Redox Reactions Electrolytic Cells Mr. Shields Regents Chemistry U14 L04

Electrolytic Cells Earlier we said there were two types of Cells: - Electrochemical (i.e. voltaic/galvanic cell) - Electrolytic cell Electrochemical cells produce electricity through a Spontaneous Redox reaction. In an ELECTROLYTIC CELL, electrons (i.e. electricity) are provided to drive a non-spontaneous Redox reaction

Electrolytic Cell Usage Electrolytic cells are typically used to: - plate metals on other metals - obtain a pure metal from it’s compounds - recharge batteries

Differences between Electrochemical Cells Electrolytic Cells are different from Voltaic Cells because … They need Electricity to force a redox rxn to occur There is an external power source req’d They don’t produce electricity The polarities are reversed (we’ll see why later) The Anode is positive The Cathode is negative Electrolytic cell reactions are also different because they usually take place in one solution/one cell

Electrolysis In this example A battery supplies Electrons to drive Let’s look at the electrical decomposition of water. This is an example Of an electrolytic cell. It’s called ELECTROLYSIS 2H20(l)  2H2(g) + O2(g) In this example A battery supplies Electrons to drive The non-spontaneous Reaction forward What If we drew this electrolytic cell, What would it look Like?

- Notice that oxidation still takes place at the anode and Reduction still takes place at the Cathode. BUT … now the Anode is positive And the cathode Is negative. 2O-2 O2 +4e- Battery feeds e- into cell + - Electrons from an External source Flow into the Cathode making it Negative. Electrons flow from The anode into The external power source Making this electrode positive O2 H2

Memory Jogger Remember our phrase for recalling characteristics of An electrochemical (Voltaic) cell? “ An Ox ate a Red Pussy Cat” Well there’s another one for an electrolytic cell to also help you remember. “A POX on Electrolytic cells” Anode Positive Oxidation The Cathode is then just the opposite Cathode / Negative / Reduction

Electrode Comparison Voltaic Cell Electrolytic Cell Anode Negative Positive Cathode Oxidation takes place Reduction takes place

Other Electrolytic Cells Remember we said earlier that Electrolytic cells are typically used to … - plate metals on other metals - obtain pure elements from compounds - recharge batteries We’re going to look at two of these processes: - Production of pure elements - Mainly Group 1 + 2 elements - Downs Cell - Metal Plating

Electroplating This is another example of an electrolytic cell. It is used To plate one metal over another conducting object. For example: To plate Ag over Stainless steel To plate Au over Cu Or Ni To plate Cr over Fe

Electroplating - + In this diagram what is: The Cathode The Anode At what electrode Does Ox. Occur At which electrode Does Red. Occur What are the half Cell reactions? Cr  Cr+2 +2e- + - What’s plated Chrome Electrode Cr+2 +2e-  Cr Chrome electrode What’s plated

Downs Cell (fused Salt) + Simplified Cell All reactive Group 1 +2 metals, F2 & Al Are produced This way. If Na is being produced At the cathode then NaCl is melted (Fused) (801 deg C) To produce a different Metal I need to use a fused salt of that metal + - -

PROBLEM A manufacturer wants to make Gold forks out of his more cheaply made stainless steel (Fe) forks. Draw An electrolytic cell that he could use to accomplish this Task. Assume Au forms Au+3. Label everything fully and specify the oxidation and reduction half cell reactions that will take place.