Oxidation States Dr. Sobers Lecture Slides The Oxidation State - - PowerPoint PPT Presentation
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,
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:
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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.
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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.
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In a chlorine molecule, the chlorine atoms are covalently bonded. The two atoms share electrons equally and the oxidation state is 0.
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The two atoms of a hydrogen chloride molecule are covalently
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.
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See the handout for the list of rules.
Free elements have an oxidation state of zero Example Oxidation State O2(g) Fe(s) O3(g) C(graphite) C(diamond)
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The oxidation state of monatomic ions is the charge of the ion Example Oxidation State O2-
Fe3+ +3 Na+ +1 I- V4+
+4
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Fluorine in a compound always has an oxidation state of -1 Example Comments and Oxidation States NaF Na is +1 F is -1 These are monatomic ions. Rule 2 applies CF4 F is -1 See rule 7 for C This is a molecular compound. Fluorine is the most electronegative element so it is -1 when bonded to other atoms.
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Hydrogen in a compound usually has an oxidation state of +1 Example Comments and Oxidation States CH4 H is +1 See rule 7 for C A molecular compound. Carbon is more electronegative. NaH Na is +1 The hydride ion is -1 This is an ionic compound - see rule 2 An exception to rule 4 is the hydride ion.
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Oxygen in a compound usually has an oxidation state of -2 Example Comments and Oxidation States MgO Mg is +2 O is -2 These are monatomic ions. See rule 2 H2O O is -2 H is +1 (rule 4) A molecular compound - oxygen is more electronegative But why is covalently bonded oxygen -2 and not -1?
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Oxygen in a compound usually has an oxidation state of -2
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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. H-O-H H-F +1 +1 +1
Compound: Oxidation State: But why is covalently bonded oxygen -2 and not -1?
Oxygen in a compound usually has an oxidation state of -2 Example Comments and Oxidation States F2O Fluorine is more electronegative than
from oxygen. Exception to rule 5: oxygen bonded to fluorine.
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F-O-F
+2
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. Exception to rule 5: oxygen in a peroxide is -1.
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H - O - O - H +1 -1 +1
Each oxygen has only one bond to a less electronegative atom
Chlorine, bromine and iodine in a compound usually have an oxidation state of -1 Example Comments and Oxidation States NaCl
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Na is +1 Cl is -1 These are monatomic ions. See rule 2 CBr4 Br is -1 See rule 7 for C This is a molecular compound and bromine is more electronegative.
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Chlorine, bromine and iodine in a compound usually have an oxidation state of -1 Example Comments and Oxidation States FCl
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F is -1 Cl is +1 Because fluorine is more electronegative, it is -1 ClBr Cl is -1 Because chlorine is more electronegative, it is -1 Br is +1 Exception to rule 6: when bonded to a more electronegative element.
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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 x = -4 4 hydrogens, each +1
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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
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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 x = +7 charge on formula = -1
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The sum of the oxidation states in a formula is equal to the charge on the formula. Example Comments and Oxidation States KMnO4 By rule 5, oxygen is -2 Oxidation state of Mn = x +1 + x + 4(-2) = 0 x = +7 Try last one again but with a cation Monatomic ion, potassium, is +1 K+ MnO4- +1
+7
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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 x = +3 charge on formula = -1
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The rules are presented so that lower numbered rules have priority. If an exception is forgotten or not realized, then the order
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
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Use the Oxidation Numbers
C3H8(g) + O2(g) → CO2(g) + H2O(g) Carbon is being oxidized Both carbon and oxygen are more electronegative than
but hydrogen is +1 when bonded to either. The
allows for recognition of oxidation and reduction. +1
+4 +1 -2 Oxygen is being reduced Increase in oxidation state Decrease in oxidation state
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CuCl2(aq) + Mg(s) → MgCl2(aq) + Cu(s) Copper is being reduced
+2 +1 Magnesium is being oxidized Decrease in oxidation state Increase in oxidation state
+2 Copper is the oxidizing agent Magnesium is the reducing agent It is oxidizing magnesium It is reducing copper
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