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Electrolysis – The Final Topic of Chemistry 30. Mr. Chapman. Electrolysis. Think of electrolysis and electrolytic cells as the opposite of electrochemical cells . In electrochemical cells , a spontaneous REDOX reaction is used to create electric current.

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Presentation Transcript
electrolysis
Electrolysis
  • Think of electrolysisand electrolytic cells as the opposite of electrochemical cells.
  • In electrochemical cells, a spontaneous REDOX reaction is used to create electric current.
  • In an electrolytic cell the reverse will occur; an

electric current is required in order for a non-spontaneous reaction to occur.

electrochemical cells and electrolytic cells opposites
Electrochemical Cells and Electrolytic Cells: Opposites

Electrochemical Cells

Electrolytic Cells

Energy conversion:

- Electrical  Chemical

Spontaneous reaction?

-No

Value of E°:

- Negative

  • Energy conversion:

- Chemical  Electrical

  • Spontaneous reaction?

- Yes

  • Value of E°:

- Positive

examples of electrolysis
Examples of Electrolysis:
  • Electrolysis of molten sodium chloride
  • Electrolysis of water
  • Electroplating
electrolysis1
Electrolysis
  • If we look at the latin roots of the word "electrolysis" we learn that it means, essentially, to "break apart" (lysis) using electricity.
  • Our first example of an electrolytic cell will examine how an electric current can be used to break apart an ionic compound into its elements.
  • The following equation represents the breaking apart of NaCl(l):
electrolysis of nacl s
Electrolysis of NaCl(s)

2NaCl(l) → 2Na(l) + Cl2 (g)

The half-reactions involved in this process are:

things to notice
Things to Notice:
  • Notice that a negative voltage (-4.07V) results when we add up the half-reactions.
  • This tells us that the overall reaction will NOT be spontaneous, and a minimum of 4.07 volts will be required for this reaction to occur.
  • This is the opposite of an electrochemical cell, in which the cell voltage is always positive.
setting up an electrolytic cell
Setting up an Electrolytic Cell:

Some key differences with an electrochemical cell set-up:

  • the two half-reactions are not separated by a salt bridge
  • an electrochemical cell (or other source of electric current) will be required in order to force the reaction to occur
other important differences
Other Important Differences:
  • The anode of the electrolytic cell is the site of oxidation and the cathode is the site of reduction, just as in an electrochemical cell.
  • In an electrochemical cell, the anode is negative and cathode positive, but this is reversed in the electrolytic cell - the anode is positive and the cathode is negative.
electrolytic cell
Electrolytic Cell

The Actual Cell:

An Explanation:

Electrons are "produced" in the battery at the anode, the site of oxidation.

The electrons leave the electrochemical cell through the external circuit.

These negative electrons create a negative electrode in the electrolytic cell which attracts the positive Na+ ions in the electrolyte. Na+ ions combine with the free electrons and become reduced (2Na+ + 2e- → Na )

Meanwhile the negative Cl- become attracted to the positive electrode of the electrolytic cell. At this electrode chlorine is oxidized, releasing electrons (Cl-→ Cl2 + 2 e-)

These electrons travel through the external circuit, returning to the electrochemical cell.