In 1982, the International Union of Pure and Applied Chemistry (IUPAC) redefined the standard state pressure to be 1 bar. As a result, standard reduction potentials for half-cell reactions must be adjusted accordingly. In this problem, we illustrate the approach to adjusting these values. Since the hydrogen electrode reaction is assigned a value of 0 V, for both the old (1 atm) and new (1 bar) standards of pressure, the change in reduction potential for a half reaction should be calculated from the balanced equation that includes the following hydrogen electrode half-cell reaction.

For example, to calculate the change in reduction potential for the half-cell reaction NO₃‾(aq) + 4 H⁺(aq) + 3 e‾ → NO(g) + 2 H₂O(l), we focus on the overall equation obtained by combining these two half-reactions.

Since the cell potential is directly proportional to the Gibbs energy of reaction, Δ_rG = -zFE_cell, the change in cell potential that results from a change in pressure (from 1 atm to 1 bar) is obtained by first calculating the corresponding change in the Gibbs energy of reaction.

Eq. 13.34

Eq. 19.14

Eq. 19.15

Table 19.1

Transcribed Image Text:

1 2 H₂(g) → H+ (aq) + e¯