When the number of atoms are the same on the left and the right side of a chemical equation?

Table of Contents Show

What is a Chemical Equation?
Fundamentals of a Chemical Equation
Steps to Writing & How to Balance Chemical Equations
Balancing Chemical Equations – Example
1. Write the Word Equation
2. Rewrite the Word Equation as a Formula Equation
3. Balance the formula equation using the law of conservation of mass.
4. Check the balanced equation by counting all the atoms.
Further Reading

In this tutorial you will learn the process of balancing chemical equations as well as the components that make up a chemical equation.

What is a Chemical Equation?

A chemical equation represents a chemical reaction using symbols and numbers. Chemical equations show the relative molar quantities and identities of different reactants and products. Balancing chemical equations can help you determine the accurate ratio of compounds involved in a reaction!

Fundamentals of a Chemical Equation

1. Basic parts of a chemical equation

There are 3 basic parts to a chemical equation: reactants, products, and the “yields” arrow.

	Reactants	Yields	Products
Definition	Reactants are compounds or elements that are needed in the reaction and that undergo chemical change during the reaction.	In reactions, “yield” means to “produce” or “form” and is usually represented by an arrow. When reactants undergo a chemical change, they yield products.	Products are the end results, or compounds produced, in a chemical reaction.
Example	CH4 (g) + 2O2 (g)	→	CO2 (g) + 2H2O (g)

The reactants and products are listed as multiple molecules added together, and this is the case for any chemical equation. The yields arrow, however, can come in a few different styles:

Forward arrow (→) shows a general reaction.
Equilibrium arrow (⇌) shows a reversible process.
Resonance arrow (↔) shows that two or more species are resonance structures of each other.
Theoretical arrow (⇢) shows a theoretical process.
Retrosynthetic arrow (⇒) shows potentially ways to get from reactants to products.

2. Numbers in chemical equations

There are two types of numbers in chemical equations: subscripts and coefficients. Subscripts represent how many atoms of each element are present in a molecule. Coefficients represent how many molecules of a specific chemical compound are present in the reaction. Only coefficients of molecules can be changed in balancing equations.

Molecules in which the subscripts of atoms of the same elements are different (for example H2O vs. H2O2) have different chemical compositions, meaning they are not the same compound. For this reason, when balancing chemical equations, subscripts of molecules cannot be changed. The same molecules having different coefficients (for example 2H2O and 5H2O) are the same compounds, present in different amounts.

In a molecule 3NO2, the coefficient is 3 (there are 3 molecules of NO2), and the subscript of oxygen is 2 (there are 2 oxygen atoms in 1 molecule of NO2). In total, there are 6 oxygen atoms (3 molecules of NO2 and 2 oxygen atoms per 1 molecule of NO2.)

The coefficients of each molecule or compound represent the relative quantity of the element in the reaction in moles or numbers of molecules. Because they are relative quantities, the numbers can also be interpreted as ratios; in the example above, the ratio would be 1 CH4 : 2O2 : 1 CO2 : 2H2O. As long as the ratios are constant, the equation can be correctly rewritten with any proportional coefficients (such as 2:4:2:4, 3:6:3:6, etc.), though it is common practice to use the smallest whole number coefficients.

3. The Law of Conservation of Mass

The law of conservation of mass states that matter (or atoms) are neither created nor destroyed in chemical reactions. This means that there must be equal amounts of each compound before and after a chemical reaction takes place. The law applies to balancing chemical equations because there must be the same relative amounts of each compound on both sides of the equation.

Here’s an example of a reaction in which methane burns in air to produce carbon dioxide and water vapor:

CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)

As you can see, both sides of the equation each have 1 carbon atom, 4 hydrogen atoms, and 4 oxygen atoms. Thus the equation is balanced.

Steps to Writing & How to Balance Chemical Equations

Identifying the names of the reactants and products. It can be helpful to write a word equation to list out all the compounds in the reaction. In a word equation, the reactants and products are represented by their names, meaning they are written in word form, instead of as a molecular formula.
1. It is important to remember that word equations only show the names of compounds, and not the quantities. This means that word equations do not show the full picture of the reaction.
Using the word equation, write a formula equation by rewriting all of the compounds into their chemical formula.
1. Formula equations are not balanced. They only show the compounds in their chemical formula.
Balance the formula equation using the law of conservation of mass to write the chemical equation.
1. Start by counting the atoms of elements that only appear once on each side of the equation and balance those first. Balance atoms that appear multiple times on each side of the equation last.
2. Usually, balance any hydrogen or oxygen atoms last.
After balancing all the atoms, you have a balanced formula equation, or chemical equation.

