Electrochemistry Oxidation-Reduction Unit 14: The Chemistry-Electricity Connection Unit 14: Electrochem LPChem: Wz

Chemistry and Electricity Electrochemistry Chemistry and Electricity Galvanic Cell: a spontaneous chemical reaction that causes electric current to be generated. Electrolytic Cell: a nonspontaneous chemical reaction caused by the application of electric current. LPChem: Wz Unit 14: Electrochemistry

An electrochemical reaction involves the transfer of electrons: Electrochemistry An electrochemical reaction involves the transfer of electrons: 2 Na + Cl2  2 NaCl 2 Na0  2 Na1+ Cl2 0  2 Cl1- LPChem: Wz + 2 e- oxidation 2 e- + reduction The electrons produced here… …Are used up here. Unit 14: Electrochemistry

An electrochemical reaction involves the transfer of electrons: Electrochemistry An electrochemical reaction involves the transfer of electrons: 2 Na0  2 Na1+ Cl2 0  2 Cl1- LPChem: Wz Loss of Electrons is Oxidation + 2 e- 2 e- + Gain of Electrons is Reduction Unit 14: Electrochemistry

An electrochemical reaction involves the transfer of electrons: Electrochemistry An electrochemical reaction involves the transfer of electrons: 2 Na0  2 Na1+ Cl2 0  2 Cl1- LPChem: Wz Oxidation Is Loss (of electrons) + 2 e- 2 e- + Reduction Is Gain (of electrons) Unit 14: Electrochemistry

Redox Reaction (a complete reaction) Electrochemistry An electrochemical reaction involves the transfer of electrons: 2 Na0  2 Na1+ Cl2 0  2 Cl1- Redox Reaction (a complete reaction) LPChem: Wz + 2 e- oxidation (half-reaction) + 2 e- + reduction (half-reaction) 2 Na0 + 2 e- + Cl20  2 NaCl + 2 e- Unit 14: Electrochemistry

Elements in their “elemental state” have a oxidation # of zero. Electrochemistry Transfer of electrons: Redox Reaction  LPChem: Wz Elements in their “elemental state” have a oxidation # of zero. oxidation +1 -1 2 Na0 + Cl20  2 NaCl reduction Elements in compounds have oxidation #s based on periodic table location. Unit 14: Electrochemistry

Ions have an oxidation number equal to their charge. Electrochemistry Transfer of electrons: 2 Na1+ + 2 Cl1-  2 NaCl Not Redox.  Redox Reaction  LPChem: Wz oxidation +1 -1 2 Na0 + Cl20  2 NaCl Ions have an oxidation number equal to their charge. reduction +1 = +1 +1 -1 +1 -1 -1 = -1 Unit 14: Electrochemistry

Electrochemistry Zn + CuSO4  ZnSO4 + Cu Zn (0  +2) Cu (+2  0) What element was oxidized? What element was reduced? Which element LOST electrons? Which element GAINED electrons? -2 -2 +2 +2 LPChem: Wz Zn (0  +2) Cu (+2  0) Zn (0  +2) Cu (+2  0) Unit 14: Electrochemistry

How do I make this reaction into a battery? Galvanic Cells: Zn + CuSO4  ZnSO4 + Cu How do I make this reaction into a battery? Physically separate the half- reactions Zn  Zn2+ + 2e- 2e- + Cu2+  Cu LPChem: Wz Unit 14: Electrochemistry

Physically separate the half- reactions Zn  Zn2+ + 2e- Galvanic Cells: Physically separate the half- reactions Zn  Zn2+ + 2e- 2e- + Cu2+  Cu LPChem: Wz Unit 14: Electrochemistry

Galvanic Cells: When the half-reactions are separate, electrons produced by the oxidation must travel through a wire to the reduction. LPChem: Wz e- e- e- e- Zn  Zn2+ + 2e- 2e- + Cu2+  Cu Unit 14: Electrochemistry

