SPEAKER: When you're working with electrochemical reactions, the Nernst equation is an important tool to help you figure out what the cell potential is. Right in front of me, I've got a cell mixed up, which is made of copper and zinc. It's using copper solution and zinc solid. And the copper is being reduced. And the zinc is being oxidized. And if I met standard conditions, then my cell potential should be 1.1 volts. If I come over and look at my voltmeter, it reads almost 1.1 volts. Pretty close. And if you look at the bottles, I only have one significant figure. So this is pretty close. And this is what we would expect for standard conditions. We mentioned the Nernst equation. The Nernst equation is an important tool because it helps us determine the cell potential when we're not at standard conditions. Remember that Q represents the reaction quotient. The ratio of the products over reactants at the conditions you're experiencing, not necessarily equilibrium. In this particular form, we're locked in to 298 Kelvin. That's embedded in this constant right here. Can you use the Nernst equation to predict the cell potential on the voltmeter? If I was to recreate this cell using-- instead of 1 molar copper sulfate and 1 molar zinc sulfate, I trade out the 1 molar copper sulfate for 0.1 molar copper sulfate, how do you predict that this value will change? OK. So we've dispensed the liquids into our cell. And let's go ahead and hook it up and see what the measurement is. It's lower. Could you have predicted that? Yes. Remember that Q is the reaction quotient. And it represents the ratio of the products over the reactants. On the product side, we had zinc 2+ ions. And in the reactant side, we had copper 2+ ions. The concentrations of these were 1 molar and 0.1 molar. That gives us a ratio of 10 for the value of Q. Well, the log of 10 is 1. And the number of electrons transferred in our redox reaction from earlier was 2. So this value divided by 2 is a little under 0.03. And that's about what we're seeing in the difference of our two measurements. Now let's look at another interesting situation. What if we were to replicate the cell, except this time we're gonna use two solutions that have the same components but different concentrations? Calculate the value that we're going to find when we measure the potential of this cell. Let's look at the chemistry and do some calculations. First, let's look at the chemistry. The copper ions in the concentrated side of our cell are being reduced and have a potential associated with the reduction of copper. But the copper ions are being oxidized in the dilute side. When we go to calculate the E out of the cell, that's gonna to come out with a value of 0. When we plug that 0 into the Nernst equation, here's the 0. The n is 2. There's 2 electrons being transferred. And the Q. How do we figure out the Q? Products over reactants. The Q is gonna equal the dilute divided by the concentrated. Dilute divided by the concentrated. And that is 0.1. So when we put 0.1 into the log of Q, that turns out to be a negative number. And overall, we have our cell potential at nonstandard conditions equaling 0.030 volts. So we will see a little bit of voltage coming from our concentration cell when we measure its potential.
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20. Electrochemistry
Cell Potential: The Nernst Equation
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