Today, we highlight some more high-yield stuff around General Chemistry and Electrochemistry. We’re joined once again by Clara from Blueprint MCAT (formerly Next Step Test Prep).
High-Yield Topic in General Chemistry: Electrochemistry
[01:55] A Brief Overview
Electrochemistry is a challenge for many students because it isn’t always taught all that well in college classes. So you need to memorize things like oxidation potentials, reduction potentials, signs, etc.
There are actually patterns that make it easy to remember which is which. A lot of times, students think they seem to get it backwards and so they get intimidated by it. But once you have it down, it becomes the easiest thing!
[Related episode: Breaking Down an Organic Chemistry MCAT Passage]
[04:50] Question 33
Zn has an oxidation potential of +0.76, while Al has an oxidation potential of +1.66. Which metal serves as the better oxidizing agent?
(A) Al, because it more readily gives up electrons
(B) Zn, because it more readily gives up electrons
(C) Al3+, because it is more prone to gaining electrons
(D) Zn2+, because it is more prone to gaining electrons
Oxidation refers to losing electrons. But with an oxidizing agent, it oxidizes something else so it reduces. So you have to recognize there’s an opposite thing happening here. Now, we’re looking for what metal to oxidize something else and which metal is best at reducing itself.
We’re looking at something that’s best at reducing itself to oxidize something else. The RIG part of OIL RIG is that reduction is gaining electrons.
Both answer choices C and D are both more prone to gaining electrons. The answer here is D. Al has its oxidation potential of +1.66. It had a higher oxidation potential because Zn is only 0.76. This means Al is better at oxidizing. But we’re looking at what’s better at reducing so that must be Zn, which is more prone to reducing.
The charges were placed there in answer choices C and D to probably only serve as a distraction. If you think about a half reaction, you might see something like Al becoming Al 3+ +3 electrons. One thing on one side of that reaction is always going to be charged.
Also, we’re looking for the thing that’s best at reducing itself. Al and Zn as metals can actually reduce themselves because to reduce, they have to gain electrons. And to gain electrons, they have to be negative.
But there’s no such thing as negative Al or negative Zn. So the only forms of Al and Zn that can actually reduce are the positive forms. Hence, the charges have been placed there only because it’s technically accurate with the charges.
[13:05] Question 37
Nickel-cadmium (NiCd) batteries are rechargeable cells used in products ranging from remote-control toys to airplanes. An NiCd battery in the process of discharging is most analogous to:
(A) a half-cell.
(B) an electrolytic cell.
(C) a galvanic cell.
(D) a nonspontaneous concentration cell.
The correct answer here is C. It’s all about definitions here. The difference between electrolytic and galvanic is super close. It totally depends on whether they’re talking about a battery that’s discharging or recharging.
An electrolytic cell is one that’s nonspontaneous. It’s one where you’re putting a lot of power in to drive the reaction.
A galvanic cell is one that is spontaneous. It’s just running by itself because it is spontaneous and producing this electricity in a spontaneous fashion.
When a battery is discharging, it can build up all of this differential and spontaneously letting that energy go. A battery in a process of discharging is always going to be similar to a galvanic cell. An electrochemical cell is just like letting its spontaneous process go.
A half-cell is one half of an electrochemical cell. It’s not going to ever be a full battery. A full battery needs both halves. Any electrochemical cell would be made up of two parts. There’s a part that’s a cathode and another part that’s an anode. Half-cell is just one of those parts. It’s like a half-reaction.
A concentration cell is a type of galvanic cell that depends on different concentrations of the reactants involved. They’re always spontaneous. A nonspontaneous concentration cell is a trick because it doesn’t exist. A concentration cell is a thing. They’re MCAT relevant.
[16:58] Question 39
A certain electrochemical cell contains a positive (+) cathode. With this in mind, which of these statements is true?
(A) The cell is galvanic.
(B) The cell is electrolytic.
(C) This cell could be either electrolytic or galvanic; we cannot decide from this information alone.
(D) This cell is neither an electrolytic nor a galvanic cell.
This question is relevant to other parts of the MCAT science as well. There are charges on electrodes. A cathode can be charged as well as an anode. The charge could be positive or negative.
It’s much easier to learn this if you could imagine what happens in an electrochemical cell in general. If ever you’ve heard the mnemonic RED CAT, AN OX. Reduction happens at the cathode and oxidation happens at the anode.
Now, if reduction happens at the cathode, that means electrons always go towards the cathode.
Reduction is the gain of electrons and electrons go towards the cathode.
If electrons are going towards the cathode and the cathode in this question is positive, it means that electrons are going in a spontaneous direction because electrons are negative.
Negative things love to go towards positive things. The question stem just confusingly describes a spontaneous reaction. Spontaneous means galvanic. So the cell must be galvanic because only the spontaneous cells are galvanic cells.
[20:45] When You Know It’s Clicking
It becomes clear if it’s clicking when you can almost immediately eliminate some answer choices. In question packs, there’s a moment where you see it’s spontaneous in the process. Right away, you can get rid of two answer choices, which those that mention electrolytic cell.
Moreover, just be able to work through all those steps. There’s that moment when you’re going to be able to go through all the steps.
Before you even get into the electrochemical cell part of all of this, just learn about redox reactions. Understand things like positive oxidation potential or negative reduction potential. Then you already have what you need to do those steps. Just stay calm to do them.
[22:28] Blueprint MCAT (formerly Next Step Test Prep)
If you’re looking for some more practice with the MCAT, @nextstepmcat does a free Question of the Day.
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