How to Write a Chemical Equation?

Updated: May 4, 2020

A chemical equation gives you the essential information about a chemical reaction, specifically:

The nature of the reactants (chemicals reacting together) and products (chemicals produced)
The quantities of reactants and products
The physical state of the reactants and products (E.g. solid, liquid, gas, aqueous)

Write down a word equation for the chemical reaction
Write down the unbalanced equation using correct chemical symbols for all the reactants and products
Balance the equation by inspection. (Ensure that the same number of each type of atom appears on both sides of the equation)
Write the state symbol after each chemical symbol

Typically, you balance H (hydrogen) atoms first, followed by O (oxygen) atoms, than any other atoms. OR you can start by balancing the most complicated molecule.
Never change the chemical symbol of a substance. (Do not change the subscripts in the chemical symbols of reactants or products)
A number in front of a formula multiplies every symbol that follows it
Remember that atoms cannot be created or destroyed in a chemical reaction (i.e. do not ‘eat’ any chemical symbols)

Formation of iron(III) chloride from the reacting iron with chlorine gas

Iron + chlorine → iron(III) chloride
Before balancing: Fe + Cl2 → FeCl3
Balance number of Cl atoms: There are 2 Cl atoms on the left and 3 Cl atoms on the right. The simplest way to balance Cl atoms is to multiply 3 on the left and 2 on the right to make a total of 6 Cl atoms.
We obtain: Fe + 3 Cl2 → 2 FeCl3
Notice that the number of Fe atoms is not the same for both sides: 1 on the left and 2 on the right. Hence, we will just have to multiply 2 on the left.
We have: 2 Fe + 3 Cl2 → 2 FeCl3
Not forgetting to include the state symbols: 2 Fe(s) + 3 Cl2(g) → 2 FeCl3(s)

Formation of carbon dioxide and water from methane and oxygen. (or known as Combustion of methane in oxygen)

methane + oxygen → carbon dioxide + water
Convert the above to chemical symbols: CH4 + O2 → CO2 + H2O
Balance H first: There are 4 H atoms on the left and 2 on the right. Hence, we will have to multiply the chemical symbol containing H on the left by 2. (i.e. multiply H2O by 2)
We have: CH4 + O2 → CO2 + 2 H2O
Balance O: There are 2 O atoms on the left and 4 O atoms on the right. We will have to multiply the O2 on the left by 2.
We have: CH4 + 2 O2 → CO2 + 2 H2O
The carbon atoms are balanced so we do not have to multiply them.
Including the state symbol: CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)

Balance: Pb3O4 + HNO3 → PbO2 + Pb(NO3)2 + H2O

Notice that there are 3 Pb atoms on the left and 2 Pb atoms on the right. There are several ways to go about balancing the Pb atoms – we can multiply PbO2 by 2 or Pb(NO3)2 by 2 or multiply both sides of the equation. Any of the ways is correct. But some ‘ideal’ way will be shorter than the others.
We shall look at multiplying Pb(NO3)2 by 2. (shortest method in this case)
We have Pb3O4 + HNO3 → PbO2 + 2Pb(NO3)2 + H2O
We have 1 N atom on the left and 4 N atoms on the right. We will have to multiply the HNO3 by 4
We have: Pb3O4 + 4HNO3 → PbO2 + 2Pb(NO3)2 + H2O
Balance H atom: Pb3O4 + 4HNO3 → PbO2 + 2Pb(NO3)2 + 2H2O
Notice that the number of O atoms is balanced. We’re done!
Including state symbol: Pb3O4(s) + 4 HNO3(aq) → PbO2(s) + 2 Pb(NO3)2(aq) + 2 H2O(l)

Important: Always express your equation as the simplest form.

For example: 2 Pb3O4(s) + 8 HNO3(aq) → 2 PbO2(s) + 4 Pb(NO3)2(aq) + 4 H2O(l)
The above equation is not correct as it is not in its simplest form. (You can divide throughout by 2)

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