5 Steps to a 5 AP Chemistry 2019

252 ❯ STEP 4. Review the Knowledge You Need to Score High

TIP

When using the Nernst equation on a cell reaction in which the overall reaction is

not supplied, only the half-reactions and concentrations, there are two equivalent meth-

ods to work the problem. The first way is to write the overall redox reaction based upon

E° values, and then apply the Nernst equation. If Ecell turns out to be negative, it indi-

cates that the reaction is not a spontaneous one (an electrolytic cell), or that the reaction

is written backwards if it supposed to be a galvanic cell. If it is supposed to be a galvanic

cell, all you need to do is reverse the overall reaction and change the sign on Ecell to

positive. The other method involves using the Nernst equation with the individual

half-reactions, then combining 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:

E

EE

n

Cr O(aq)14H (aq) 6e 2Cr(aq)7HO(l) 1.33V

0.0592

log

[Cr]

[CrO ][H]

(ignoreHO)

1.33 V

0.0592

6

log

[.20]

[.10][1.0 10 ]

0.78V

27

23

2

32

27

214 2

2

414

++→+ °=

=°−

=−

×

=

−+−+

+

+

−

−

Experiments

Electrochemical experiments are normally concerned with standard cell voltages.

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 mea-

surements 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

solutions 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 con-

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

Review Chapter 13 on Solutions and Colligative Properties for solution techniques.

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. When diagramming an electrochemical cell, be sure the electrons go from anode to cathode.


5. Be sure that for a galvanic cell, the cell potential is greater than 0.


6. In cell notation, be sure to write anode, anode compartment, salt bridge, cathode com-
   partment, cathode in this specific order.


7. When using a multiplier to equalize electron loss and gain in reduction half-cell poten-
   tials, do not use the multiplier on the voltage of the half-cell.