5 Steps to a 5 AP Chemistry

and then apply the Nernst equation. If Ecellturns out to be negative, it indicates that the

reaction is not a spontaneous one (an electrolytic cell), or that the reaction is written back-

wards if it supposed to be a galvanic cell. If it is supposed to be a galvanic cell, all you need

to do is to reverse the overall reaction and change the sign on Ecellto positive. The other

method involves using the Nernst equation with the individual half-reactions, then com-

bining them depending on whether or not it is a galvanic cell. The only disadvantage to the

second method is that you must use the Nernst equation twice. Either method should lead

you to the correct answer.

Let’s practice. Calculate the potential of a half-cell containing 0.10 M K 2 Cr 2 O 7 (aq),

0.20 M Cr^3 +(aq), and 1.0 × 10 −^4 M H+(aq).

Answer:

The following half-reaction is given on the AP Exam

Experimental

Electrochemical experiments fall into two broad categories. Some experiments are con-

cerned with standard cell voltages, while other experiments use the Nernst equation directly

or indirectly. Experiment 21 in the Experimental chapter uses these concepts.

Measurements of the cell potential are essential and require a voltmeter (potentiometer).

These measurements may be taken from different combinations of half-cells, or from meas-

urements before and after changes of some aspect of the cell were made.

Using measurements of different half-cell combinations, a set of “standard” reduction

potentials may be constructed. This set will be similar to a table of standard reduction

potentials. The solutions used in the half-cells must be of known concentration. These solu-

tions are produced by weighing reagents and diluting to volume. The measurements will

require a balance and a volumetric flask. It is also possible to produce known concentra-

tions by diluting solutions. This method requires a pipette and a volumetric flask. Review

the Solutions and Colligative Properties chapter for solution techniques.

Experiments involving the Nernst equation are primarily concerned with concentra-

tions. One or more of the concentrations in the Qportion of the Nernst equation are cal-

culated by measuring the nonstandard cell potential and comparing this to the standard cell

potential. Remember, you calculate the concentration from a measured voltage. Once the

concentration is determined, it may be combined with other concentrations and used to

calculate an equilibrium constant.

Common Mistakes to Avoid

1.Be sure your units cancel to give the unit wanted in your final answer.

2.Be sure to round your answer off to the correct number of significant figures.

3.Remember that oxidation is the loss of electrons and reduction the gain, and that in

redox reactions the same number of electrons is lost and gained.

4.Be sure that what you determine to be the oxidizing and reducing agents are reactantspecies.

Cr O 2723 −+−+()a q ++→+ °= 14 H a q() 6 e 2 Cr ()a q 7 H O l 2 ()E 1. 333

0 0592^2

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log

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250  Step 4. Review the Knowledge You Need to Score High

STRATEGY

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