Announcements Moving on to Chapter 21 Electrochemistry in Silberberg today. Skoog has a similar Chapter but Silberberg is better for you guys as it assumes no previous knowledge. Do Problems: 2,9,10,11,13,17,22,29,31,38,40,44,46,50,53,58,62,64,65,70,7 2,73,82,85,87 Laboratory Presentation March 11 at 7:30AM sharp Exam 3 March 17 Comprehensive Final Exam: March 24 7:30AM - 9:30 C114
Electrochemistry Chapter 21 (Silberberg) and Chapter 16 Skoog)
This Chapter concerns itself with a family of chemical reactions called oxidation-reduction reactions where electrons are transferred from a donar to an acceptor. Redox reactions are responsible for many things we take for granted...energy battery that can do work. We can supply electricity to a solution of dissolved metals (Cr for example) and force reduction of ions to the corresponding metal. We call this plating or electroplating.
Corrosion is an observable, troublesome and expensive reality in our world. Redox reactions form the basis for all corrosive processes. 2Fe(s) + 3/2O2(g) + xh2o(aq) -> Fe2O3.xH2O(s) Rust
Photosynthesis is the bio-chemical pathway that converts light into glucose (energy) is a myriad of oxidation-reduction reactions.
Let s ask some reasonable questions and answer them to get us started. 1. How can I distinguish a redox reaction from an acid-base or precipitation reaction? 2. What is oxidation-reduction reaction? 3. What are oxidation numbers or oxidation states and how do I assign them and use them? 4. What is the activity series? Where did it come from and how is it useful? 5. How do I balance redox chemical equations?
Precipitation Involves Acid-Base Neutralization Involves Oxidation Reduction Involves Cations Anions H + ions OH - ions Combine to Form Combine to Form Oxidation Reduction Which is the Insoluble Precipitate Predicted by Salt and H2O Loss of e - Gain of e - Which is called Solubility Rules Reducing Agent Oxidizing Agent
Acids and bases were viewed as proton (H + ) donars/acceptors with an acid strength characterized by Ka. Similarly, redox reactions occur when e - are donated by a specie and simultaneously accepted by another, characterized by standard reduction potential E. HA + B <=======> A - + BH + Acid a H + donor Base, a H + acceptor Conjugate Base Conjugate Acid Zn(s) + Cu 2+ (aq)! Zn 2+ (aq) + Cu(s) Reductant a e - donor Oxidant, a e - acceptor Conjugate Oxidant Conjugate Reductant
A redox reaction is a type of chemical reaction whereby an element, or a compound, looses an e - (is oxidized) to another element or another different compound that accepts the electron (is reduced). Solid Zn rod Cu 2+ ions Zn 2+ ions Cu(s) Corroded Zn bar Zn(s) + Cu 2+ (aq)! Zn 2+ (aq) + Cu(s)
The element or compound that looses electrons is oxidized and called the reducing agent or reductant. The element or compound that gains e - is reduced and called the oxidizing agent or oxidant. Zn Zn 2+ + 2e - Zn 2+ ions Cu(s) Corroded Zn bar Zn is oxidized or the reducing agent Cu 2+ + 2e - Cu(s) Cu 2+ is reduced or the oxidizing agent Zn(s) + CuSO 4 (aq) ZnSO 4 (aq) + Cu(s)
We assign oxidation numbers to understand what specie is being oxidized and reduced in a redox reaction. There are 7-basic rules... 1. The oxidation of all free elements in a reaction = 0 2. In monatomic ions, the oxidation number is equal to the charge on the ion (Look at the Group Number) 3. The sum of the ox. numbers of all the atoms in a molecule or ion is equal to the charge on the molecule or ion. 4. Fluorine has an oxidation state of 1 in its compounds 5. The oxidation number of hydrogen is +1 except when it is bonded to metals in binary compounds ( 1). 6. The oxidation number of oxygen is usually 2. In H 2 O 2 and O 2 2- it is 1 (exceptions). 7. When there is a conflict the lower numbered rules take priority over the higher numbered rules.
Examples: Assigning Oxidation Numbers HCO - 3 O = -2 H = +1 3x(-2) + 1 +? = -1 C = +4 K 2 Cr 2 O 7 O = -2 K = +1 7x(-2) + 2x(+1) + 2x(?) = 0 Cr = +6 IF 7 F = -1 7x(-1) +? = 0 I = +7 H2O2 H = +1 O = -1 NaIO 3 Na = +1 O = -2 3x(-2) + 1 +? = 0 I = +5
Oxidation Reduction reactions, precipitation reactions and acid-base and all chemical reactions occur because matter moves from higher chemical potential to lower chemical potential. Reactants ===> Products + free energy (that can do work) Potential Reactants ==> Products
Copper wire reacts with silver nitrate to form silver metal. What is the oxidizing agent in the reaction? Cu (s) + 2AgNO 3 (aq) Cu(NO 3 ) 2 (aq) + 2Ag (s) Cu Cu 2+ + 2e - Cu 2+ is oxidized (reducing agent) Ag + + 1e - Ag Ag + is reduced (oxidizing agent)
Recognizing Oxidizing and Reducing Agents Identify the oxidizing agent and reducing agent in each of the following: (a) 2Al(s) + 3H 2 SO 4 (aq) (b) PbO(s) + CO(g) (c) 2H 2 (g) + O 2 (g) Al 2 (SO 4 ) 3 (aq) + 3H 2 (g) Pb(s) + CO 2 (g) 2H 2 O(g)
Recognizing Oxidizing and Reducing Agents Assign an O.N. to each atom and determine which gained and which lost electrons in going from reactants to products. An increase in O.N. is associated with the oxidized species (the reducing agent) and a decrease in O.N. is associated with the reduced species (oxidizing agent). SOLUTION: 0 +1 +6-2 +3 +6-2 0 (a) 2Al(s) + 3H 2 SO 4 (aq) Al 2 (SO 4 ) 3 (aq) + 3H 2 (g) The O.N. of Al increases; it is oxidized; it is the reducing agent. The O.N. of H decreases; it is reduced; it is the oxidizing agent.
(continued) +2-2 +2-2 0 +4-2 (b) PbO(s) + CO(g) Pb(s) + CO 2 (g) The O.N. of C increases; it is oxidized; it is the reducing agent. The O.N. of Pb decreases; it is reduced; it is the oxidizing agent. 0 0 +1-2 (c) 2H 2 (g) + O 2 (g) 2H 2 O(g) The O.N. of H increases; it is oxidized; it is the reducing agent. The O.N. of O decreases; it is reduced; it is the oxidizing agent.
When balancing redox reactions we make two equations called half-reactions one for oxidation the other for reduction. Cu(s) + 2Ag + (aq) Cu 2+ (aq) + 2Ag(s) Cu(s) Cu 2+ + 2e - Oxidation half-reaction (lose e - ) 2Ag+ + 2e - 2Ag(s) Reduction half-reaction (gain e - ) Reduction half-reactions electrons as reactants and the Oxidation half-reaction the electrons are products. By convention we like to write and tabulate reactions as reductions rather than oxidations (somebody had to pick one).