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Oxidation State Slides.Key Oxidation States Dr. Sobers’ Lecture Slides The Oxidation State Also known as the oxidation number The oxidation state is used to determine whether an element has been oxidized or reduced. The oxidation state is not always a real, quantitative, physical constant. The oxidation state can be the charge on an atom: 2+ - MgCl2 Mg Cl Oxidation State: +2 -1 2 The Oxidation State For covalently bonded substances, it is not as simple as an ionic charge. A covalent bond is a sharing of electrons. The electrons are associated with more than one atomic nuclei. This holds the nuclei together. The electrons may not be equally shared. This creates a polar bond. The electronegativity of a covalently bonded atom is its ability to attract electrons towards itself. 3 Example: Chlorine Sodium chloride is an ionic compound. In sodium chloride, the chloride ion has a charge and an oxidation state of -1. The oxidation state of sodium is +1. 4 Example: Chlorine In a chlorine molecule, the chlorine atoms are covalently bonded. The two atoms share electrons equally and the oxidation state is 0. 5 Example: Chlorine The two atoms of a hydrogen chloride molecule are covalently bonded. The electrons are not shared equally because chlorine is more electronegative than hydrogen. There are no ions but the oxidation state of chlorine in HCl is -1 and the oxidation state of hydrogen is +1. 6 7 Assigning Oxidation States See the handout for the list of rules. Rule 1: Free Elements Free elements have an oxidation state of zero Example Oxidation State O2(g) 0 Fe(s) 0 O3(g) 0 C(graphite) 0 C(diamond) 0 9 Rule 2: Monatomic Ions The oxidation state of monatomic ions is the charge of the ion Example Oxidation State O2- -2 Fe3+ +3 Na+ +1 I- -1 V4+ +4 10 Rule 3: Fluorine in Compounds Fluorine in a compound always has an oxidation state of -1 Example Comments and Oxidation States NaF These are monatomic ions. Rule 2 applies F is -1 Na is +1 CF4 This is a molecular compound. F is -1 See rule 7 for C Fluorine is the most electronegative element so it is -1 when bonded to other atoms. 11 Rule 4: Hydrogen in Compounds Hydrogen in a compound usually has an oxidation state of +1 Example Comments and Oxidation States CH4 A molecular compound. Carbon is more electronegative. H is +1 See rule 7 for C NaH This is an ionic compound - see rule 2 Na is +1 The hydride ion is -1 An exception to rule 4 is the hydride ion. 12 Rule 5: Oxygen in Compounds Oxygen in a compound usually has an oxidation state of -2 Example Comments and Oxidation States MgO These are monatomic ions. See rule 2 Mg is +2 O is -2 H2O A molecular compound - oxygen is more electronegative O is -2 H is +1 (rule 4) But why is covalently bonded oxygen -2 and not -1? 13 Rule 5: Oxygen in Compounds Oxygen in a compound usually has an oxidation state of -2 But why is covalently bonded oxygen -2 and not -1? After all, fluorine is more electronegative but only -1 in compounds. Oxygen usually makes two bonds and fluorine makes one. The electronegative oxygen pulls electron density from two atoms. Compound: H-F H-O-H Oxidation State: +1 -1 +1 -2 +1 14 Rule 5: Oxygen in Compounds Oxygen in a compound usually has an oxidation state of -2 Example Comments and Oxidation States F2O Fluorine is more electronegative than oxygen so electron density is pulled away from oxygen. F-O-F -1 +2 -1 Exception to rule 5: oxygen bonded to fluorine. 15 Rule 5: Oxygen in Compounds Oxygen in a compound usually has an oxidation state of -2 Example Comments and Oxidation States H2O2 In hydrogen peroxide, each oxygen atom is bonded to another oxygen and to one less electronegative element. H - O - O - H +1 -1 -1 +1 Each oxygen has only one bond to a less electronegative atom Exception to rule 5: oxygen in a peroxide is -1. 16 Rule 6: Halogens other than fluorine in Compounds Chlorine, bromine and iodine in a compound usually have an oxidation state of -1 Example Comments and Oxidation States NaCl These are monatomic ions. See rule 2 Na is +1 Cl is -1 CBr4 This is a molecular compound and bromine is more electronegative. Br is -1 See rule 7 for C 17 Rule 6: Halogens other than fluorine in Compounds Chlorine, bromine and iodine in a compound usually have an oxidation state of -1 Example Comments and Oxidation States FCl Because fluorine is more electronegative, it is -1 F is -1 Cl is +1 ClBr Because chlorine is more electronegative, it is -1 Cl is -1 Br is +1 Exception to rule 6: when bonded to a more electronegative element. 18 Rule 7: The Sum of the Oxidation States in a formula The sum of the oxidation states in a formula is equal to the charge on the formula. Example Comments and Oxidation States CH4 By rule 4, hydrogen is +1 Assign the variable x to carbon Create an algebraic equation x + 4(+1) = 0 4 hydrogens, each +1 x = -4 19 Rule 7: The Sum of the Oxidation States in a formula The sum of the oxidation states in a formula is equal to the charge on the formula. Example Comments and Oxidation States C3H8 By rule 4, hydrogen is +1 Average oxidation state of C = x 3x + 8(+1) = 0 3x = -8 x = -3/8 The average for carbon is -3/8 20 Rule 7: The Sum of the Oxidation States in a formula The sum of the oxidation states in a formula is equal to the charge on the formula. Example Comments and Oxidation States - MnO4 By rule 5, oxygen is -2 Oxidation state of Mn = x x + 4(-2) = -1 charge on formula = -1 x = +7 21 Rule 7: The Sum of the Oxidation States in a formula The sum of the oxidation states in a formula is equal to the charge on the formula. Example Comments and Oxidation States KMnO4 Try last one again but with a cation +1 +7 -2 + - K MnO4 Monatomic ion, potassium, is +1 By rule 5, oxygen is -2 Oxidation state of Mn = x +1 + x + 4(-2) = 0 x = +7 22 Rule 7: The Sum of the Oxidation States in a formula The sum of the oxidation states in a formula is equal to the charge on the formula. Example Comments and Oxidation States - ClO2 By rule 5, oxygen is +2 Oxidation state of Cl = x x + 2(-2) = -1 charge on formula = -1 x = +3 23 Lower Number Rules Take Precedence The rules are presented so that lower numbered rules have priority. If an exception is forgotten or not realized, then the order of the rules can help. In FCl the rule about fluorine being -1 in a compound takes precedence over the rule about other halogens usually being -1 in a compound. In H2O2 the sum must be zero. The rule for hydrogen takes precedence over the rule for oxygen so the oxidation state of -1 is calculated. 24 Recognizing Oxidations and Reductions Use the Oxidation Numbers Oxidation and Reduction C3H8(g) + O2(g) → CO2(g) + H2O(g) -3/8 +1 0 +4 -2 +1 -2 Carbon is being oxidized Increase in oxidation state Oxygen is being reduced Decrease in oxidation state Both carbon and oxygen are more electronegative than hydrogen. Oxygen is more electronegative than carbon but hydrogen is +1 when bonded to either. The oxidation state is not quantitatively meaningful but allows for recognition of oxidation and reduction. 26 Oxidation and Reduction CuCl2(aq) + Mg(s) → MgCl2(aq) + Cu(s) +2 -1 0 +2 -1 +1 0 Copper is being reduced Decrease in oxidation state Magnesium is being oxidized Increase in oxidation state Copper is the oxidizing agent It is oxidizing magnesium Magnesium is the reducing agent It is reducing copper 27.
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