Balancing Chemical Equations – Example

Balance the reaction: methane burns in air and combines with oxygen to produce carbon dioxide and water vapor

1. Write the Word Equation

Write the reaction as a word equation: methane + oxygen → carbon dioxide + water

2. Rewrite the Word Equation as a Formula Equation

Rewrite as formula equation: CH4 (g) + O2 (g) → CO2 (g) + H2O (g)

3. Balance the formula equation using the law of conservation of mass.

Count the atoms of elements that appear once in the equation. In this example, carbon and hydrogen atoms appear once, and oxygen appears twice on the right side.
Balance hydrogen and oxygen atoms last. Since oxygen appears twice, you would balance oxygen last. In the example, you would start by balancing carbon, then hydrogen, and oxygen last.
There is one carbon atom on the left side of the equation (CH4), and one on the right side (CO2). Therefore, carbon is already balanced. Both compounds containing carbon (CH4 and CO2) should have a coefficient of 1, because they must exist equally to have an equal number of carbon atoms.
There are 4 hydrogen atoms on the left side of the equation (CH4), and 2 on the right side (H2O). You will need 2 more hydrogen atoms on the right side to balance the hydrogen atoms. Multiplying the coefficient of H2O by 2 gives 2H2O, which has 4 hydrogen atoms.
1. It is important not to try to change H2O to H4O in an effort to balance the hydrogen atoms. Doing so changes the chemical identity, and H4O is not water. Multiplying the entire compound (H2O) changes the number of water molecules in the reaction, without altering its chemical identity.
The formula equation is now: 1CH4 (g) + 1O2 (g) → 1CO2 (g) + 2H2O (g)
There are now 2 oxygen atoms on the left side (O2) and 4 on the right side (2H2O and CO2). Multiplying the coefficient of O2 by 2 gives 2O2, which has 4 oxygen atoms.
After all the atoms are balanced, you have a chemical equation: CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)

4. Check the balanced equation by counting all the atoms.

Left Side	Right Side
Carbon atoms – 1 (CH4)	Carbon atoms – 1 (CO2)
Hydrogen atoms – 4 (CH4)	Hydrogen atoms – 4 (2H2O)
Oxygen atoms – 4 (2O2)	Oxygen atoms – 4 (2 from CO2, 2 from 2H2O)

Each element in the reaction has the same number of atoms on both sides — therefore the balanced chemical equation is correct!

where the + sign means that the two substances interact chemically with each other and the → symbol implies that a chemical reaction takes place. But substances can also be represented by chemical formulas. Remembering that hydrogen and oxygen both exist as diatomic molecules, we can rewrite our chemical change as

This is an example of a chemical equationA concise way of representing a chemical reaction., which is a concise way of representing a chemical reaction. The initial substances are called reactantsAn initial substance in a chemical equation., and the final substances are called productsA final substance in a chemical equation..

Unfortunately, it is also an incomplete chemical equation. The law of conservation of matter says that matter cannot be created or destroyed. In chemical equations, the number of atoms of each element in the reactants must be the same as the number of atoms of each element in the products. If we count the number of hydrogen atoms in the reactants and products, we find two hydrogen atoms. But if we count the number of oxygen atoms in the reactants and products, we find that there are two oxygen atoms in the reactants but only one oxygen atom in the products.

What can we do? Can we change the subscripts in the formula for water so that it has two oxygen atoms in it? No; you cannot change the formulas of individual substances because the chemical formula for a given substance is characteristic of that substance. What you can do, however, is to change the number of molecules that react or are produced. We do this one element at a time, going from one side of the reaction to the other, changing the number of molecules of a substance until all elements have the same number of atoms on each side.

