An ionic formula balances positive and negative charge. A chemical equation balances atoms, and an ionic equation balances atoms and charge while leaving out the ions that do nothing. Get these right and every calculation that follows starts from the correct ratio.
In an ionic compound the total positive charge equals the total negative charge, so Al3+ and SO42− combine 2 : 3 as Al2(SO4)3. A balanced equation has the same number of each atom on both sides; balance only with the large numbers in front, never by changing a formula. An ionic equation shows only the species that change, and its charges must balance as well as its atoms.
Learn the charges of the common polyatomic ions. Then choose the smallest numbers of each ion that make the total charge zero, and put brackets round a polyatomic ion when there is more than one of it.
| Ion | Formula | Ion | Formula |
|---|---|---|---|
| ammonium | NH4+ | hydroxide | OH− |
| nitrate | NO3− | hydrogencarbonate | HCO3− |
| sulfate | SO42− | carbonate | CO32− |
| phosphate | PO43− | sulfite | SO32− |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is the formula of aluminium sulfate?
In the balanced equation for the complete combustion of propane, C3H8 + xO2 → 3CO2 + 4H2O, what is x?
The state symbol (aq) means the species is:
In the reaction between hydrochloric acid and sodium hydroxide solution, the spectator ions are:
Write the correct formula of every reactant and product first, then balance by changing only the coefficients. Add (s), (l), (g) or (aq) to show the state of each species.
Split every soluble ionic compound in solution into its ions, cross out the ions that appear unchanged on both sides (the spectator ions), and write what is left.
Never balance an equation by changing a subscript. Writing H2O2 to make H2 + O2 → H2O balance turns water into hydrogen peroxide, a different substance. And an ionic equation that balances atoms but not charge is still wrong: check the total charge on each side.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Barium chloride solution is added to sodium sulfate solution and a white precipitate forms. What is the ionic equation?
Which equation is correctly balanced?
What is the formula of calcium phosphate?
Aluminium reacts with dilute hydrochloric acid. What is the ionic equation?
This skill is now recorded on your profile. Keep the chain going.