How do you balance the chemical equations for double replacement reactions?

1 Answer

You balance double replacement equations by pairing each cation with the anion from the other compound in the correct ratios.



Write a balanced equation for the reaction between Fe(NO₃)₂ and Na₃PO₄.


  1. Identify the cations and anions in each compound:
    Fe(NO₃)₂ has Fe²⁺ and NO₃⁻
    Na₃PO₄ has Na⁺ and PO₄³⁻

  2. Pair up each cation with the anion from the OTHER compound:
    Fe²⁺ pairs with PO₄³⁻
    Na⁺ pairs with NO₃⁻

  3. Write two new (CORRECT!!) formulas using the pairs from
    Step 2.
    Fe₃(PO₄)₂, since Fe is +2 and PO₄ is -3
    NaNO₃ since Na is +1 and NO₃ is -1

  4. Write the unbalanced equation
    Fe(NO₃)₂ + Na₃PO₄ → Fe₃(PO₄)₂ + NaNO₃

  5. Balance the equation.
    The Fe₃(PO₄)₂ has 3 Fe and 2 PO₄ on the right, so you need 3 Fe and 2 PO₄ on the left:
    3Fe(NO₃)₂ + 2Na₃PO₄ → Fe₃(PO₄)₂ + NaNO₃

Now we have 6 NO₃ on the left, so we need 6 NO₃ on the right.

The balanced equation is
3Fe(NO₃)₂ + 2Na₃PO₄ → Fe₃(PO₄)₂ +6NaNO₃

Here is a video to help with balancing double replacement reactions.