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Unformatted text preview: 2/24/2010 Announcements: Wednesday, February 24, 2010
Homework WebAssign #8- Due Wednesday, March 3 at 8:00 AM Recitation Week 7 – Redox Reactions Lab- Hypochlorite (ClO-) in Bleach Complete your prelab before recitation. Suggested Textbook Questions Begin working Chapter 4.5 and Chapter 21 additional problems.
You are cordially invited to attend an Informational Session to learn more about becoming a Learning Assistant with the STEM-Colorado Program. When: Wednesday, February 24, 2010, at 6 p.m. Where: UMC 235 (hall right of Reception Desk) Refreshments will be served, while they last. Applications for Fall 2010 available Feb 24 - Mar 10 at http://stem.colorado.edu/Apps Get more information from faculty and LAs in these departments:
CHEM will be hiring for: Applied Math Math • General Chemistry MCDB Astronomy • Physical Chemistry with Physics Chemistry Biochemistry Applications Geological Sciences Electrochemical Cells Voltaic or Galvanic Cell- an electrochemical cell that contains a spontaneous reaction (DG < 0). Ex. batteries Electrolytic Cell- an electrochemical cell that contains a non-spontaneous reaction (DG > 0)that must be driven by an external power source. Ex. Re-charging old batteries How can you predict if a cell is galvanic or electrolytic? Calculate the cell potential. (More on this later.) Voltaic or Galvanic Cells To get any useful work out of a redox reaction, we need the electrons to travel through an external wire. Separate the two half reactions into “half cells”: Both begin with vowels! e– ANODE Cu(s) Cu2+ + 2e– oxidation + Both begin with consonants! CATHODE Ag+ + e– Ag(s) reduction Electrons go from the anode to the cathode. Voltaic or Galvanic Cells ANODE Cu(s) Cu2+ + 2e– oxidation e– a– V + CATHODE Ag+ + e– Ag(s) reduction February 24, 2010 - CT#1
For the following redox reaction, which reaction is occurring at the anode? ClO3– + 3H2O + 6I– Cl– + 3I2 + 6OH- A. ClO3– + 6H+ + 6e– Cl– + 3H2O B. Cl– + 3H2O ClO3– + 6H+ + 6e– C. 2I– I22+ 2e– – C. 2I – I + 2e D. I2 + 2e– 2I–
Oxidation occurs at the anode & oxidation is the loss of electrons. If this were the whole cell, electrons would “build up” in the cathode compartment. Nature won’t allow this! Complete the circuit with a “salt bridge”. Allows ions to flow between the half cells and keep charge neutrality. Electrons passing through the wire produce a voltage = work! E. ClO3– + 3H2O + 6I– Cl– + 3I2 + 6OH- 1 2/24/2010 Figure 21.3: General Characteristics of Voltaic and Electrolytic Cells February 24, 2010 - CT#2
Which of the following is NOT true about a volatic (Galvanic) cell? A. Oxidation occurs at the anode.
Notice the sign convention changes! B. Electrons travel through a wire from the anode to the cathode. C. Reduction occurs at the cathode. D. Anionstravel through the the bridge from Anions travel through salt salt bridge the anode to the to the from the anodecathode.cathode.
E. It contains a spontaneous reaction. Shorthand Notation anode|anode compartment||cathode compartment|cathode Example: Cu2+(aq) + Zn(s) → Cu(s) + Zn2+(aq)
reductioncathode oxidationanode February 24, 2010 - CT#3
Which of the following is the correct balanced redox reaction for Ni(s)|Ni2+||Mn2+|Mn(s)? A. Mn(s) + Ni2+(aq) → Mn2+(aq) + Ni(s) B. Ni(s)+ Mnn2+(aq) → 2+(aq) + Mn(s) Ni(s) + M 2+(aq) → Ni Ni2+(aq) + Mn(s) C. 2Mn(s) + Ni2+(aq) → Mn2+(aq) + 2Ni(s) D. 2Ni(s) + Mn2+(aq) → Ni2+(aq) + 2Mn(s) E. Ni(s) + Ni2+(aq) → Mn2+(aq) + Mn(s)
Anode reaction: Ni(s) → Ni2+ + 2eCathode reaction: Mn2+ + 2e- → Mn(s) In shorthand: Zn(s)|Zn2+(aq)||Cu2+(aq)|Cu(s) Another one: Cu(s) + 2Ag+(aq) → Cu2+(aq) + 2Ag(s)
oxidationanode reductioncathode In shorthand: Cu(s)|Cu2+(aq)||Ag1+(aq)|Ag(s) Shorthand Notation anode|anode compartment||cathode compartment|cathode If a half cell does not contain a conductive metal, an inert metal or a “passive electrode” must be used. Typically this would be platinum (Pt) or graphite. For example: Pt(s)|ClO4-(aq), ClO3-(aq), OH-(aq)||H+(aq), O2(g)|Pt(s)
Anode reaction: ClO3- + 2OH- → ClO4- + H2O(l) + 2eNet reaction: Cathode reaction: O2(g) + 4H+ + 4e- → 2H2O 2ClO3-(aq) + 4OH-(aq) + O2(g) + 4H+(aq) → 2ClO4-(aq) + 4H2O(l) 2 ...
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