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Electrolysis – The Final Topic of Chemistry 30

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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Electrolysis – The Final Topic of Chemistry 30

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  1. Electrolysis – The Final Topic of Chemistry 30 Mr. Chapman

  2. 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.

  3. 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

  4. Examples of Electrolysis: • Electrolysis of molten sodium chloride • Electrolysis of water • Electroplating

  5. 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):

  6. Electrolysis of NaCl(s) 2NaCl(l) → 2Na(l) + Cl2 (g) The half-reactions involved in this process are:

  7. 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.

  8. 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

  9. 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.

  10. 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.

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