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Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
209
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 20
2. Calculate the standard cell potentials of voltaic cells that contain the following pairs of half-cells. Use E 0 values from Table
21-1 in your textbook.
a. iron in a solution of Fe 2ϩ ions; silver in a solution of Ag ϩ
ions
b. chlorine in a solution of Cl Ϫ ions; zinc in a solution of Zn 2ϩ
ions
Determining reaction spontaneity An important use of standard reduction potentials is to determine if a proposed reaction is
spontaneous under standard conditions. To predict whether a proposed redox reaction is spontaneous, write the reaction in the form
of half-reactions and look up the standard reduction potentials. Use
the values to calculate the standard potential of a voltaic cell with
the two half-cell reactions. If the calculated voltage is positive, the
reaction is spontaneous. If the value is negative, the reaction is not
spontaneous; however, the reverse reaction would be spontaneous
because it would have a positive cell voltage.
Example Problem 20-2
Predicting the Spontaneity of a Reaction
Predict whether the following redox reaction will occur
spontaneously.
2Cr 3ϩ (aq) ϩ 3Sn 2ϩ (aq) 0 2Cr(s) ϩ 3Sn 4ϩ (aq)
Write the half-reactions. Note that the coefficients are simplified.
Cr 3ϩ (aq) ϩ 3e Ϫ 0 Cr(s) (reduction)
Sn 2ϩ (aq) 0 Sn 4ϩ (aq) ϩ 2e Ϫ (oxidation)
Find the standard cell potential, using E 0 values from Table 20-1 in
your textbook.
E 0
cell ϭ E 0
Cr 3ϩ ΈCr Ϫ E 0
Sn 4ϩ ΈSn 2ϩ
ϭ Ϫ0.744V Ϫ (ϩ0.151 V)
ϭ Ϫ0.895 V
The voltage is negative, so the reaction is not spontaneous. The
reverse reaction will occur spontaneously.
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