Writing correct formulas of Products in a Double Replacement reaction or Precipitation Reaction.

Lets take a look at K₂CrO₄ and NaOH as an example. The assumption here is that you have an aqueous solution of each of these compound and you want to find out what the correct formulas of the products will be.

If you notice that I put cations (positive ions) in green and the anions (negative ions) in red. What we are going to do is replace greens with the red. This type of reaction is a double replacement reaction or if a precipitate forms a precipitation reaction. Lets approach this in steps:

1. Swap the cations with the anions and write the two possible combinations out without including the subscripts. (Do not change polyatomic ions)

KOH + NaCrO₄ (notice that polyatomic ions stays in tact but I did not bring the subscript over with the potassium, “K”)

2. The next step is to make sure that the formulas are correct. Checking the charges of the each of the ions and making sure the charges equal zero will complete this step.

KOH + NaCrO₄

By looking at the charges of each of the ions the KOH is fine as is, but the “Na” in the NaCrO₄ needs to be doubled to make it Na₂CrO₄.

3. At this point we can put most of our equation together. The original reactants are placed first and the recombined compounds are place second.

NaOH + K₂CrO₄ ® KOH + Na₂CrO₄

Leave some room in front and after each compound for balancing and states of matter.

For help on determining states of matter and determining if a precipitate will form click here.