Learning focus
Use chemical formulae and equations accurately, convert measured quantities into moles, apply balanced-equation ratios and present multi-step calculations with units and checks.
Neutralisation
For a strong acid reacting with a strong alkali, the essential change is H+(aq) + OH-(aq) -> H2O(l). Other ions remain spectators and form the dissolved salt.
Precipitation
When two aqueous solutions form an insoluble ionic compound, the ionic equation contains the ions that combine and the solid product. Examples include Ag+ + Cl- -> AgCl and Ba2+ + SO4 2- -> BaSO4.
Gas-forming ionic equations
Acid-carbonate reactions can be represented as 2H+(aq) + CO3 2-(aq) -> CO2(g) + H2O(l). The salt ions are spectators.
Coefficients and charge
Balance charge explicitly. Two H+ ions are needed for one carbonate ion because the left side must have net charge zero, matching neutral products.
Worked example
For barium chloride and sodium sulfate: Ba2+(aq) + SO4 2-(aq) -> BaSO4(s).
Practical or data skill
Match molecular equations to a small set of recurring net ionic equations and identify spectators.
Examination tip
An ionic equation should show only species that actually change.
Review questions and suggested answers
Question 1
Write the ionic equation for acid-alkali neutralisation.
Suggested answer
H+(aq) + OH-(aq) -> H2O(l).
Question 2
Write the ionic equation for calcium carbonate with acid.
Suggested answer
2H+(aq) + CO3 2-(s or aq as specified) -> CO2(g) + H2O(l); if solid calcium carbonate is used, keep CaCO3(s) intact in the appropriate ionic form.
Question 3
Why is Na+ omitted from many net ionic equations?
Suggested answer
It often remains unchanged as a spectator ion.
