2015 AP Chemistry free response 1 b c

All right, part B. A fresh zinc-air cell is weighed on an analytical balance before being placed in a hearing aid. For use, as the cell operates, does the mass of the cell increase, decrease, or remain the same? Justify your answer to part B1 or B1.

In terms of the equation for the overall cell reaction, so let's just think about it a little bit. Does the mass of the cell increase, decrease, or remain the same? Let's go to our original reaction over here. This is our total reaction. The zinc is part of the metal air cell, so that would contribute to its weight beforehand, and then it reacts with the oxygen.

Well, where is this oxygen coming from? Well, this oxygen is coming from the air, and essentially it gets incorporated into the zinc. So, before the oxygen is in the air, but then after the reaction, after the total reaction, it gets incorporated with the zinc. I guess I should say not into the zinc or it gets incorporated with the zinc as zinc oxide.

So now the oxygen is part of the cell, part or contributes to the weight of the cell. It doesn't contribute before the reaction but contributes after the reaction. So the mass, and I guess you could say the weight, is going to increase. So I'll just write increase.

Part two, we need to justify our answer in terms of the equation for the overall cell reaction, so let me just write the overall cell reaction. So, two zinc molecules react with one molecular oxygen (1 O2) to yield two zinc oxide (2 ZnO) molecules.

Well, what we could say is that this was not part of the cell before the reaction. Not part of the cell before the reaction is coming from the air before reaction, and then you could say it's part of the cell after the reaction. Part of the cell after reaction. And we could write it out in words if we like.

After a reaction, and so I could say oxygen comes from the air to be incorporated into the zinc oxide, which then contributes to the weight of the cell or to the mass of the cell, which then contributes to the mass of the cell. All right, I feel pretty good about that.

Now, let's do part C. The zinc-air cell is taken to the top of a mountain where the air pressure is lower. Will the cell potential be higher, lower, or the same as the cell potential at the lower elevation? Justify your answer to the first part based on the equation for the overall cell reaction and the information above.

Well, let me just write the overall cell reaction again. So, let me write that a little bit neater. Two zinc molecules, one molecular oxygen. We're going to have this ingrained in our brain by the time this is done. I'm writing it so frequently. We get two zinc oxides.

So what's going to happen if we go to the top of a mountain where the air pressure is lower? We have just fewer air molecules in general. That means we're also going to have lower oxygen molecules, so we can say that the partial pressure of oxygen is going to be lower.

So we could say partial pressure of the reactant will be lower. There's just going to be fewer oxygen molecules from the air being able to bump and react with the zinc molecules. Well, actually it's not reacting with the zinc molecules. As we saw in the original reactions when we look at the half reactions, fewer oxygens from the air are just bumping in the right way with the water molecules, and, actually, the electrons that are coming in through the circuit.

You're going to have fewer of them bouncing around being able to do this reaction. So you could say partial pressure of reactant, or maybe I'll say partial pressure, or you could also do the concentration of the reactant. There's going to be just less of it around. Partial pressure, or we could say concentration.

Lower cell potential. So actually, I just did it in reverse. I answered part two first, and so part one is lower. So we're going to have a lower cell potential. Lower cell potential.

It's really important to get the conceptual understanding of why it's happening. Remember when we look at the half-reaction, what's happening? What is happening at our cathode? We have oxygen coming in from the air, and it gets into the kind of the pores of this porous substance.

When the oxygen, the O2 molecules bump in just the right way with the water and the electrons coming in, they just kind of go together and react. You get your hydroxide formed. Now, if you have less, you know, at a lower elevation, you're going to have a lot of oxygen. You're going to have a high partial pressure of the molecular oxygen.

So a lot of them are, any moment in time, going to be available to bounce around and react. But if you go to a higher elevation, you're just going to have fewer of these characters. So you're going to have a lower partial pressure of oxygen, or you could think of it as having a lower concentration of oxygen inside the pores.

So it's going to be harder for this reaction to move forward. You can think about the extreme situation: what if you kept going to a super high elevation? If you eventually go into space, where you have very little oxygen or no oxygen at all, well then the reaction is going to stop altogether.

So hopefully that makes sense. Let me just go back to part C here. I'm having trouble. My...oh, let's go back to part C, and there you go.