Writing Balanced Ionic Equations
- In aqueous solutions ionic compounds dissociate into their ions, meaning they separate into the component ions that formed them, e.g. hydrochloric acid and potassium hydroxide dissociate as follows:
HCl (aq) → H+ (aq) + Cl-(aq)
KOH (aq) → K+ (aq) + OH- (aq)
- It is important that you can recognise common ionic compounds and their constituent ions
- These include:
- Acids such as HCl and H2SO4
- Group I and Group II hydroxides e.g. sodium hydroxide
- Soluble salts e.g. potassium sulfate, sodium chloride
- Follow the example below to write ionic equations
Worked example
Write the ionic equation for the reaction of aqueous chlorine and aqueous potassium iodide.
Answer:
Step 1: Write out the full balanced equation:
2KI (aq) + Cl2 (aq) → 2KCl (aq) + I2 (aq)
Step 2: Identify the ionic substances and write down the ions separately
2K+ (aq) + 2I- (aq) + Cl2 (aq) → 2K+ (aq) + 2Cl- (aq) + I2 (aq)
Step 3: Rewrite the equation eliminating the ions which appear on both sides of the equation (spectator ions ) which in this case are the K+ ions:
2I- (aq) + Cl2 (aq) → 2Cl- (aq) + I2 (aq)
Half Equations
- A half equation represents what happens to one of the reactants in a chemical reaction
- For example if you take the reaction between sodium and chlorine:
2Na (s) + Cl2 (g) → 2NaCl (s)
- This reaction can be split into two half equations which require electrons to balance them
- The sodium reaction involves the sodium losing an electron
Na → Na+ + e-
- The chlorine reaction involves the chlorine molecule gaining two electrons
Cl2 + 2e- → 2Cl-
- Notice that the half equations do not have to have the same coefficients as the overall equation, but they do need to be balanced in the atoms and charges
Examiner Tip
An electron is negatively charged, so why do we have to write the charge next to it? Actually, not all chemists do. In a shell diagram or electrolysis circuit, for example, sometime people write just the symbol, e. However, when writing half equations its good practice to include the minus sign as it helps you to check that the charges are balanced on both sides.
