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Electrochemistry - PowerPoint PPT Presentation


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Electrochemistry. The study of the exchange of chemical and electrical energy. Oxidation-Reduction reactions take place in electrochemical cells. Each electrochemical cell consists of two solid conductors called electrodes. Two types of electrochemical cells: Voltaic Cell Electrolytic Cell.

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Presentation Transcript
slide2
The study of the exchange of chemical and electrical energy.
  • Oxidation-Reduction reactions take place in electrochemical cells.
  • Each electrochemical cell consists of two solid conductors called electrodes.
slide3
Two types of electrochemical cells:
  • Voltaic Cell
  • Electrolytic Cell
voltaic cell
Voltaic Cell
  • Reactions are spontaneous (Table J)
  • Redox reactions produce electrical energy
  • Lets look at an example:

Cu+2(aq) + Zn (s) -------> Cu (s) + Zn+2(aq)

slide6

The anode, is the electrode where the oxidation of zinc occurs:

Zn (s) ----------> Zn+2(aq) + 2e-

As zinc is oxidized, the zinc electrode will slowly dissolve to produce zinc ions, which enter into the solution.

Anode

(negative)

slide7

The cathode is the electrode where the reduction of copper occurs:

Cu+2(aq) + 2e- -------> Cu (s)

As copper is reduced, copper atoms accumulate on the surface of the solid copper electrode.

Cathode

(positive)

looking at the two cells can a redox reaction take place
Looking at the two cells, can a redox reaction take place?

The reaction will not occur.There is no way for the electrons lost by Zinc to get over to the Copper ions and reduce them to Copper metal.

slide9
What if we provide a path for the electrons released by the oxidation of the zinc to get over to the copper?

Now that the electrons have a path to the Cu/Cu2+ side it would appear that the reaction can proceed,

Why?

but it does not.

slide10
As the redox reaction proceeds:
  • There is a build up of Zn2+ ions in the solution where the Zinc is being oxidized
  • At the same time, Cu2+ ions are removed from solution where the Copper ion is being reduced
  • As a result, there is a build up of a positive charge in the zinc solution, and a negative charge in the copper solution. These charges do not allow electrons to flow.
  • The positive charge in the zinc solution makes it harder for the negative electrons to leave and the negative charge in the copper solution repels the electrons that are trying to come over from the zinc side

We need a way to neutralize the charge build-up in the solutions due to the change in ion concentration

slide11

What if we had a tube filled with aqueous solution that connected the two redox reactions?

Electron flow

This tube contains

an electrolyte and is called a salt bridge.

slide12
The salt bridge allows the electrons to flow, keeping the overall concentration of ions in balance between the two sides.
  • By connecting the salt bridge we have movement of both types of ions:
    • anions are going into the oxidation side and
    • cations are going into the reduction side
slide14
Standard Cell potential
  • Electrical energy produced by a redox reaction is measured as voltage
  • Some standard electrode potentials are listed below.

Electrode Process Eo/V

Li+(aq) + e-  Li(s) -3.03

K+(aq) + e-  K(s) -2.92

Zn2+(aq) + 2e-  Zn(s) -0.76

Cu2+(aq) + 2e-  Cu(s) +0.34

electrolytic cell
Electrolytic Cell
  • Reactions are non spontaneous
  • Redox reactions require electrical energy to occur
anions are attracted towards the anode where they undergo oxidation
Anions are attracted towards the anode where they undergo oxidation.

Electrons flow from the anode to the cathode where cations undergo reduction.

use of electrochemical cell
Use of Electrochemical Cell
  • A common dry

cell battery.

use of electrolytic cell
Use of Electrolytic Cell
  • Electroplating
  • Electrolysis of water