbols are italicized letters in parentheses that appear after each species in a chemical equation to indicate the physical state of the species: gas = (g), liquid = (l), solid = (s), and aqueous = (aq).
We can see the use of state symbols in the chemical equations below:
Gases, liquids, and solids are represented by (g), (l), and (s) respectively. Species may also have the state symbol (aq) for aqueous. This means that the species is dissolved in water.
Chemical equations can be written with chemical symbols, but they can also be written as word equations:
Word equations follow the same structure as symbol equations, with the reactants on the left-hand side of the arrow and the products on the right-hand side of the arrow. Word equations can be an easier way to write a chemical equation as they do not require any chemical formulas. However, they are often more difficult to interpret as they do not indicate the number of atoms in each species, the coefficients, or the state symbols.
In order to convey a chemical equation, we should be able to write the chemical symbols and read the chemical statement aloud. When reading a chemical equation, we read each plus sign as the word “and” and the forward arrow as “react(s) to produce.” Let us look at the following chemical equation:
We should read this chemical equation as “sodium and chlorine react to produce sodium chloride.” We can include the coefficients and state symbols if we wanted to be even more specific: “Two atoms of solid sodium metal and one molecule of chlorine gas react to produce two units of sodium chloride, a solid.”
Example 2: Identifying the Chemical Statement That Corresponds to a Chemical Equation
Pick the statement that describes the chemical reaction shown in the following symbolic equation:
- Carbon dioxide reacts with water to produce methane and oxygen.
- Methane decomposes under heat to produce oxygen, carbon dioxide, and water.
- Methane burns in carbon dioxide to produce oxygen and water.
- Methane burns in oxygen to produce carbon dioxide and water.
- Oxygen reacts with water to produce methane and carbon dioxide.
Answer
In a chemical equation, the species that appear on the left-hand side of the arrow are the reactants. On the left-hand side of this chemical equation, we see , methane, and , oxygen. The species that appear on the right-hand side of the arrow are the products. On the right-hand side of this chemical equation, we see , carbon dioxide, and , water.
We can read the chemical equation as a statement by replacing the plus sign with the word “and” and the reaction arrow with the phrase “react to produce.” Reading the chemical equation gives us the statement, “Methane and oxygen react to produce carbon dioxide and water.”
The answer choice that most closely resembles this statement is answer choice D. Note that the phrase “methane burns in oxygen” describes a reaction between methane and oxygen.
We should also be able to write a chemical equation from a chemical statement. There are a few keywords that can help us determine the reactants and products. Phrases such as “reacts with” or “burns in” indicate that the species performing these actions are the reactants. Words such as “yields” or “decomposes into” indicate that species before this word in the sentence are the reactants and species after this word in the sentence are the products.
Phrases such as “is produced by” and “is generated” indicate that species before this phrase in the sentence are the products and species after this phrase in the sentence are the reactants. Common phrases used in chemical statements and their relationship to reactants and products are shown in the table below.
| Reactant A | Reacts with
Is mixed with Combines with Is added to Burns in | Reactant B |
| Reactants | Yield
Produce Form Decompose into | Products |
| Products | Are produced by
Are generated from | Reactants |
We can also gather information regarding the state of matter. The words “solution” or “soluble” indicate that the state symbol of that species should be (aq) while the words “insoluble” and “precipitate” indicate that the state symbol of that species should be (s).
Let us write a chemical equation for the reaction described by the following chemical statement: A sample of solid copper metal is added to a flask containing a nitric acid solution (). The reaction produces soluble copper (II) nitrate (), nitrogen dioxide gas, and water.
The copper metal is added to the nitric acid. The key phrase “is added to” indicates that these two species are the reactants. They can be written on the left-hand side of the reaction arrow with a plus sign between them. Furthermore, we know that the copper is a solid and should have the state symbol (s). The nitric acid is a solution, meaning that it is dissolved in water and should have the state symbol (aq). We can therefore write the following:
The keyword “produces” indicates that the copper(II) nitrate, nitrogen dioxide, and water are all products. They can be written on the right-hand side of the reaction arrow with a plus sign between them. We know that nitrogen dioxide is a gas and water is a liquid, meaning that they should have the state symbols (g) and (l) respectively. Copper(II) nitrate is described as being soluble. This means that the copper(II) nitrate is dissolved in water and should have the state symbol (aq). Adding the products gives us the unbalanced chemical equation
Example 3: Identifying the Chemical Equation That Corresponds to a Chemical Statement
An experiment shows that hydrogen peroxide decomposes to form water and oxygen gas. Which of the following word equations describes the chemical reaction that has occurred in this experiment?
