Question

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

Answer 1

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

Explanation:

Example

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

Solution

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.