• More Balancing of Equations

The following is one of many emails I have recieved basically asking the same question...

"I am a little confused on why when I work out the formula for Cr3+ and F- it comes out to CrF3 instead of Cr3F3.  In a sample that is shown, Al3+ and O2- comes out to be Al2O3. We have steps to go through to get the answer, but I am really confused..."

• The Answer is shown here...

Cr ionises to Cr³⁺ and F ionises to F^-

To neutralise the 3 positive charges on the Cr³⁺ will require 3 negative charges. In this case, to obtain 3 negative charges will require 3 ionised fluorines.

So, 1(Cr³⁺) reacts with 3(F^-). Now, when they react together, the charges are neutralised, so the ³⁺ on the Cr and the ^- on the F are no longer shown. What you have now is 1Cr3F (written as Cr₁F₃) BUT the 1 is never written in the molecular formula, so it becomes CrF₃ (which is the correct answer).

So, you must balance the positive and negative charges on the ions to cancel each other out.

In the second equation mentioned, Al ionises to Al³⁺ and O ionises to O^2-. Multiply the number of Al³⁺ by 2 (2Al³⁺), and we now have 6+ charges for Al. Multiply the O^2- by 3 (3O^2-) and we now have 6^- charges for O. The 6+ and 6^- cancel each other out. So, it took 2 Al³⁺ and 3 O^2- this is written as Al₂O₃. To balance ionic equations you are cancelling out the positive and negative charges.