OB: Redox Class #4 Voltaic vs. Electrolytic cells Voltaic cells has chemistry spontaneously produce electricity. The other kind of electrochemical cell, the electrolytic cell, uses electricity to make redox chemistry happen.

When you set up an electrochemical cell, called a voltaic cell, or battery, the chemistry happens and the electricity is produced until the cell dies out. It just happens spontaneously because it can happen. If you want a “reverse” redox to occur, one that should not happen spontaneously, you can force it to happen if you push it with electricity. For example: If we put a hunk of copper metal into a beaker containing silver nitrate solution, what happens? Think now. AgNO 3(AQ) copper Cu (S) + AgNO 3(AQ) CuNO 3(AQ) + Ag (S)

The spontaneous reaction is that the copper ionizes into solution, replacing the silver ions. The silver ions pick up the electrons, forming into silver atoms, precipitating into solids. The copper is “higher on table J” than the silver is. But what if you wanted to put silver plating onto copper? What if you wanted to FORCE a non-spontaneous reaction to occur? Could you? YES! With some smarts, and some equipment, and some electricity, you can force a redox reaction that would not occur spontaneously. This is called forming an ELECTROLTIC CELL, or electroplating. In this case we’d be silver plating the copper with silver.

AgNO 3(AQ) Cu (S) Ag (S) battery Let’s try to label the flow of electricity (electrons) through this whole system. Think about anode and cathode too.

AgNO 3(AQ) Cu (S) Ag (S) battery Ag +1 e -1 The electricity pushes electrons onto the copper bar. The silver cations pick up these electrons, forming more silver on the copper. The silver bar oxidizes into more silver cations for the solution. The electrons from this oxidation replace the electrons from the battery, completing the redox in the battery.

AgNO 3(AQ) Cu (S) Ag (S) battery Ag +1 e -1 The difficulty with this is “seeing” that the silver bar oxidizes into solution, and the silver cations in solution reduce into silver on the copper. The electrons leaving the battery are replaced with different electrons. The battery electrons become more copper, the silver electrons become part of the reduction inside the battery.

With electrolytic cells, it is the electricity that produces the redox reaction to occur. It makes the reaction that would not occur spontaneously happen. It’s a forced redox. If we look at copper and silver on table J, it’s the copper that should oxidize into solution, and the silver cations that should be reduced to silver atoms. The push of the electricity makes the reverse happen. This process is how you can plate any metal (silver, gold, etc., onto a more durable but more chemically reactive metal (like silver onto steel which is mostly iron). Electroplating is a big industry in the world, for art, jewelry, and technology.

In the next slide we will attempt to plate gold onto an aluminum spoon. Aluminum is more reactive (higher on table J) than gold, so Al should go into solution, gold cations should become gold atoms and precipitate. To avoid this, we’ll connect an outside electrical source to make the redox we want to happen, not the spontaneous redox that would otherwise occur. Then, we’ll work out the half reactions. FINALLY: which is the anode and cathode?? Is it the spoon, or the gold bar? Hint, Leo is STILL a red cat.

Au (S) battery AuCl (AQ) Aluminum spoon Let’s try to label this up, electrons from the battery, to the spoon, what happens in the solution, what happens on the gold bar, etc.

Au (S) battery Au +1 e -1 AuCl (AQ) Aluminum spoon Electrons from the battery land on the spoon, which causes the reduction of the Au +1 cations, plating the spoon gold. The gold bar oxidizes gold atoms into cations, and the free electrons from this oxidation replace the electrons from the battery. ½ ox: Au° (S) Au +1 (AQ) + 1e -1 ½ red: Au +1 (AQ) + 1e -1 Au° (S) NET: Au°+ Au +1 Au° + Au +1

Au (S) battery Au +1 e -1 AuCl (AQ) RED CAT… Reduction happens at the cathode, so the spoon is the cathode. Electrons are “lost” at the anode, so the gold bar is the anode.

Cu (S) battery Cu +2 CuSO 4(AQ) No one I know would like to cover their silver with a copper coating, but they could if they were smart enough. Putting silver into the copper II sulfate solution should result in nothing happening because the copper is higher on table J, the silver can’t react with the copper. Adding the electricity changes things up totally. The electrons show up on the silver, drawing the Cu +2 cations to it, forming a copper atom coating. The copper bar oxidizes into solution, releasing electrons that go up to the battery. This is NON SPONTANEOUS redox. This is an electrolytic cell. Ag (S)

Cu (S) battery Cu +2 e -1 CuSO 4(AQ) No one I know would like to cover their silver with a copper coating, but they could if they were smart enough. Putting silver into the copper II sulfate solution should result in nothing happening because the copper is higher on table J, the silver can’t react with the copper. Adding the electricity changes things up totally. The electrons show up on the silver, drawing the Cu +2 cations to it, forming a copper atom coating. The copper bar oxidizes into solution, releasing electrons that go up to the battery. This is NON SPONTANEOUS redox. This is an electrolytic cell. Ag (S)

For homework tonight: Redox HW #1 Redox HW #2 is due Thursday. (Organic Celebration Wednesday) If I were you I’d do both tonight + get it out of the way. That is unless you’re going to Barnes & Noble to buy your review book, which means I’d do the first one only. That is, unless I already had plans for tomorrow night, so the I would stay up late to both assignments tonight. And I would start reading my diary and my review book, no matter what!