Writing ionic equations
Writing ionic equations is one of the most challenging aspects of O level chemistry. Topics such as Redox and electrochemistry required the student to have detailed information on how to write ionic equations. Also as the student progresses from O level to A level, ions and ionic equation gradually become the pivot around which the whole curriculum revolves.
This tutorial is about writing ionic equations and but before we start writing ionic equations let us start by revisiting ions and ionic compounds.
Ions and ionic compounds
An ion is a charged entity of an atom or compound. A positively charged ion has a deficiency of electrons and it is called a cation. A negatively charged ion has an excess of electrons and it is called an anion.
An ionic compound is a compound consisting of positively charged and negatively charged ions held together by a strong electrostatic force. Ionic compounds dissociate into ions when they dissolve in water. For example:
CuSO4(aq) → Cu2+(aq) + SO42-(aq)
An ionic equation is a summary of how the ions behave in a chemical reaction. A balanced ionic equation provides the following information:
- nature of reacting ions and the produced ions.
- ratio of moles of reacting ions and of the produced ions.
- physical states of the reacting ions and of the produced ions.
- changes in the charges between the reacting ions and the products.
Writing an ionic equation
To properly write an ionic equation:
- write a balanced chemical equation including state symbols.
- split the chemical formula of each aqueous compound into its respective ions
- remove the spectator ions, i.e. ions that do not take part in the reaction. Spectator ions can be identified by the fact that they have the same charge on both sides of the equation.
- the ionic equation produced must be balanced in terms of:
- number of atoms of each element on both sides of the equation.
- total charges carried by the ions on both side of the equation.
Example of writing ionic equations
You are given that sodium hydroxide neutralises sulphuric acid to produce a salt and water. Write down a balanced ionic equation for the reaction.
First start by writing the word equation:
sodium hydroxide + sulphuric acid → sodium sulphate + water
Convert the word equation to formula equation:
NaOH + H2SO4 → Na2SO4 + H2O
Balance the chemical equation and add state symbols:
2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l)
Split all the aqueous substances into their ions:
2Na+(aq) 2OH–(aq) + 2H+(aq) SO42-(aq) → 2Na+ SO42-(aq) + 2H2O(l)
Remove spectator ions (they have the same charge on both sides of the equation:
2Na+(aq) 2OH–(aq) + 2H+(aq) SO42-(aq) → 2Na+(aq) SO42-(aq) + 2H2O(l)
The ionic equation becomes:
2OH–(aq) + 2H+(aq) → 2H2O(l)
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