How to write fully balanced chemical equations

Balancing only

The first exercise here is balancing equations when you have been provided with all the formulae. In order to do this, you need to keep a tally of the number of each element (or grouping of atoms). The number in front of a formula multiplies all the atoms within that formula. You need to change these numbers so the number of atoms of each element in the products is equal to the number of that element's atoms in the reactants. A tally table is helpful whilst you are working this out. You can do working on rough paper if you wish, but the web page also provides you with a blank table in which you can write. The webpage does not check what you have written in the tally table.

The webpage can also help you by automatically keeping a tally of all the atoms as you change the coefficients in front of formulae. However, you only get half marks for balancing, if you have opted to get help.

Write fully balanced formula equations

You should have gone through all the formula writing exercises before you do this.

  • Step 1 is to write the formulae of reactants and products. Numbers in formulae need to be correctly formatted (i.e. as subscripts). Do not think about balancing at this stage.
  • Step 2 is to balance the equation. You cannot meaningfully balance an equation until all the formulae are correct and you should never try balancing an equation by changing a formula. This web page does not allow you to change a correct formula.
  • As you work through these equations, your previous answers are recorded in reverse order.

If you ask the webpage makes a tally table, it does so for each different element. Whilst this is sometimes appropriate, there are also some reactions in which it makes sense to treat a whole group of atoms (such as a spectator molecular ion) as one entity. With practice and experience you will become better at spotting when this shortcut can be done.

Ionic Equations

The most important thing to understand about ionic equations is their purpose. Ionic equations are sometimes much simpler than full equations.

Ionic equations tell you exactly which species (atoms, molecules, or ions) are actually reacting and changing in a reaction.

When an ionic substance is dissolved in water, every ion is surrounded by water molecules. This means that the positive ions (cations) and negative ions (anions) behave independently of one another in chemical reactions.

Here are the rules for writing ionic equations on this site:

  • If a substance is made of ions (i.e. metal compounds, strong acids, and ammonium compounds) and is dissolved in water, state symbol = (aq), then each ion is considered separately. If an ion remains unchanged, dissolved in water at the end of the reaction then it is called a spectator ion and it is left out of the equation. It doesn't matter if it has changed partners with a different oppositely charged ion.
  • Do not write the full formula of every substance - only those that are covalent, metallic elements and solids. Only write the relevant ions of dissolved ionic compounds that will change in the reaction and leave out the spectator ions. You can list the reactants in any order you wish. Similarly, you can list the products in any order you wish.
  • All other substances (covalent compounds, covalent elements, metal elements and solid ionic compounds) are written in full.
  • Use the formatting tools to ensure that the number of atoms is in subscript and the charge of an ion is superscript. Number formatting and capitalisation must be correct. As soon as the formula has been entered correctly, the correct formula automatically replaces the input box.
  • A state symbol needs to be entered in normal brackets following each item in the equation.
  • Balancing is done on this site once all the formulae and state symbols have been correctly entered.

In order to maintain consistency, this site follows these rules strictly and if you do the same in your own work, you should not be faulted. You will sometimes see a more relaxed approach taken in textbooks in order to explain a point and this is perfectly reasonable.

The questions here come in a variety of forms, for example a formula equation, a word equation, or just a text description of the reaction.

Ionic equation guidelines

When starting to learn how to write ionic equations, you are advised to do this one topic at a time.
You can select the topics using the link on the page.
  • If a strong acid is reacting with a base, then the relevant active species from the acid is H+ (aq).

    • Hydrochloric, nitric and sulfuric acids are strong.
    • Weak acids include ethanoic, phosphoric, carbonic, and sulfurous acids. They are mainly undissociated into ions when dissolved in water so in the ionic equation, the formula of the whole molecule is written.
  • If a strong alkali is being neutralised by an acid, then the relevant active species from the alkali is OH- (aq).

  • Aqueous solutions of group 1 and group 2 hydroxides are strong.
  • 'Strong' means that the compound fully dissociates into ions when dissolved in water.

    For example, the full equation for neutralisation of sulfuric acid by potassium hydroxide is:
    H2SO4 (aq) + 2 KOH (aq) → H2SO4 (aq) + H2O (l)
    Sulfuric acid is a strong acid so the relevant ions from sulfuric acid in the ionic equation reaction are the 2 H+ (aq) ions. The SO42- (aq) ion is not going to change. Potassium hydroxide is a strong alkali so it is the 2 OH- (aq) that react, not the 2 K+ (aq) ions. Therefore we leave out the SO42- from the sulfuric acid reactant and potassium sulfate product. We also leave out the 2 K+ from the potassium hydroxide reactant and potassium sulfate product. This gives us our ionic equation:
    2 H+ (aq) + 2 OH- (aq) → 2 H2O (l) which simplifies even further down to:
    H+ (aq) + OH- (aq) → H2O (l)

  • In a metal displacement reaction, the active species in a dissolved salt is the metal ion.

    For the reaction:
    CuSO4 (aq) + Zn (s) → Cu (s) + ZnSO4 (aq)
    it is only the metal atoms and metal ions that are reacting so the sulfate ions are spectators and are omitted from the ionic equation.
    Cu2+ (aq) + Zn (s) → Cu (s) + Zn2+ (aq)

  • In a halogen displacement reaction, the active species in a dissolved salt is the halide ion.

    For the reaction:
    2 KI (aq) + Cl2 (aq) → 2 KCl (aq) + I2 (aq)
    it is only the halogen and halide ions that are reacting so the potassium ions are spectators and are omitted from the ionic equation.
    2 I- (aq) + Cl2 (aq) → 2 Cl- (aq) + I2 (aq)

  • In a precipitation reaction, the active reactant species are just the dissolved ions that will make up the formula of the precipitate.

    For example:
    KI (aq) + AgNO3 (aq) → KNO3 (aq) + AgI (s)
    the K+ and NO3- ions remain dissolved and unchanged: therefore they do not appear in the ionic equation. It is the I- and Ag+ that combine together to make the precipitate.
    I- (aq) + Ag+ (aq) → AgI (s)

Equations come up in completely random order. If you wish, you can choose the type of reactions for which you wish to practise writing ionic equations, or you can have the full range. This page will be helpful, to Year 12 as well as GCSE students.

If you come across an error on any of these pages, please let me know which equation number it is using the email link in the help section.