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Electrical Potential

Electrical Potential. Electrical Potential. The measure of the cell’s ability to produce an electric current Measured in volts Can’t measure an individual half-cell When half-cells are connected, the difference between two cells can be measured

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Electrical Potential

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  1. Electrical Potential

  2. Electrical Potential • The measure of the cell’s ability to produce an electric current • Measured in volts • Can’t measure an individual half-cell • When half-cells are connected, the difference between two cells can be measured • Reduction potential: the tendency of a half-reaction to occur as a reduction • Difference between reduction potentials is called the cell potential

  3. Standard Cell Potential • Standard cell potential (E0cell) is measured when the ionconcentrations are 1M • We assign an arbitrary value of zero to the hydrogen half cell under std. conditions. 2H+(aq) + 2e- H2(g) E0red = 0.00 volts • 1 M H+ solution, H2(g) bubbled in around a Pt electrode at std. P and 25oC. • Can use to measure E0red for other half-cells

  4. F2(g) + 2e- 2F-(aq) Cl2(g) + 2e- 2Cl-(aq) MnO2(g) + 4H+(aq) + 2e- Mn2+(aq) + 2H2O(l) NO3-(aq) + 4H+(aq) + 3e- NO(g) + 2H2O(l) Ag+(aq) + e- Ag(s) Fe3+(g) + e- Fe2+(aq) O2(g) + 2H2O(l) + 4e- 4OH-(aq) strength of reducing agent Cu2+(aq) + 2e- Cu(s) strength of oxidizing agent N2(g) + 5H+(aq) + 4e- N2H5+(aq) Fe2+(aq) + 2e- Fe(s) 2H2O(l) + 2e- H2(g) + 2OH-(aq) Na+(aq) + e- Na(s) Li+(aq) + e- Li(s) Selected Standard Electrode Potentials (298K) Half-Reaction E0(V) +2.87 +1.36 +1.23 +0.96 +0.80 +0.77 +0.40 +0.34 0.00 2H+(aq) + 2e- H2(g) -0.23 -0.44 -0.83 -2.71 -3.05 • Part of Table 20.1 on pg 712 in your text

  5. Standard Reduction Potentials • Remember  E0H+ is ALWAYS 0.00V • If we look at the voltaic cell between H and Zn: • First determine what is oxidized and what is reduced: • Zn (oxidized), H (reduced) • Write ½ reactions: • Zn (s) Zn2+ (aq) + 2e- • 2H+ (aq) + 2e-  H2(g) • Combine: Zn + 2H+ Zn2+ + H2 • The cell potential was measured at +0.76 V • Plug into E0cell = E0red – E0oxid 0.76 V = 0.00V – E0Zn2+ E0Zn2+ = -0.76V

  6. Calculating Standard Cell Potentials • The ½ cell with the more positive reduction potential will be reduced • If the cell potential is positive, the reaction will be spontaneous • BIG HINT the e- MUST BE EQUAL to create overall cell reaction • Use Table 20.1 to help in this process

  7. Calculating Standard Cell Potentials • Find the cell potential for the following: • Mg(s)|Mg2+(aq)||Zn2+(aq)|Zn(s) • Zn (s)|Zn2+(aq) ||Mg2+(aq)|Mg(s) • Cell potential: -0.76 – (-2.37) = 1.61V -2.37 – (-0.76) = -1.61V • Which one is the spontaneous reaction?

  8. Calculating Standard Cell Potentials • Determine the cell reaction, standard cell potential and the half cells that acts as the cathode for a voltaic cell composed of the following half cells: Fe3+ + e-  Fe2+ E0Fe3+ = +0.77V Ni2+ + 2e-  Ni E0Ni2+ = -0.25V

  9. Now you try some! • A voltaic cell is constructed using the following half-reaction: Ag+ + e-  Ag E0 = +0.80 V Cu2+ + 2e-  Cu E0 = +0.34 V • Determine the cell reaction and the standard cell potential • Calculate the standard cell potential to determine whether this redox reaction will occur spontaneously: 3Zn+ + 2Cr  3Zn + 2Cr3+

  10. Now what?? • Work on the half-sheet with a partner. When completed, check your work with Mrs. Etnyre. • When you get the answers correct, you may work on homework for the rest of the hour.

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