Oxidation Numbers

Question 1

In Reaction 2, what are the oxidation numbers on carbon in CaCO3 (s) and CO2 (g), respectively?

Answer 1

+4 and +4

To find the oxidation number on carbon, we can start by examining the oxidation numbers on the other atoms in the compounds. For CaCO3, oxygen always has an oxidation number of -2 unless it is in a peroxide or superoxide, so we know that the total oxidation number on the three oxygens in CaCO3 is -6. Alkaline metals usually have oxidation numbers of +2, so Ca has an oxidation number of +2. The non-carbon elements add up to -4, and the compound is neutral, so the oxidation number on carbon must be +4. For CO2, oxygen again has an oxidation number of -2, so the total oxidation number on the two oxygens is -4. Because the compound is neutral, carbon has to have an oxidation number of +4.

Question 2

True or False: oxygen always has an oxidation number of -2 unless it is in a peroxide or superoxide

Answer 2

True

Question 3

Potassium permanganate (KMnO4) is used as a disinfectant in swimming pools and wastewater plants. What is the oxidation state of manganese in KMnO4?

Answer 3

To find the oxidation state of Mn, let’s start with the atoms with known oxidation numbers. Oxygen generally has an oxidation state of -2 (with some exceptions, as in H2O2). Potassium has an oxidation state of +1. Since the compound as a whole is neutral, we are left with (+1) + Mn + (4)(-2) = 0, or Mn = +7.

Question 4

Zn has an oxidation potential of +0.76, while Al has an oxidation potential of +1.66. Which metal serves as the better oxidizing agent?

Answer 4

Zn2+, because it is more prone to gaining electrons