Galvanic Cells: The salt bridge completes the circuit by allowing ions to transfer. This is necessary to equalize the charge (so the beakers don’t end up charged). LPChem: Wz KCl Cl- K+ Zn  Zn2+ + 2e- 2e- + Cu2+  Cu Unit 14: Electrochemistry

Galvanic Cells: Anode Cathode At the Anode is Oxidation Galvanic Cells are typically drawn in alphabetical order: (This makes the electrons move left to right through the wire.) LPChem: Wz Anode Cathode Bridge At the Anode is Oxidation KCl At the Cathode is Reduction Cl- K+ Zn  Zn2+ + 2e- 2e- + Cu2+  Cu Unit 14: Electrochemistry

Galvanic Cells: The anode (+) attracts anions. The cathode (-) attracts cations. LPChem: Wz Anode Cathode Bridge At the Anode is Oxidation KCl At the Cathode is Reduction Cl- K+ Zn  Zn2+ + 2e- 2e- + Cu2+  Cu Unit 14: Electrochemistry

Galvanic Cells: If you put something in the middle of the wire, the electrons go through it on their way from anode to cathode. Electricity! LPChem: Wz Separating half-reactions makes galvanic reactions into batteries. KCl Now let’s calculate its voltage! Cl- K+ Zn  Zn2+ + 2e- 2e- + Cu2+  Cu Unit 14: Electrochemistry

Voltage Voltage is the “push” behind electric current. It depends on the two half-reactions used: Because every element has different electron affinity, the voltage is different for each element. LPChem: Wz Zn  Zn2+ + 2e- 2e- + Cu2+  Cu Unit 14: Electrochemistry

Voltage Standard Reduction Potential Voltage (AKA Electrochem. Potential) is found on a chart like this. LPChem: Wz The chart is all REDUCTION half-reactions. Unit 14: Electrochemistry

Voltage Standard Reduction Potential The REDUCTION half-reactions from our battery was: Cu2+ + 2e-  Cu LPChem: Wz Unit 14: Electrochemistry

Voltage Standard Reduction Potential The chart is in order of potential/ voltage. Eo (V) Elements at the top are most desperate to reduce (gain electrons). LPChem: Wz Reactions with the most positive voltages are the most spontaneous reductions. Reactions with the least positive (most negative) voltages are least spontaneous reductions. Hydrogen is zero. Unit 14: Electrochemistry

Voltage Standard Reduction Potential If charts are always of reduction potential, how do we find the voltage for our oxidation half-reaction? LPChem: Wz If oxidation is the opposite of reduction, then the opposite of the original chart should do the trick! Unit 14: Electrochemistry

Voltage Standard Reduction Potential The OXIDATION half-reactions from our battery was: Zn  Zn2+ + 2e- Its opposite would be: Zn2+ + 2e-  Zn LPChem: Wz Unit 14: Electrochemistry

Voltage Standard Reduction Potential The reduction potential of: Zn2+ + 2e-  Zn = - 0.76 V Its opposite Zn  Zn2+ + 2e- would be worth the opposite voltage: = 0.76 V LPChem: Wz Unit 14: Electrochemistry

+ Voltage The two half-reactions used: Reduction: Oxidation: Added together: Complete Reaction = 2e- + Cu2+  Cu 0.34 V LPChem: Wz + 0.76 V Zn  Zn2+ + 2e- 1.10 V Unit 14: Electrochemistry

Voltage The further apart the two half reactions are on the chart, the greater the voltage of the overall reaction. Voltage is a potential– like gravitational potential energy. Larger separation between the half-reactions means the electrons have further to “fall.” More potential! LPChem: Wz Unit 14: Electrochemistry

Voltage The anode (oxidation) half reaction will always need to be “flipped” because the chart is for reductions. That means changing the sign on its voltage. LPChem: Wz Unit 14: Electrochemistry

Voltage If the anode (oxidation) half-reaction were above the cathode half-reaction on the chart, what would happen?? Things don’t fall “up.” No reaction would take place. Unless… …external current were applied. (Electrolysis.) LPChem: Wz Unit 14: Electrochemistry