Balanced chemical equations

A balanced equation models a chemical reaction using the formulae of the reactants and products. It shows the number of units of each substance involved.

Balancing an equation

If you just write an equation replacing names with formulae, it may not be balanced. The numbers of atoms of each element on the left must be the same as they are on the right.

To balance an unbalanced equation, you need to add numbers to the left of one or more formulae. Here is one way to work out how to do this for the reaction between nitrogen and hydrogen.

StepResult
Check to see if there are equal numbers of atoms of each element on both sides. Here there aren’t.N2 + H2 → NH3
There are two nitrogen atoms on the left but only one on the right, so a big 2 is added to the left of the NH3.N2 + H2 → 2NH3
There are two hydrogen atoms on the left but (2 × 3) = 6 on the right, so a big 3 is placed in front of the H2.N2 + 3H2 → 2NH3
Check to see if there are equal numbers of each element of both sides. There are.(two nitrogen atoms and six hydrogen atoms)
Add the state symbols if they are requested.N2(g) + 3H2(g) → 2NH3(g)
curriculum-key-fact
Balanced equations only show formulae, not names. A balancing number multiplies all the atoms in the substance next to it.

State symbols

Balanced equations often include state symbols in brackets after each formula. They show the physical state of that substance.

State symbolMeaning
(s)Solid
(l)Liquid
(g)Gas
(aq)Aqueous solution

An aqueous solution forms when a substance dissolves in water.

State symbols are useful because they show what a substance is like. For example:

  • H2O(l) is liquid water, but H2O(g) is steam
  • HCl(g) is hydrogen chloride gas, but HCl(aq) is hydrochloric acid
Question

Sodium metal reacts with water to form sodium hydroxide solution and hydrogen gas. Write a balanced equation for the reaction, including state symbols.

2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g) or

Na(s) + H2O(l) → NaOH(aq) + ½H2(g)