How do I calculate the cell potential?

In summary: It should be 4Fe3+ + H2 --> 2Fe2+ + 2H+.In summary, the conversation discusses a problem involving a voltaic cell and finding its cell potential. The solution involves balancing the reaction equation and using the Nernst equation. The pH of both compartments also needs to be considered, and the solubility of Fe(OH)3 may affect the validity of the problem. The conversation also suggests ignoring pH in the iron solution and using pressure in the calculation.
  • #1
Newton25
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< Mentor Note -- thread moved to HH from the technical chemistryforum, so no HH Template is shown >

I was given this problem:
A voltaic sell uses the reaction 4Fe3+ + H2 --> 2Fe2+ +2H+.
Find the cell potential when [Fe3+]=1.50 M, PH2=.50 atm, and [Fe2+]=.00100 M and the pH of both anode and cathode compartments of the cell=5.00.
The reduction potentials are
Fe3+ +1e --> Fe2+ E=.771V
H2--> H+ + 2e E=0 V

I'm having quite a bit of difficulty with it. My first step was to balance the reaction equation and I split it into the half reactions. For the oxidation half, I got 2H2--> 4H+ + 4e. For the reduction, I got 4Fe3+ +4e --> 4Fe2+. And my E initial was .771 V. My next step was to use the Nernst equation. I had E=.771 - ((.0592)/4)logQ. Solving Q is the part that's giving me problems. First, I wasn't sure if I could use the pressure of H2 as everything else was a concentration. Second, the pH of both compartments being 5.00 is messing me up. If it was just the anodic compartment, I would use the antilog to determine the concentration of hydrogen ion. However, since it's 5.00 in both compartments, I have no clue what to do with that. If anybody could help with anything, it'd be greatly appreciated! Thanks.
 
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  • #2
Would it help if you ignore pH in the iron solution, and use it only for the hydrogen electrode? And yes, you should use pressure in Q, standard state calls for hydrogen pressure of 1 bar.

Actually question doesn't look good to me - if memory serves me well, solubility of Fe(OH)3 is so low Fe3+ can exist only in quite acidic solutions. pH 5 is way too high.
 
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  • #3
Borek said:
Would it help if you ignore pH in the iron solution, and use it only for the hydrogen electrode? And yes, you should use pressure in Q, standard state calls for hydrogen pressure of 1 bar.

Actually question doesn't look good to me - if memory serves me well, solubility of Fe(OH)3 is so low Fe3+ can exist only in quite acidic solutions. pH 5 is way too high.
Yes, that would help. Thank you. I wasn't sure if I was able to ignore it or not.

So, would the Q then be equal to ([.00100]^4 * [antilog(-5.00)]^4)/ ([.50]^2 * [1.50]^4) ?
 
  • #4
You are on the right track, but check if your reaction equation is balanced.
 

1. How do I calculate the cell potential?

To calculate the cell potential, you will need to use the standard reduction potentials of the two half-reactions involved in the cell. Simply subtract the reduction potential of the anode from the reduction potential of the cathode to obtain the cell potential.

2. What is the standard reduction potential?

The standard reduction potential is a measure of the tendency of a species to gain electrons and undergo reduction. It is measured in volts (V) and is referenced to the standard hydrogen electrode (SHE) which has a potential of 0 V.

3. What is the difference between anode and cathode?

The anode is the electrode where oxidation occurs and electrons are lost, while the cathode is the electrode where reduction occurs and electrons are gained. In a galvanic cell, the anode is the negative electrode and the cathode is the positive electrode.

4. Can I calculate cell potential without knowing the concentrations of the species involved?

Yes, you can calculate cell potential without knowing the concentrations of the species involved. This is because the standard reduction potentials are independent of concentration. However, if the concentrations are not 1 mol/L, you will need to use the Nernst equation to adjust for the concentration effect.

5. How can I use the cell potential to predict the direction of a redox reaction?

If the calculated cell potential is positive, then the reaction will proceed spontaneously in the forward direction (from left to right in the cell notation). If the cell potential is negative, then the reaction will proceed in the reverse direction (from right to left in the cell notation). A zero cell potential indicates that the reaction is at equilibrium.

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