Chapter 15 Lecture Chapter 15 15.1 Oxidation Reduction Reactions Oxidation and Reduction Fifth Edition Oxidation and Reduction An oxidation-reduction reaction provides us with energy from food provides electrical energy in batteries occurs when iron rusts 4Fe(s) + 3O 2 (g) 2Fe 2 O 3 (s) Rust forms on a set of tools when oxygen reacts with iron. Learning Goal Identify a reaction as an oxidation or a reduction. Assign and use oxidation numbers to identify elements Basic that Chemistry are oxidized or reduced. 2 Electron Loss and Gain In an oxidation-reduction reaction, one reactant loses electrons and another reactant gains electrons. electrons are transferred from one reactant to another oxidation is a loss of electrons (oil) Zn(s) Zn 2+ (aq)+ 2 e (loss of electrons) reduction is a gain of electrons (rig) Cu 2+ (aq) + 2 e Cu(s) (gain of electrons) Examples of Oxidized and Reduced Substances 3 4 Identify each of the following as (O) oxidation or (R) reduction: A. Sn(s) B. Fe 3+ (aq) + 1 e C. Cl 2 (g) + 2 e Sn 4+ (aq) + 4 e Fe 2+ (aq) 2Cl (aq) Oxidation and Reduction in Photochromism When these glasses are exposed to the Sun, there is an oxidation reduction reaction when the UV light is absorbed by AgCl, and Cl and Ag atoms are produced. It is the silver atoms that are black and darken the lenses. Glasses with photochromic lenses darken when exposed to sunlight. 5 1
Writing Oxidation and Reduction Reactions Write each of the reactants with its product as a half-reaction. The Cl ion is oxidized because it loses an electron. The Ag + ion is reduced because it gains an electron. The half-reaction is called a reduction because the addition of an electron (negative charge) reduces the positive charge on Ag +. For each of the following, identify the reaction as an oxidation or a reduction: Write the oxidation reaction and the reduction reaction for the following: 2Cs(s) + F 2 (g) 2CsF(aq) Oxidation Numbers An oxidation number is assigned to each atom in a compound or ion for the more electronegative atom is negative identifies oxidation or reduction keeps track of the loss and gain of electrons 9 10 Assigning Oxidation Numbers 1. The sum of the oxidation numbers in a molecule is equal to zero, or for a polyatomic ion, to its charge. 2. The oxidation number of an element is zero. 3. The oxidation number of a monatomic ion is equal to its charge. 4. In compounds, the oxidation number of Group 1A(1) elements is +1 and of Group 2A(2) elements is +2. Assigning Oxidation Numbers (continued) 5. In compounds, the oxidation number of fluorine is always 1. Other elements in Group 7A(17) also are 1 except when combined with oxygen or fluorine. 6. In compounds, the oxidation number of oxygen is 2 except in OF 2. 7. In compounds with nonmetals, the oxidation number of hydrogen is +1; in compounds with metals, the oxidation number of hydrogen is 1. 11 12 2
Assign oxidation numbers to the elements in each of the following: A. N 2 B. PCl 3 C. SO 4 2 Determine the oxidation number of each of the following: A. C in CO 2 1) +2 2) +4 3) -4 B. Cl in Cl 2 O 7 1) +2 2) +7 3) -14 C. P in H 3 PO 4 1) +5 2) -5 3) -3 D. Mn in MnO 4 2 1) +4 2) +6 3) +8 17 18 3
Identifying Oxidation-Reduction When the oxidation number for an element increases, oxidation (loss of e ) has occurred decreases, reduction (gain of e ) has occurred Example: increase in oxidation number = oxidation of Fe Changes in Oxidation Numbers Identify Oxidation or Reduction 2Fe(s) + 3Cl 2 (g) 2FeCl 3 (s) 0 0 +3-1 decrease in oxidation number = reduction of Cl 19 20 Identify the reactant that is oxidized and the reactant that is reduced in each equation. (Hint: Assign oxidation numbers to each atom.) 1. S(s) + F 2 (g) SF 6 (g) oxidized reduced 2. MnO 4 (aq) + Fe 2+ (aq) Mn 2+ (aq) + Fe 3+ (aq) oxidized reduced Use the increase or decrease in oxidation number to identify the reactant that is oxidized and the reactant that is reduced. A. 4Fe(s) + 3O 2 (g) 2Fe 2 O 3 (s) B. 6Na(s) + N 2 (g) 2Na 3 N(s) C. 2K(s) + I 2 (g) 2KI(s) 21 22 Terms Used in Oxidation and Reduction Reactions Oxidizing and Reducing Agents In the reaction Mg(s) + Cl 2 (g) MgCl 2 (aq) Mg is the reducing agent, because it provides electrons when oxidized Mg(s) Mg 2+ (aq) + 2 e oxidation Cl 2 is the oxidizing agent, because it accepts electrons when reduced Cl 2 (s) + 2 e 2Cl (aq) reduction 23 24 4
Identify the oxidizing agent and the reducing agent in each of the following: A. Mg(s) + 2H + (aq) Mg 2+ (aq) + H 2 (g) B. 2Al(s) + 3Br 2 (g) 2AlBr 3 (s) 25 5