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Chapter 21. Electrochemistry : Energy conversion between chemical and electrical forms. Voltaic Cells. Type of electrochemical cell (machine that uses redox to produce electrical energy, or uses electrical energy to cause a chem Rx)

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chapter 21

Chapter 21

Electrochemistry:

Energy conversion between chemical and electrical forms.

voltaic cells
Voltaic Cells
  • Type of electrochemical cell (machine that uses redox to produce electrical energy, or uses electrical energy to cause a chem Rx)
  • Converts chemical energy to electrical energy via redox reactions
parts of voltaic cells
Parts of Voltaic Cells
  • Half-cells
    • physical separation of half-reactions
    • Each contains a different metal
  • Electrodes:
    • Anode: where oxidation occurs (neg.)
    • Cathode: where reduction occurs (pos.)
  • Salt bridge: pathway for ions between half-cells
  • Wires: pathway for electrons
slide4

Cathode

Anode

direction of electron flow
Direction of Electron Flow
  • Through wire
  • Oxidation occurs at the anode
  • Electrons move from oxidized metal to reduced metal
  • So, electrons flow from anode to cathode
direction of ion flow
Direction of Ion Flow
  • Ions flow through salt bridge
    • (+) cations migrate to cathode.
    • (-) anions migrate to anode.
standard reduction potential e
Standard Reduction Potential (E°)
  • E° shows how easily a substance gains electrons
  • Standard Reduction Potential Chart:
    • The greater the E°, themore easily a substance is reduced.
    • The smaller the E°, the more easily a substance is oxidized.
  • Chart shows:
    • Substances at the top are most easily reduced.
    • Substances at the bottom are more easily oxidized.
standard reduction potential e1
Standard Reduction Potential (E°)

TO FIND the E° (the voltage) of a voltaic cell:

  • Determine which substance is reduced and which one is oxidized.
    • The substance that is HIGHER in the chart is REDUCED.
    • The substance that is LOWER in the chart is OXIDIZED.
standard reduction potential e2
Standard Reduction Potential (E°)
  • Change the sign of the E° that represents OXIDATION (the 1 that is lower on the chart) and
  • Then just ADD the 2 values of E°.
standard reduction potential e3
Standard Reduction Potential (E°)

Ex 1 : Given a voltaic cell consisting of a Ag electrode in one half-cell and a Zn electrode in the other half-cell.

a. Determine the reduction potential for this cell.

  • Identify the anode and cathode. (Cathode is reduced & anode is oxidized.)
  • Which electrode gains mass?

(Hint: FAT RED CAT) The cathodegains mass. The metal ion gains electrons because it is reduced.

d. Which electrode loses mass?

(Hint: THIN OXAN) The anode loses mass. The metal is loses electrons, because it is oxidized.

e. Write the equation for the oxidation half reaction and write the equation for the reduction half reaction.

standard reduction potential e4
Standard Reduction Potential (E°)

Line Notation = shorthand way to show a voltaic cell.

Oxidation oxidation reduction reduction

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