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### Course: Class 12 Chemistry (India)>Unit 2

Lesson 3: Cell potentials at nonstandard conditions

# Using the Nernst equation

Using the Nernst equation to calculate the cell potential when concentrations are not standard conditions.  Created by Jay.

## Want to join the conversation?

• Once we find the cell potential, E how do we know if it is spontaneous or not?
• For a reaction to be spontaneous, ΔG should be negative.
Since ΔG= -nF*E (n=>n-factor or number of electrons transferred, F=96500 C= 1 Faraday
=>If E is positive, ΔG is negative. If E is negative, ΔG is positive. So, E should be positive for the reaction to be spontaneous.
• I still don't understand about the n. What does it represent?
• It's when you're doing redox reactions and trying to cancel out the number of electrons to balance each side. That number would be n. In other words, it would be the number of electrons you're transferring, as Andrews had said.
• What if we are dealing with an equation like 3 moles of Solid Iodine reacting with 2 moles of Aluminum(3+) giving 6 moles of Iodine(-) and 2 moles of Solid Aluminum
Would the Reaction Quotient be
[Iodine(-)]^6/[Aluminum(3+)^2
• You got it. Remember the reaction quotient only depends on aqueous ions, not solids, so your equation, after looking through it, seems correct.
• What happens to the cell potential if the temperature is increased and vice versa?

E_cell = E_standard_cell_potential - (RT)/(nF) * ln([products]/[reactants])

So it depends on the sign of the log term.

If concentrations of the products are greater than the concentrations of the reactants, then the cell potential will become more negative as temperature increases. (the log bit keeps the second term negative).

If the concentrations of the products are less than the concentrations of the reactants, then the cell potential becomes more positive as temperature increases (the log bit makes the second term positive).
• why do leave uot concentration of pure solids while writing nernst equation??
• Using concentrations in the Nernst equation is a simplification. In reality, what we care about is the activity. For solutions, the activity is equal to the concentration, which is why we can get away with just writing concentrations for these species. Pure solids and liquids have an activity of 1, so we can ignore them (since multiplying by 1 doesn't change the value). If you're interested in learning more about activity, it is sometimes also called "chemical activity" or "thermodynamic activity".
• What is the cell potential when Q is greater than 0 and less than 1, or the concentration of zinc ions is smaller than the concentration of copper ions? Is this cell potential greater than the standard potential? What would happen if there is no zinc ion in the beginning of the reaction (the concentration of zinc ions is 0)? That means Q is 0, and cell potential will be infinite. How could that be?
• Where does the number above n come from ? This wasn't shown. I have tried multiplying R by T and I do not get the same answer.

Thanks
• The number has been obtained from thermodynamic relationship (RT)/F and then multiplied by ln(10) to convert it to a log base 10. It is explained in the previous video called 'Nernst equation.' I hope this helps!
• What if we have a galvanic cell with 1-molar zink and copper solutions, but are working at a tempetature not equal to 25 degrees celcius? If we plug everything into the Nernst-equation, we would still get 1.1 V. But is this correct? Because i thougt the voltage depends on the temperature too?
• If you are not at 25*C,

Nernst says: E_cell = E_standard_cell_potential - (RT)/(nF) * ln([products]/[reactants])

so when Jay made the simplification of the second term to 0.0592/n * log_10(q), it is at 25*C. This is a good generalization if working with standard conditions, but you should use the first equation, plugging in all the conditions that are required.
(1 vote)
• when you write the equation with log, do you mean ln acturally?because the calculated value indicated this way.