BioPHYSICAL chemistry

Since the electron must come from somewhere, there must be an electron
donor, Y, producing an oxidized species, Y+:

Y →Y++e− (6.2)

The actual reaction is written by summing these two equations:

X +e−+Y→X−+Y++e− or X +Y →X−+Y+ (6.3)

Reduction/oxidation reactions are similar to acid/base reactions.
In both cases a charge, namely an electron or proton, is trans-
ferred. For acids and bases the parameter pKAdescribes the
affinity of a molecule for a proton. The equivalent parameter
for oxidation/reduction reactions is the midpoint potential; that
is, the potential at which half of a molecule is oxidized.

Electrochemical cells

Electrochemical reactions are often driven in electrochemical
cells, such as a battery. An electrochemical cell is a system
that has two electrodes (Figure 6.1). These electrodes are con-
nected electrically with an electrolyte – the solution where the
oxidation/reduction reaction occurs. As the reaction proceeds,
electrons are released and enter the anode. Electrons re-enter
the cell through the second electrode, termed the cathode. The
electrodes are either in the same solution or connected by a
solution that is termed a salt bridge. In a Galvanic cell, a spont-
aneous reaction generates electricity, whereas an electrolytic
cell is one that needs an external voltage to drive a non-
spontaneous reaction.
In order to calculate the energy of an electrochemical cell, a standard
must be chosen in order to address the pairing of ions. The standard
chosen may vary, but the most common one is the hydrogen electrode.
This reaction involves the transfer of two electrons to two protons in
solution, forming hydrogen gas:

2H+(aq) +2e−→H 2 (gas) (ΔG)°rec= 0 (6.4)

As a standard, the Gibbs energy of reaction,ΔG°reaction, is defined as zero. With
this definition, it is possible to determine relative reaction Gibbs energies
for any half reaction.
In electrical circuits, energies are usually described in terms of voltages
rather than Gibbs energies. The unit of voltage, namely a Volt (V), is a Joule
per Coulomb (J C−^1 ) and so voltage is a measure of the energy required

CHAPTER 6 REDOX REACTIONS AND BIOENERGETICS 115

Anode

Galvanic

cell

Oxidation Reduction



Cathode

Electrons

Figure 6.1

A galvanic cell with

electrons being

transferred from

solution to the anode

and traveling from

the anode to the

cathode, where the

reduction occurs.