How do you balance the chemical equations for double replacement reactions?
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.
Identify the cations and anions in each compound:
Fe(NO₃)₂ has Fe²⁺ and NO₃⁻
Na₃PO₄ has Na⁺ and PO₄³⁻
Pair up each cation with the anion from the OTHER compound:
Fe²⁺ pairs with PO₄³⁻
Na⁺ pairs with NO₃⁻
Write two new (CORRECT!!) formulas using the pairs from
Fe₃(PO₄)₂, since Fe is +2 and PO₄ is -3
NaNO₃ since Na is +1 and NO₃ is -1
Write the unbalanced equation
Fe(NO₃)₂ + Na₃PO₄ → Fe₃(PO₄)₂ + NaNO₃
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.