Chem 112 Week of February 22-26 Electrochemistry Notes
Chem 112 Week of February 22-26 Electrochemistry Notes Chem 112
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This 3 page Class Notes was uploaded by Rebecca Plummer on Friday February 26, 2016. The Class Notes belongs to Chem 112 at Pennsylvania State University taught by Dr. Raymond Shaak in Spring 2016. Since its upload, it has received 35 views. For similar materials see General Chemistry in Chemistry at Pennsylvania State University.
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Date Created: 02/26/16
Rebecca Plummer Dr. Raymond Schaak 26 February 2016 Electrochemistry Redox Reactions: Something that is oxidized loses electrons. OIL (oxidation is losing) - o Cl (aq) Cl2(s) Something that is reduced gains electrons. RIG (Reducing is gaining) 2+ o Fe (aq) Fe (s) When combining half-reactions, you must cancel out all electrons o - I is being oxidized because it goes from a negative charge to a zero charge (It loses electrons) Mn is being reduced because it goes from a 4+ charge to a 2+ charge (it gains 2 electrons) Oxidation Reaction: 2I - I + 2e - - 2- 2+ Reduction Reaction: MnO + 2e 4 Mn In order to balance the reduction half reaction, you must add 4 waters to the right side of the equation to account for the four oxygens on the left side. - - 2+ MnO +42e Mn + 4H 2 Now the oxygens are balanced, but the hydrogens are + not, so you need to add 8 H to the left side to make it balanced - - + 2+ MnO +42e + 8H Mn + 4H 2 Now that all the elements are balanced, you need to balance the charges. T There is a 5+ charge on the left because there is a 3- and an 8+ charge. There is a 2+ Charge on the right because the water has a 0 charge and Mn has 2+ charge In order to make the left side equal the right side, you need to add 3 more electrons to the left side. - - + 2+ MnO +45e + 8H Mn + 4H 2 Now you need to multiple the oxidation reaction by 5 and the final reduction reaction by 2 so the electrons can cancel out completely from both sides. The overall reaction, once the electrons cancel out will be: 2MnO 4- + 10I - + 16H + 2Mn 2+ + 5I 2 + 8H O 2 Voltaic Cells The reactions in a voltaic cell occur spontaneously. Anode: o The reaction in the anode is an Oxidation reaction o The electrode in the anode loses mass o Electrons flow from the anode to the cathode Rebecca Plummer Dr. Raymond Schaak 26 February 2016 o Anode has a negative charge Cathode: o The reaction in the cathode is a Reduction reaction o The electrode in the cathode gains mass o Cathode has a positive charge Electrolytic Cell The reaction in electrolytic cells need an energy source to make the reaction occur (the reactions are not spontaneous) Anode: o Anode has a positive charge o The reaction in the anode is an Oxidation reaction o Electrons flow from the anode to the cathode Cathode: o Cathode has a negative charge o The reaction in the cathode is a Reduction reaction Table of Standard Reduction Potentials E cell E reduction+ E oxidation The table of standard reduction potentials will tell you the values forrednd Eoxiyour job is to add them to get Ecell Remember to switch the sign of the number if the reaction is written the opposite way as in the table (just like in thermochemistry when you flip the reaction, you have to flip the sign of the ∆H value) Spontaneity and Cell Potential ∆G = -nFE or ∆G° = -nFE° The above equation is used to predict spontaneity under standard conditions ∆G is the free energy that tells whether a reaction is spontaneous or not 3+ - n is the number of electrons transferred so, Al (aq) + 3e Al(s) n=3 F is Faraday’s Constant which is 96485 J/Vmol or 96.485kJ/Vmol E is the cell potential (V) A reaction is spontaneous when ∆G < 0, so E has to be > 0. 0 RT E=E − lnQ nF The above equation is known as the Nernst Equation It is used the predict spontaneity under nonstandard conditions Rebecca Plummer Dr. Raymond Schaak 26 February 2016
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