What balanced means

A word equation names the reactants and products. A chemical equation uses formulae instead, so it can show the exact ratio of particles that react and form.

For example, hydrogen reacts with oxygen to form water. The unbalanced equation is:

$\ce{H2 + O2 -> H2O}$

Count atoms on each side:

Hydrogen is balanced, but oxygen is not. A balanced equation must fix the oxygen count without changing the formula of water.

Using coefficients

To balance an equation, place whole-number coefficients in front of formulae. A coefficient multiplies every atom in that formula.

$\ce{2H2O}$ means two water molecules. It contains:

Do not change the small numbers inside formulae. Changing $\ce{H2O}$ to $\ce{H2O2}$ would make a different substance. In balancing, you change the number of particles, not the formula of each particle.

For hydrogen and oxygen forming water:

$\ce{2H2 + O2 -> 2H2O}$

The coefficient 1 is not usually written. $\ce{O2}$ means $\ce{1O2}$.

A reliable balancing method

Use the same method each time:

1. Write the formula equation from the information given.
2. Count atoms of each element on both sides.
3. Add or adjust coefficients to balance one element at a time.
4. Recount after every change.
5. Check that the final coefficients are whole numbers in the simplest ratio.

Worked example: balance magnesium reacting with oxygen to form magnesium oxide.

Start with:

$\ce{Mg + O2 -> MgO}$

Oxygen is not balanced because the reactants contain $\ce{O2}$ but each $\ce{MgO}$ contains one oxygen atom. Put 2 before $\ce{MgO}$:

$\ce{Mg + O2 -> 2MgO}$

Now oxygen is balanced, but magnesium is not. Put 2 before $\ce{Mg}$:

$\ce{2Mg + O2 -> 2MgO}$

Final atom count:

This is balanced because every element has the same atom count on both sides.

State symbols

State symbols show the physical state of each substance in the equation. Write the state symbol immediately after the formula.

The symbol (aq) does not mean "liquid". It means the substance is dissolved in water to form an aqueous solution. For example, hydrochloric acid is written as $\ce{HCl (aq)}$, while liquid water is written as $\ce{H2O (l)}$.

State symbols do not balance an equation. They are extra information added to a balanced formula equation.

Example:

$\ce{2Mg (s) + O2 (g) -> 2MgO (s)}$

This shows solid magnesium reacting with oxygen gas to form solid magnesium oxide.

Writing complete equations

The aim is to write the balanced chemical equation, including state symbols when they are required. The formulae or reaction information may be given, but you still need to balance the equation and place each state symbol correctly.

Worked example: methane burns completely in oxygen to form carbon dioxide and water. The water is collected after cooling to a liquid. The formulae and states are:

• methane: $\ce{CH4 (g)}$
• oxygen: $\ce{O2 (g)}$
• carbon dioxide: $\ce{CO2 (g)}$
• water: $\ce{H2O (l)}$

Start unbalanced:

$\ce{CH4 (g) + O2 (g) -> CO2 (g) + H2O (l)}$

Carbon is balanced. Hydrogen is not: there are 4 H atoms on the left and 2 H atoms in one water molecule, so put 2 before water:

$\ce{CH4 (g) + O2 (g) -> CO2 (g) + 2H2O (l)}$

Now oxygen on the products is 4 atoms in total: 2 in $\ce{CO2}$ and 2 in $\ce{2H2O}$. Put 2 before oxygen:

$\ce{CH4 (g) + 2O2 (g) -> CO2 (g) + 2H2O (l)}$

Check the atom count:

This equation is balanced and includes the state symbols.

When a coefficient is fractional during working, multiply every coefficient by the same number to give the smallest whole-number ratio. For ethane combustion:

C2H6 + 7/2O2 -> 2CO2 + 3H2O

Multiplying through by 2 gives:

$\ce{2C2H6 + 7O2 -> 4CO2 + 6H2O}$

There are 4 carbon atoms, 12 hydrogen atoms and 14 oxygen atoms on each side. Fractions and larger multiples can represent the same balanced ratio; use the smallest whole numbers when requested.