Unit 14: ElectrochemLPChem: Wz

Unit 14: Electrochemistry

LPChem: Wz Unit 14: Electrochemistry + 2 e - oxidation 2 e - + reduction The electrons produced here… …Are used up here.

LPChem: Wz Unit 14: Electrochemistry + 2 e - Loss of Electrons is Oxidation 2 e - + Gain of Electrons is Reduction

LPChem: Wz Unit 14: Electrochemistry + 2 e - Oxidation Is Loss (of electrons) 2 e - + Reduction Is Gain (of electrons)

LPChem: Wz Unit 14: Electrochemistry + 2 e - 2 e - + oxidation (half- reaction) reduction (half- reaction) 2 Na e -  + Cl NaCl + 2 e -

LPChem: Wz Unit 14: Electrochemistry 2 Na 0  + Cl NaCl Redox Reaction oxidation reduction Elements in their “elemental state” have a oxidation # of zero. Elements in compounds have oxidation #s based on periodic table location.

LPChem: Wz Unit 14: Electrochemistry 2 Na 0  + Cl NaCl Redox Reaction oxidation reduction +1 Ions have an oxidation number equal to their charge. +1 = = -1

Unit 14: Electrochemistry LPChem: Wz Zn (0  +2) Cu (+2  0) Zn (0  +2)

Unit 14: Electrochemistry LPChem: Wz

Unit 14: Electrochemistry LPChem: Wz

Unit 14: Electrochemistry LPChem: Wz Zn  Zn e - 2e - + Cu 2+  Cu e-

Unit 14: Electrochemistry LPChem: Wz Zn  Zn e - 2e - + Cu 2+  Cu KCl Cl - K+K+

Unit 14: Electrochemistry LPChem: Wz Zn  Zn e - 2e - + Cu 2+  Cu KCl Cl - K+K+ Anode Bridge Cathode At the Anode is Oxidation At the Cathode is Reduction

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

Unit 14: Electrochemistry LPChem: Wz Zn  Zn e - 2e - + Cu 2+  Cu KCl Cl - K+K+ Separating half-reactions makes galvanic reactions into batteries. Now let’s calculate its voltage!

Unit 14: Electrochemistry LPChem: Wz Zn  Zn e - 2e - + Cu 2+  Cu

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

Unit 14: Electrochemistry LPChem: Wz The REDUCTION half-reactions from our battery was: Cu e -  Cu

Unit 14: Electrochemistry LPChem: Wz The chart is in order of potential/ voltage. E o (V) Elements at the top are most desperate to reduce (gain electrons).  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 LPChem: Wz If charts are always of reduction potential, how do we find the voltage for our oxidation half-reaction? If oxidation is the opposite of reduction, then the opposite of the original chart should do the trick!

Unit 14: Electrochemistry LPChem: Wz The OXIDATION half-reactions from our battery was: Zn  Zn e - Its opposite would be: Zn e -  Zn

Unit 14: Electrochemistry LPChem: Wz The reduction potential of: Zn e -  Zn = V Its opposite Zn  Zn e - would be worth the opposite voltage: = 0.76 V

Unit 14: Electrochemistry LPChem: Wz Zn  Zn e - 2e - + Cu 2+  Cu0.34 V 0.76 V V

Unit 14: Electrochemistry LPChem: Wz 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!

Unit 14: Electrochemistry LPChem: Wz  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.

Unit 14: Electrochemistry LPChem: Wz  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.)