Oxidation Number

Oxidation Number In the previous lesson we have studied the development of the concept of redox reactions, one of which is the reaction of increasing and decreasing oxidation numbers. What is meant by oxidation number and how do we determine it? 1. Definition of Oxidation Numbers Oxidation number is a number that indicates the ability of an atom to lose or gain electrons in the formation of a compound. The value of the oxidation number indicates the number of electrons released or captured, so the oxidation number can be either positive or negative. 2. Determination of the Oxidation Number of an Element We can determine the magnitude of the oxidation number of an element in compound by following the following rules. The rules for determining the oxidation numbers of elements are: a. Free elements (eg H2 , O2 , N2 , Fe and Cu) have numbers oxidation = 0. b. In general, elemental H has an oxidation number = +1, except in hydride compounds, the oxidation number of H = –1. Example: - The oxidation number of H in H2 O, HCl, and NH3 is +1 - The oxidation number of H in LiH, NaH, and CaH2 is –1 c. In general, element O has an oxidation number = –2, except in peroxide compounds, the oxidation number of O = –1 Example: - The oxidation number of O in H2 O, CaO, and Na2 O is –2 - The oxidation number of O in H2 O2 , Na2 O2 is –1 d. Element F always has an oxidation number = –1. e. Metal elements have an oxidation number that is always positive. Example: - Group IA (alkali metals: Li, Na, K, Rb, and Cs) oxidation number = +1 - Group IIA (alkaline earth: Be, Mg, Ca, Sr, and Ba) say its oxidation state = +2 f. The oxidation number of a single ion = its charge. Example: - The oxidation number of Fe in Fe2+ ion is +2 g. The sum of the oxidation numbers of the elements in the compound = 0. Example: - In the compound H2 CO3 : 2 oxidation states of H + 1 oxidation state of C + 3 oxidation states of O = 0 h. The sum of the oxidation numbers of the elements in a polyatomic ion = ionic charge. Example: - In the NH4 ion + applies to 1 oxidation state of N + 4 oxidation states of H = + 1 Example 1. Determine the oxidation numbers of the underlined elements in the following compounds. a. Fe2 O3 b. H2 c. MnO4 – Answer: a. Fe2 O3 oxidation number O = –2 (rule c) 2 oxidation number Fe + 3 oxidation number O = 0 2 oxidation number Fe + 3(–2) = 0 2 oxidation number Fe – 6 = 0 2 oxidation state Fe = +6 bioxFe = Fe oxidation = +3 b. H2 O2 oxidation state H = +1 (rule b) 2 oxidation state H + 2 oxidation state = 0 2(+1) + 2 oxidation state = 0 +2 + 2 oxidation state c. MnO4 – oxidation state of O = –2 (c rule) Mn + 4 oxidation states of O = –1 (h rule) Mn + 4(–2) = –1 oxidation state Mn – 8 = –1 oxidation state Mn = –1 + 8 oxidation states Mn = +7 Exercise 1 Determine the oxidation numbers of the underlined elements in the following compounds. a. NH4 b. H3+PO4 c. Cu(NO3 )2 d. NH4 NO2