Gibbs Free Energy and Electrical Work (9.6-9.8)

1
Gibbs Free Energy and Electrical Work (9.6-9.8)

• When reversible nonexpansion work is being
  performed (e.g., moving charges through an
  electric field) the Gibbs free energy takes on a
  new meaning
• Spontaneity is defined in terms of work
• Gibbs free energy difference is related to the
  potential difference (E) and the number of
  charged species transported in the chemical
  process
• The electric potential is now a measure of the
  extent of the reaction
• Potential is dependent on the composition of the
  mixture through the reaction quotient (QP)
• We can define a standard potential (E0) in such a
  way that it is related to the standard Gibbs free
  energy and the equilibrium constant
• The Nernst equation relates the potential of a
  system to the standard potential and the reaction
  quotient

2
Electrochemical Cells and Potentials (9.8-9.9)

• Electrochemical cells can be used to extract
  electrons through chemical reactions
• An electrical potential is created when two
  separate redox reactions come into contact
• Oxidation occurs at the anode, reduction occurs
  at the cathode
• The overall potential is dependent of the
  oxidation and reduction potentials in each
  half-cell
• Half-reaction standard potentials are typically
  expressed as reduction potentials
• The overall standard potential is the sum of the
  oxidation and reduction potentials
• Standard potentials can be used instead of Gibbs
  energy to understand reactivity
• Positive potentials mean that the process is
  spontaneous
• Oxidation potential is opposite in sign to the
  reduction potential
• When determining overall cell potential, the half
  potentials are NOT multiplied by the integers
  when balancing equations

3
Biochemical Standard State (9.12)

• When defining the reaction quotient (or
  equilibrium constant), there is a subtle rule one
  uses that is not convenient for biochemical
  applications
• Concentrations are made unitless by dividing by a
  standard concentration (C0) of 1 M
• For proton concentration, the standard
  concentration is more conveniently described in
  terms of neutral solutions
• Standard proton concentration is related to
  solutions of pH 7
• This modification to a biochemical standard state
  means other thermodynamic quantities will be
  defined differently
• Standard Gibbs energy and electrochemical
  potentials are different in the biochemical
  standard state (prime symbol is used to signify
  biochemical standard state)

4
Electrochemical Cell
5
Electrochemical Potentials in the Biochemical
Standard State