Answer
In the chemical statement, we find the keywords “decomposes” and “form.” Both of these keywords indicate that the species that appear before the keyword in the sentence are the reactants and the species that appear after the keyword are the products.
Before the keywords, we see only one species, hydrogen peroxide. This is the only reactant. It should appear in the chemical equation to the left of the reaction arrow:
After the keywords, we see the species water and oxygen. These are the products. They should appear after the reaction arrow in the chemical equation separated by a plus sign:
The answer choice that matches our chemical equation is answer choice E.
Example 4: Identifying the Chemical Equation, with State Symbols, That Corresponds to a Chemical Statement
Solid calcium carbonate decomposes upon heating to produce carbon dioxide and solid calcium oxide. Which of the following symbolic equations, with state symbols, describes this chemical reaction?
Answer
In the chemical statement, we find the keyword “decomposes.” This keyword indicates that the species that appear before it in the sentence are the reactants and the species that appear after it are the products.
Before the keyword, we see only one species, calcium carbonate. This is the only reactant. It should appear in the chemical equation to the left of the reaction arrow. After the keyword, we see carbon dioxide and calcium oxide. These are the products. They should appear to the right of the reaction arrow separated by a plus sign:
With this information, we can eliminate answer choices D and E, as both have more than one reactant.
We know from the chemical statement that calcium carbonate and calcium oxide are solids. The state symbol (s) should be written after each of these species in the chemical equation to indicate that they are in the solid state:
Furthermore, we should recognize that carbon dioxide is a gas that has the chemical formula . It should appear in the chemical equation with the state symbol (g) to indicate that it is in the gaseous state:
Looking at the three remaining answer choices, we can rule out answer choice A because the reactant has the state symbol (g) instead of (s). We can also rule out answer choice C because both products were given the state symbol (g) and we know that calcium oxide must be a solid. This leaves us with the correct answer being answer choice B:
All of the reactions thus far have been irreversible reactions, meaning that the reactants react to produce the products and the products cannot convert back into reactants. Irreversible reactions use the forward reaction arrow and are said to proceed in the forward direction, that is, from reactants (left) to products (right).
Definition: Irreversible Reaction
An irreversible reaction is one where reactants produce the products and the products cannot convert back into reactants. The chemical equation uses the forward reaction arrow .
In a reversible reaction, the reactants react to produce the products and the products can convert back into reactants. This reaction is said to proceed in both the forward direction (reactants produce products) and the reverse direction (products produce reactants) with both reactions occurring simultaneously. Let us take a look at the reaction below:
This reaction is reversible. If we read the reaction from left to right, nitrogen gas reacts with hydrogen gas to produce ammonia. However, if we read the reaction from right to left, ammonia decomposes to produce nitrogen gas and hydrogen gas. Both of these reactions can occur simultaneously. The double arrow , also called an equilibrium arrow, is used to indicate that the reaction is reversible.
Definition: Reversible Reaction
A reaction where reactants produce the products and the products can simultaneously convert back into reactants. The chemical equation uses the equilibrium arrow .
Example 5: Understanding the Types of Reaction Arrows
The reaction of hydrogen and iodine to produce hydrogen iodide is shown below:
What is the meaning of the arrow in this chemical reaction equation?
- Heat is needed for the reaction to work.
- There is a change in the state of matter.
- The reaction is reversible.
- The reaction is irreversible.
- Ultraviolet light is needed for the reaction to work.
Answer
The arrow used is called the equilibrium arrow. It separates the reactants (left-hand side of the arrow) from the products (right-hand side of the arrow) in a reversible reaction, that is, a reaction that can proceed from reactants to products and from products to reactants simultaneously.
The answer choice that correctly states the meaning of the equilibrium arrow is answer choice C.
Key Points
- A chemical equation shows all of the species involved in a reaction.
- Reactants are written to the left of the reaction arrow and products are written to the right.
- State symbols (g, l, s, and aq) can be used to indicate the physical state of each species.
- Keywords such as “reacts,” “decomposes,” and “produces” in a chemical statement can help us write a chemical equation.
- Reactions may be irreversible, indicated by a forward arrow , or reversible, indicated by an equilibrium arrow .
