No school. Grading day.
Went over old redox test to review terms and concepts.
Build a battery lab. Groups of two built certain combinations of half cells and then everyone recorded the voltage of each cell. You drew your half cell and then after doing the first part of CA 50 and watching this video you added some details: half reactions, direction of electron flow, anode and cathode labels, and direction of ions in salt bridge. Then we looked at the chart of standard reduction potentials in your textbook and talked about what they mean. I introduced the idea of the standard hydrogen electrode and explained where the voltages came from. HW is to finish CA 50/ read chapter 19/ do textbook problems (unit outline)
Today we started with a practice AP problem from your packet. This led to a discussion of the effect of concentration on cell potential. If the concentration at the cathode is decreased this gets you closer to equilibrium so the cell potential goes down. You can also use Le Chatelier’s principle if you think about the reactant concentration decreasing, this suggest the reaction shifts in the reverse direction which would lower the cell potential (not as much driving force for the forward reaction). If you decrease the concentration at the anode, though, the cell potential will go up. This is like removing product which would cause the rxn to shift towards the products, increasing the driving force for the forward reaction.
Next we learned how to use the cell potential to calculate the equilibrium constant. I introduced the delta G = -n F E and reminded you of delta G = -RTln K. Just remember when determining n, the moles of electrons transferred in the reaction, you must have a balanced reaction. E.g. their are 6 moles of electrons transferred in the reaction between Al and Fe2+
(2Al + 3Fe2+ –> 3Fe + 2Al3+)
Next we looked at how to calculate the cell potential under non-standard conditions using the Nernst equation. We did 19.85 to practice.
During the 2nd half of class we talked about electrolysis and practiced predicting products for molten and aqueous solutions (see power point near the end)
HW: Textbook problems. Study for electrochem test on Monday