5 Steps to a 5 AP Chemistry

Cell Potential

In the discussion of the Daniell cell we indicated that this cell produces 1.10 volts.

This voltage is really the difference in potential between the two half-cells. There are

half-cell potentials associated with all half-cells A list of all possible combinations of

half-cells would be tremendously long. Therefore, a way of combining desired half-cells has

been developed. The cell potential (really the half-cell potentials) depends on concentration

and temperature, but initially we’ll simply look at the half-cell potentials at the standard

temperature of 298 K (25°C) and all components in their standard states (1 M concentra-

tion of all solutions, 1 atmosphere pressure for any gases, and pure solid electrodes).

All the half-cell potentials are tabulated as the reduction potentials, that is, the potentials

associated with the reduction reaction. The hydrogen half-reaction has been defined

as the standard and has been given a value of exactly 0.00 V. All the other half-reactions

have been measured relative to it, some positive and some negative. The table of standard

reduction potentials provided on the AP exam is shown in Table 16.1 and in the back of

this book.

Here are some things to be aware of in looking at this table:

 All reactions are shown in terms of the reduction reaction relative to the standard hydro-

gen electrode.

 The more positive the value of the voltage associated with the half-reaction (E°), the

more readily the reaction occurs.

 The strength of the oxidizing agent increases as the value becomes more positive, and the

strength of the reducing agent increases as the value becomes more negative.

This table of standard reduction potentials can be used to write the overall cell reaction and

to calculate the standard cell potential, E°the potential (voltage) associated with the cell

at standard conditions. There are a few things to remember when using these standard

reduction potentials to generate the cell reaction and cell potential:

1.The standard cell potential for a galvanic cell is a positive value, E°>0.

2.Because one half-reaction must involve oxidation, one of the half-reactions shown in the

table of reduction potentials must be reversed to indicate the oxidation. If the half-reaction

is reversed, the sign of the standard reduction potential must be reversed. However, this

is not necessary to calculate the standard cell potential.

3.Because oxidation occurs at the anode and reduction at the cathode, the standard cell

potential can be calculated from the standard reduction potentials of the two half-reactions

involved in the overall reaction by using the equation:

But remember, both E°cathodeand E°anodeare shown as reduction potentials, used

directly from the table without reversing.

Once the standard cell potential has been calculated, the reaction can be written by

reversing the half-reaction associated with the anode and adding the half-reactions together,

using appropriate multipliers if needed to ensure that the numbers of electrons lost and

gained are equal.

Suppose a galvanic cell was to be constructed utilizing the following two half-reactions

taken from Table 16.1:

Ni a q Ni s

aq Ag(s)

2 + →−

→

() ()

()

E

E

°= 0.25V

Ag+ °=0.80V

EE E°° °cell=−>cathode anode 0

Electrochemistry  245

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