LE 4.10 The Nernst Equation
Question Which way will the electrons flow? A. From left to right B. From right to left C. The cell is already at equilibrium so there will be no electron flow (SEE PIC)
(SEE PIC)
example of using Nernst equation with a concentration cell (SEE PIC)
(SEE PIC)
read pt. 2 (see pic)
(see pic)
mV to V
1 mV = 1000 Volts (millivolts)
Concentration Cells (from video)
1) definition ish: electrode in each half-cell is the same and the solution has the same ions but different concentrations 2) Anytime the two half cells are not identical, there is a potential difference between them. 3) This is true even if both cells contain the same half-reaction, as long as the concentrations are different
my version of calculating with the nernst equation
1) start w/ multiplying log and Q 2) multiply that by 0.0591/n 3) subtract E°cell - ____(number you get from first 2 steps)____
STEPS for how to use the Nernst Equation (6)
1. Write out the redox reaction from the shorthand notation 2. Balance the redox equation 3. Calculate E°cell from the standard reduction potentials 4. How many electrons are transferred in the overall reaction? This is "n" in the Nernst equation 5. Place the product and reactant concentrations in the Q expression. Don't forget the exponents! 6. Solve for Ecell
Question #1 from LE: An electrochemical cell is constructed using the following spontaneous redox reaction: X(s) + Y2+(aq) ⇌ X2+(aq) + Y(s) When the Y2+ and X2+ concentrations are equal, the cell generates a positive voltage of 1.00 v. How will the cell potential change if the concentrations are changed so that [X2+] = 10 × [Y2+]? A. The voltage of the cell will decrease. B. The voltage of the cell will increase. C. The voltage of the cell will increase.
A. The voltage of the cell will decrease. Correct. There are two ways you can think about this: 1) LeChatelier's principle tells us that if you increase the concentration of product in an equilibrium reaction, you will cause the equilibrium to shift to the left. Shifting to the left in the case of a redox reaction means producing a lower potential. 2) The Q term in the Nernst equation is products over reactants. Increasing the ratio of products to reactants ([X2+] = 10 × [Y2+]) means the log(Q) term will become larger, but since that number is being subtracted from Eocell, Ecell is decreasing. In other words, increasing Q decreases Ecell, and decreasing Q increases Ecell.
READ pt. 3
As with any new equation you encounter, you should look to see what other variables the equation can be used to solve for. For example, the Nernst equation also includes Eocell, n, and Q as variables -- and Q is a ratio of products to reactants. If you dig deeper into the Nernst equation you'll find that Eocell is based on Eored and Eoox, so we could conceivably give you a question in which an unknown reaction is happening in one of the half-cells and you have to work backward from the Nernst equation to determine Eored or Eoox, and identify the half-reaction occurring in that cell. But don't worry, we won't hit you with a question like that this semester. Just make sure you are aware of the hidden variables that are buried in each new equation you encounter. We'll learn in the video that you can also use the Nernst equation to calculate the potential of a concentration cell, which is an electrochemical cell that uses the exact same chemical species in both half-cells. For example, this electrochemical cell generates a positive voltage even though the oxidation half-reaction is simply the reverse of the reduction half-reaction. SEE PIC
read pt. 1
Until now we have been working only with standard cells -- that is, cells in which the aqueous concentrations are all 1 M, and gases are 1 atm, and the temperature is 25 oC. IRL, we rarely use standard cells. Room temperature and 1 atm pressure are convenient enough, but 1 M is a pretty stout concentration and we normally work with concentrations that are lower than that. But the table of standard reduction potentials only provides potentials at standard conditions. So if we're using a concentration other than 1 M, we have to tweak the standard potential somehow to account for the nonstandard conditions. That's where the Nernst equation comes in.
concentration cell
an electrochemical cell that uses the exact same chemical species in both half-cells but each chemical species has a different concentration (there are two Cu slides but one has a concentration of 0.100 M and the other has a concentration of 0.001 M)
Nernst equation 1) name all parts (SEE PIC)
memorize this version i think: Ecell= E°cell - (0.0591/n) x log (Q) E°cell = E°red - E°ox n is the number of electrons (e^-) transferred Ecell= E°cell - (RT/nF) (lnQ) (SEE PIC)
ANYTIME you have a concentration cell the E°cell is ______ (remember this is not the overall Ecell just E°cell which is a component of the Nernst equation)
zero
