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Balancing Chemical Equations Chemistry Tutorial

Key Concepts

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Identifying Balanced and Unbalanced Chemical Equations

A chemical equation is said to be balanced when the number of atoms of each element on the left hand side of the equation is the same as the number of atoms of each element on the right hand side of the equation

If there is no number before the molecular formula in a chemical equation then that means that only 1 molecule of this compound is present.

Consider the chemical equation shown below:

general form of equation: reactants → product
chemical equation: 2Mg + O2 → 2MgO

How many atoms of each type of element are present on the reactant side of the chemical equation?

2 atoms of Mg (2 lots of Mg atoms, 2Mg)
2 atoms of O (each O2 molecule is made up of 2 atoms of O and there is 1 O2 molecule)

How many atoms of each type of element are present on the product side of the chemical equation?

There are 2 "molecules" of MgO.(4)
Each "molecule" of MgO is made up of 1 "atom" of Mg and 1 "atom" of O
Therefore there are a total of:

2 atoms of Mg (from 2 lots of MgO )
2 atoms of O (from 2 lots of MgO)

Is this chemical equation balanced?

general form of equation: reactants → product
chemical equation: 2Mg + O2 → 2MgO
number of Mg atoms: 2     = 2
number of O atoms:     2 = 2

Yes, this chemical equation is balanced.
The number of Mg atoms on the reactant side of the chemical equation is the same as the number of Mg atoms on the product side of the chemical equation.
AND
The number of O atoms on the reactant side of the chemical equation is the same as the number of O atoms on the product side of the chemical equation.

Now consider the chemical equation shown below:

general form of equation: reactants → product
chemical equation: H2 + O2 → H2O

How many atoms of each type of element are present on the reactant side of the chemical equation?

2 atoms of H (each H2 molecule is made up of 2 atoms of H and there is 1 H2 molecule)
2 atoms of O (each O2 molecule is made up of 2 atoms of O and there is 1 O2 molecule)

How many atoms of each type of element are present on the product side of the chemical equation?

1 H2O molecule is present on the product side of the chemical equation.
Each H2O molecule is composed of 2 atoms of H and 1 atom of O
Therefore, on the product side of the chemical equation there are

2 atoms of H (each H2O molecule is made up of 2 atoms of H and there is 1 H2O molecule)
1 atom of O (each H2O molecule is made up of 1 atom of O and there is 1 each H2O molecule)

Is this chemical equation balanced?

general form of equation: reactants → product
chemical equation: H2 + O2 → H2O
number of H atoms: 2     = 2
number of O atoms:     2 ≠ 1

No, this chemical equation is NOT balanced, it is said to be unbalanced.
There are more atoms of O on the reactant side of the chemical equation than there are on the product side of the chemical equation.

What could we do to change this unbalanced chemical equation into a balanced chemical equation?

First, we cannot change any of the chemical formula.
H2 must stay as H2
O2 must stay as O2
H2O must stay as H2O

But we can change the number of each molecule present.
For example, if you could somehow halve an oxygen molecule you would have 1 atom of O on the left hand side of the chemical equation:
number of O atoms in ½O2 = ½ × 2 = 1
Would this balance the chemical equation?

general form of equation: reactants → product
chemical equation: H2 + ½O2 → H2O
number of H atoms: 2     = 2
number of O atoms:     ½ × 2 = 1 = 1

Yes, the chemical equation is now balanced.
However, we can't really halve a molecule of O2 so easily.
Which is why we use whole numbers (integers) to balance chemical equations.
To clear the balanced chemical equation of the fraction of ½ all we need to do is multiply all numbers in front of molecular formula in the equation by 2, as shown below

general form of equation: reactants → product
chemical equation: (2 × 1)H2 + (2 × ½)O2 → (2 × 1)H2O
chemical equation: 2H2 + O2 → 2H2O

Is this chemical equation balanced?

general form of equation: reactants → product
chemical equation: 2H2 + O2 → 2H2O
number of H atoms: 2 × 2 = 4     = 2 × 2 = 4
number of O atoms:     1 × 2 = 2 = 2 × 1 = 2

Yes, this chemical equation is balanced.

So now consider the chemical equation below:

general form of equation: reactants → product
chemical equation: 6H2 + 3O2 → 6H2O

Is the chemical equation balanced?

Yes it is.
6 × 2 = 12 atoms of H on the reactant side of the chemical equation AND 6 × 2 = 12 atoms of H on the product side of the chemical equation.
3 × 2 = 6 atoms of O on the reactant side of the chemical equation AND 6 × 1 = 6 atoms of O on the product side of the chemical equation.

BUT, this is not the preferred to way to balance the chemical equation because we are not using the lowest whole number ratio of molecules.
If we divide throughtout by 3 we would arrive at the lowest whole number ratio of molecules.
This is shown below:

general form of equation: reactants → product
chemical equation: (6 ÷ 3 = 2)H2 + (3 ÷ 3 = 1)O2 → (6 ÷ 3 = 2)H2O
chemical equation: 2H2 + O2 → 2H2O

The ratio of H2 : O2 : H2O is 2:1:2 which is the lowest whole number ratio we can have for this chemical reaction.

The balanced chemical equation for the reaction between H2 and O2 to produce H2O is therefore:

2H2 + O2 → 2H2O

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Worked Example of How to Balance a Chemical Equation

Question: When zinc metal (Zn) reacts with hydrochloric acid (HCl), hydrogen gas (H2) and zinc chloride (ZnCl2) are produced.

Write a balanced chemical equation for this reaction.

Solution:

  1. Identify the reactants and products:

    reactants are zinc and hydrochloric acid

    products are hydrogen and zinc chloride

  2. Write the word equation:

    general form of equation: reactants → products
    word equation for this reaction: zinc + hydrochloric acid → hydrogen + zinc chloride

  3. Write the chemical formula for all reactants and products (as given in the question):

    zinc, Zn

    hydrochloric acid, HCl

    hydrogen gas, H2

    zinc chloride, ZnCl2

    Once you have written the formula for each reactant and product you cannot change them during the process of balancing the equation.

  4. Write the unbalanced chemical equation by replacing the words in the word equation with the formulae above:

    general form of equation: reactants → products
    word equation for this reaction: zinc + hydrochloric acid → hydrogen + zinc chloride
    unbalanced chemical equation: Zn + HCl → H2 + ZnCl2

  5. Balancing the equation:

The balanced chemical equation for this reaction is :

Zn + 2HCl → H2 + ZnCl2

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Footnotes:

(1) In this discussion we will be assuming all reactions go to completion, that is, the reactions are spontaneous and irreversible in the forward direction so we will be using the arrow →
Please note that you will balance equilibrium reactions in exactly the same way, but the arrow to use will be ⇋

(2) We will be using the term "molecular formula" rather loosely to refer to both:
molecular compounds
ionic compounds

(3) You will probably meet exceptions to this "rule" quite quickly.
When a fuel combusts in air, for example, the assumption is that there is excess oxygen gas available to sustain the forward reaction.
Because we are mostly interesting in how much energy we will get as a result of burning the fuel we usually balance combustion reactions so that we have 1 molecule of the fuel. This means we often balance a combustion reaction with fractions of an oxygen molecule.
For more information go to Heat of Combustion (fuels)

(4) We will be using the term "molecule" to refer to covalent molecules and also to the formula unit of ionic compounds.
We will also be using the term "atom" to refer to both atoms of elements as well as to their corresponding ions.