Learn The Oxidation Number

QUESTION 1

What is the oxidation state of chromium in Cr²⁺?

SOLUTION

For a simple ion such as this, the oxidation state equals the charge on the ion: +2 (by convention, the + sign is always included to avoid confusion)

What is the oxidation state of chromium in CrCl₃?

This is a neutral compound, so the sum of the oxidation states is zero. Chlorine has an oxidation state of -1 (no fluorine or oxygen atoms are present). Let n equal the oxidation state of chromium:

n + 3(-1) = 0

n = +3 

The oxidation state of chromium is +3.

QUESTION 2

What is the oxidation state of chromium in Cr(H₂O)₆³⁺?

SOLUTION

This is an ion and so the sum of the oxidation states is equal to the charge on the ion. There is a short-cut for working out oxidation states in complex ions like this where the metal atom is surrounded by electrically neutral molecules like water or ammonia.

The sum of the oxidation states in the attached neutral molecule must be zero. That means that you can ignore them when you do the sum. This would be essentially the same as an unattached chromium ion, Cr³⁺. The oxidation state is +3.

What is the oxidation state of chromium in the dichromate ion, Cr₂O₇^2-?

The oxidation state of the oxygen is -2, and the sum of the oxidation states is equal to the charge on the ion. Don't forget that there are 2 chromium atoms present.

                                                                                2n + 7(-2) = -2

                                                                                            n = +6

QUESTION 3

What is the oxidation state of copper in CuSO₄?

SOLUTION

Unfortunately, it isn't always possible to work out oxidation states by a simple use of the rules above. The problem in this case is that the compound contains two elements (the copper and the sulfur) with variable oxidation states.

In cases like these, some chemical intuition is useful. Here are two ways of approaching this problem:

·         Recognize CuSO₄ as an ionic compound containing a copper ion and a sulfate ion, SO₄^2-. To form an electrically neutral compound, the copper must be present as a Cu²⁺ ion. The oxidation state is therefore +2.

·         Recognize the formula as being copper(II) sulfate (the (II) designation indicates that copper is in a +2 oxidation state, as discussed below).