Writing Balanced Ionic Equations (OCR Gateway GCSE Chemistry: Combined Science)

• 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 

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