Oxidation & Reduction Reactions Section 1

Oxidation-reduction reactions involve a transfer of electrons Oxidation involves the loss of electrons Reduction involves the gain of electrons

Oxidation States An oxidation number is assigned to an element in a molecule based on the distribution of electrons in that molecule Rules for assigning oxidation numbers Rule Example The oxidation number of a lone element is 0 Na = 0 N2 = 0 The oxidation number of an lone ion is the charge of the ion Cl- = -1 The more electronegative element in a binary compound is assigned the number equal to the charge if it were an ion The O in NO = -2 The oxidation number of fluorine is always -1 F in LiF = -1

Oxidation States (cont.) Rule Example Oxygen has an oxidation number of -2 unless combined with F, the it is +1 or +2, or in a peroxide, then it is -1 O in NO2 = -2 O in H2O2 = -1 Hydrogen’s oxidation number is +1 unless it is combined with a metal, then it is -1 H in LiH = -1 In compounds group 1, 2 elements, and aluminum have oxidation numbers of +1, +2, and +3 respectively Ca in CaCO3 = +2 The sum of the oxidation numbers of all atoms in a neutral compound is 0 C in CaCO3 = +4 The sum of the oxidation numbers in a polyatomic ion equals the charge of the ion P in H2PO4- = +5

Oxidation Processes in which the atoms or ions of an element experience an increase in oxidation state are oxidizing processes 2Na + Cl2  2NaCl Each sodium atom loses an electron changing the oxidation state from 0 to +1 An element whose oxidation number increases is oxidized Na is oxidized

Reduction Processes in which the atoms or ions of an element experience an increase in oxidation state are oxidizing processes 2Na + Cl2  2NaCl Each chlorine atom accepts an electron changing its oxidation state from 0 to -1 An element that undergoes a decrease in oxidation state is reduced Cl is reduced

Redox as a Process Just like mass has to be conserved in reactions, so does the number of electrons This means that for oxidation to occur, reduction must also occur Any chemical process in which elements undergo changes in oxidation number is an oxidation-reduction reaction Or redox reaction for short

Redox as a Process (cont.) The redox reactions can be written showing either just the oxidation, or just the reduction alone These are called half-reactions 3Na  3Na+ + 3e- (oxidation) Al3+ + 3e-  Al (reduction) 3Na + Al3+  3Na+ + Al (redox reaction)

If none of the atoms in a reaction change oxidation state, the reaction is not a redox reaction SO2 + H2O  H2SO4 Na+ + Cl- + Ag+ + NO3-  Na+ + NO3- + AgCl

Redox Reactions & Covalent Bonds Oxidation numbers are assigned based on the electronegativity of an atom relative to the other atoms to which it is bonded in a molecule It is not based on a real charge

Redox in Covalent Bonds (cont.) When hydrogen burns with chlorine, a covalent bond forms from the sharing of two electrons The electrons in the bond are not shared equally and are more strongly attracted to the chlorine. Why? H2 + Cl2  2HCl Use the oxidation rules to assign the oxidation numbers