1. a. Zn
2. The water and hydroxide are recycled.
3. They choose the path of no(almost) resistance; there's far less resistance through metal than there is in the solution where excess charge is being neutralized. Note that the Zn solid is not in contact with the MnO2 solid because the plastic membrane separating them is only permeable to water and dissolved ions, not to solids
Alkaline cells last longer because they recycle two of their reactants: hydroxide and water and primarily because all of the MnO2 ( the receiver of electrons)contributes to generating electricity. In dry cells, some MnO2 is put into the battery just to get rid of unwanted hydrogen, which never forms in alkaline batteries.
4. Znà Zn+2 + 2e-
5. 1.2 V + 0.76 V = 1.96 V
6. a. Zn + 2 MnO2 à ZnO + Mn2O3
b. Technically the reaction is not really at equil’m, but we’ll pretend.
(1) If the MnO2 would dissolve, then the forward reaction would be favoured and more products would result. If the solid does not dissolve, adding more will have no effect on equilibrium. (Adding gaseous or aqueous ions always increases concentration and hence increases the number of effective collisions. This strategy fails in the case of liquids and solids.)
(2) no effect. Contrary to what said in class, changing OH-‘s concentration is not the way to manipulate voltage of alkaline batteries. (For other cells, however, the concentration will affect voltage.) To increase voltage, manufacturers instead place several cells in series within the battery.
(3) no effect
7. Specifically what role do oxygen and water play in rusting?
They themselves are reduced, which means that they oxidize iron.
2H2O + O2 + 4e-
8. Is it important to fix car dents immediately? Why?
Yes. Any stress on the metal allows oxidation to occur faster by letting out electros faster.
9. Why doesn’t the rust accumulate at the site of oxidation?
Oxidation occurs at the anode, where the iron turns into ions and leaves a hole behind. As usual, mass accumulates at the cathode where reduction takes place. At the cathode we get production of hydroxide which binds to iron ions and precipitates as rust.