1.1 What is Chemistry?

http://www.ck12.org Chapter 23. Electrochemistry

E°cell= +0.77 - +0.34

E°cell= +0.43

Note: We can also think of this as reversing the sign of the oxidation half-reaction (from +0.34 to -0.34) and then
adding the two values together ((+0.77) + (-0.34) = +0.43). However, do not get mixed up and reverse the sign twice!

Next, we can determine Q, which has the following form for this reaction:

Q=[Cu

2 +][Fe 2 +] 2

[Fe^3 +]^2

Q=[^0.^25 ][^0.^20 ]

2

[ 1. 0 × 10 −^4 ]^2

Q= 1. 0 × 106

Finally, plug all of these values into the Nernst equation:

E=E◦−^0 .05619 voltsn log Q

E= 0 .43 V−^0 .05619 V 2 log( 1. 0 × 106 )

E= 0 .43 V− 0 .17 V

E= 0 .26 V

Under the given conditions, this electrochemical cell has a potential of +0.26 V.

Practice Problem 23.1

Calculate the cell potential at 25°C for the following reaction:

Fe(s) + Co^2 +(aq)→Fe^2 +(aq) + Co(s)

The ions are present in the following concentrations: 0.050 M Fe^2 +and 0.020 M Co^2 +.

Lesson Summary

• The ability of a particular electrochemical cell to generate an electric current is called its electrical potential.
  Reduction potentials measure the tendency of a substance to be reduced in a redox reaction.


• The standard hydrogen electrode is arbitrarily assigned a standard reduction potential of 0.00 V, and it serves
  as a reference by which all other half-cell potentials are measured.


• The standard cell potential for any electrochemical cell can be determined by finding the difference in reduc-
  tion potentials between the two half-cells. The cell potential must be positive for the overall reaction to be
  spontaneous.


• Reduction potentials can be used to make predictions about whether reactions will occur and whether a
  particular oxidizing or reducing agent is strong enough for a given purpose.


• The Nernst equation allows for the calculation of cell potentials under non-standard conditions.

Lesson Review Questions

1. What does standard reduction potential measure?


2. Under what conditions is a particular standard cell potential (E;cell) determined?


3. Explain the operation and usefulness of the standard hydrogen electrode.


4. Distinguish the anode adn the cathode in a galvanic cell.