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CHM 170 Notes Chapter 18 part 2

by: Joseph Notetaker

CHM 170 Notes Chapter 18 part 2 46657 - CHM 170 - 002

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These notes outline the half reaction method and go over the properties of voltaic cells
General Chemistry 2
Mark M Richter
Class Notes
General Chemistry
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This 3 page Class Notes was uploaded by Joseph Notetaker on Saturday April 16, 2016. The Class Notes belongs to 46657 - CHM 170 - 002 at Missouri State University taught by Mark M Richter in Spring 2016. Since its upload, it has received 7 views.


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Date Created: 04/16/16
CHM 170 Chapter 18 part 2 The Half Reaction Method in an acidic medium (As demonstrated by this equation) SO +2Cr O 2 72-------------- SO42-+ Cr 3+ First assign oxidation values: +4 -2 +6 -2 +6 -2 +3 S O 2 +Cr 2 O7------- S O 4 + Cr Next split the reaction into two half reactions: Oxidation: S O +4 2-2--------- S O 4-2 +6 -2 +3 Reduction: Cr 2 O 7 -------- Cr Now we have to balance the atoms do this by adding an H O for every Oxyg2n atom and 2 hydrogens on the opposite side to balance out the equation like so: Oxidation: SO + 22 O ----2------------- SO 4-2+ 4H + Reduction: Cr O 2 7-2 + 14H -------------- 2Cr +3 + 7H O 2 Add electrons to make the charges even: -2 + - Oxidation: SO + 22 O ----2--------------- SO 4 + 4H + 2e -2 + - +3 Reduction: Cr O 2 7 + 14H + 6e ---------------- 2Cr + 7H O 2 Next we use multiplication to make the electrons equal: -2 + - 3[SO +22H O --2----------- SO 4 + 4H + 2e ] -2 + - 3SO +26H O --2------------ 3SO 4 + 12H + 6e Finally we combine the equations: 3SO +26H O --2-- 3SO 4-2 + 12H + 6e - + Cr O2 7-2 + 14H + 6e ------ 2Cr +3 + 7H O 2 -2 + -2 +3 3SO +2Cr O 2 7 + 2H -------- 3SO 4 + H O2+ 2Cr A reaction in a basic solution uses the same method except in a basic solution you must add OH to the H in order to neutralize any acid Electricity is caused by the flow of electrons through a circuit Redox reactions involve the flow of electrons A voltaic cell is an electrochemical cell that produces electrical current from a spontaneous chemical reaction The electrode allows for the flow of electrons through the circuit The salt bridge balances out the positive and negative charges allowing the reaction to continue This reaction occurs because different metals have different orbital energy. This potential is measured in volts volt= joule coulmn The cell potential is a measure of the overall tendency of the redox reaction to occur spontaneously. If the cell potential is positive then the reaction is spontaneous. Oxidation occurs in the anode and reduction occurs in the cathode A shorthand for this concept has been developed: Zn(s)|Zn (aq)||Cu (aq)|Cu(s) This equation can be used to calculate overall cell potential: E o =E o +E o Cell Cathode Anode You’ll find that the values of the cathodes and anodes have been calculated already Eocell 0 means that the reaction is spontaneous. The amount of material in the cell has no effect on the cell potential You can calculate the free energy of a cell using this equation: o o ∆ G =−nF E cell N = number of moles F = faraday’s constant (96,485) The Nernst equation calculates cell potential outside of standard conditions: E =E o − 0.059v logQ cell cell n Batteries are voltaic cells that use spontaneous reactions as a source of energy Electrolytic cells are the opposite, their purpose is to induce nonspontaneous reactions by consuming energy


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