To accommodate the two oxygen atoms as reactants, let us assume that we have two water molecules as products:

The 2 in front of the formula for water is called a coefficientA number in a chemical equation indicating more than one molecule of the substance.. Now there is the same number of oxygen atoms in the reactants as there are in the product. But in satisfying the need for the same number of oxygen atoms on both sides of the reaction, we have also changed the number of hydrogen atoms on the product side, so the number of hydrogen atoms is no longer equal. No problem—simply go back to the reactant side of the equation and add a coefficient in front of the H2. The coefficient that works is 2:

There are now four hydrogen atoms in the reactants and also four atoms of hydrogen in the product. There are two oxygen atoms in the reactants and two atoms of oxygen in the product. The law of conservation of matter has been satisfied. When the reactants and products of a chemical equation have the same number of atoms of all elements present, we say that an equation is balancedA condition when the reactants and products of a chemical equation have the same number of atoms of all elements present.. All proper chemical equations are balanced. If a substance does not have a coefficient written in front of it, it is assumed to be 1. Also, the convention is to use all whole numbers when balancing chemical equations. This sometimes makes us do a bit more “back and forth” work when balancing a chemical equation.

Write and balance the chemical equation for each given chemical reaction.

Hydrogen and chlorine react to make HCl.
Ethane, C2H6, reacts with oxygen to make carbon dioxide and water.

Solution

Let us start by simply writing a chemical equation in terms of the formulas of the substances, remembering that both elemental hydrogen and chlorine are diatomic:
H2 + Cl2 → HCl
There are two hydrogen atoms and two chlorine atoms in the reactants and one of each atom in the product. We can fix this by including the coefficient 2 on the product side:
H2 + Cl2 → 2HCl
Now there are two hydrogen atoms and two chlorine atoms on both sides of the chemical equation, so it is balanced.
Start by writing the chemical equation in terms of the substances involved:
C2H6 + O2 → CO2 + H2O
We have two carbon atoms on the left, so we need two carbon dioxide molecules on the product side, so that each side has two carbon atoms; that element is balanced. We have six hydrogen atoms in the reactants, so we need six hydrogen atoms in the products. We can get this by having three water molecules:
C2H6 + O2 → 2CO2 + 3H2O
Now we have seven oxygen atoms in the products (four from the CO2 and three from the H2O). That means we need seven oxygen atoms in the reactants. However, because oxygen is a diatomic molecule, we can only get an even number of oxygen atoms at a time. We can achieve this by multiplying the other coefficients by 2:
2C2H6 + O2 → 4CO2 + 6H2O
By multiplying everything else by 2, we don’t unbalance the other elements, and we now get an even number of oxygen atoms in the product—14. We can get 14 oxygen atoms on the reactant side by having 7 oxygen molecules:
2C2H6 + 7O2 → 4CO2 + 6H2O
As a check, recount everything to determine that each side has the same number of atoms of each element. This chemical equation is now balanced.

Test Yourself

Write and balance the chemical equation that represents nitrogen and hydrogen reacting to produce ammonia, NH3.

Answer

N2 + 3H2 → 2NH3

Many chemical equations also include phase labels for the substances: (s) for solid, (ℓ) for liquid, (g) for gas, and (aq) for aqueous (i.e., dissolved in water). Special conditions, such as temperature, may also be listed above the arrow. For example,

From the statement “nitrogen and hydrogen react to produce ammonia,” identify the reactants and the products.
From the statement “sodium metal reacts with water to produce sodium hydroxide and hydrogen,” identify the reactants and the products.
From the statement “magnesium hydroxide reacts with nitric acid to produce magnesium nitrate and water,” identify the reactants and the products.
From the statement “propane reacts with oxygen to produce carbon dioxide and water,” identify the reactants and the products.
Write and balance the chemical equation described by Exercise 1.
Write and balance the chemical equation described by Exercise 2.
Write and balance the chemical equation described by Exercise 3.
Write and balance the chemical equation described by Exercise 4. The formula for propane is C3H8.
Balance: ___NaClO3 → ___NaCl + ___O2
Balance: ___N2 + ___H2 → ___N2H4
Balance: ___Al + ___O2 → ___Al2O3
Balance: ___C2H4 + ___O2 → ___CO2 + ___H2O
How would you write the balanced chemical equation in Exercise 10 if all substances were gases?
How would you write the balanced chemical equation in Exercise 12 if all the substances except water were gases and water itself were a liquid?

reactants: nitrogen and hydrogen; product: ammonia
reactants: magnesium hydroxide and nitric acid; products: magnesium nitrate and water
Mg(OH)2 + 2HNO3 → Mg(NO3)2 + 2H2O