CONCEPT 8.2: The free-energy change of a reaction tells us whether or not the reaction
occurs spontaneously
• Biologists follow the energy and entropy changes during chemical reactions to determine
whether they require an input of energy or occur spontaneously
Free-Energy Change, G
• Gibbs free energy, G, can be simplified and referred to as free energy
• Free energy is the portion of a system’s energy that can do work when temperature and
pressure are uniform throughout the system, as in a living cell
• Change in free energy during a reaction is related to temperature and changes in enthalpy
and entropy
ΔG = ΔH – TΔS
– ΔG = change in free energy
– ΔH = change in enthalpy (total energy)
– ΔS = change in entropy
– T = Temperature in Kelvin (K)
• The ΔG for a process can be used to determine whether it is spontaneous or not
– ΔG is negative for all spontaneous processes
– ΔG is zero or positive for nonspontaneous processes
• Every spontaneous process decreases the system’s free energy
• Spontaneous processes can be harnessed by the cell to perform work
Free Energy, Stability, and Equilibrium
• ΔG represents the difference between the free energy of the final state and the free energy
of the initial state
ΔG = G final state – G initial state
• If a reaction has negative ΔG, the system loses free energy and becomes more stable
• Free energy can be thought of as a measure of a systems stability; unstable systems
(higher G) tend to become more stable (lower G)
– For example, a diver on a platform is less stable than when floating in the water
– A drop of concentrated dye is less stable than when it is dispersed randomly
through a liquid
– A glucose molecule is less stable than the simpler molecules into which it can be
split
, • Equilibrium, the point at which forward and reverse reactions occur at the same rate,
describes a state of maximum stability
• Systems never spontaneously move away from equilibrium
• A process is spontaneous and can perform work only when it is moving toward
equilibrium
Free Energy and Metabolism
• The concept of free energy can be applied to the chemistry of life’s processes
Exergonic and Endergonic Reactions in Metabolism
• Chemical reactions can be classified based on their free-energy changes
– An exergonic reaction (“energy outward”) proceeds with a net release of free
energy to the surroundings
– An endergonic reaction (“energy inward”) absorbs free energy from the
surroundings
occurs spontaneously
• Biologists follow the energy and entropy changes during chemical reactions to determine
whether they require an input of energy or occur spontaneously
Free-Energy Change, G
• Gibbs free energy, G, can be simplified and referred to as free energy
• Free energy is the portion of a system’s energy that can do work when temperature and
pressure are uniform throughout the system, as in a living cell
• Change in free energy during a reaction is related to temperature and changes in enthalpy
and entropy
ΔG = ΔH – TΔS
– ΔG = change in free energy
– ΔH = change in enthalpy (total energy)
– ΔS = change in entropy
– T = Temperature in Kelvin (K)
• The ΔG for a process can be used to determine whether it is spontaneous or not
– ΔG is negative for all spontaneous processes
– ΔG is zero or positive for nonspontaneous processes
• Every spontaneous process decreases the system’s free energy
• Spontaneous processes can be harnessed by the cell to perform work
Free Energy, Stability, and Equilibrium
• ΔG represents the difference between the free energy of the final state and the free energy
of the initial state
ΔG = G final state – G initial state
• If a reaction has negative ΔG, the system loses free energy and becomes more stable
• Free energy can be thought of as a measure of a systems stability; unstable systems
(higher G) tend to become more stable (lower G)
– For example, a diver on a platform is less stable than when floating in the water
– A drop of concentrated dye is less stable than when it is dispersed randomly
through a liquid
– A glucose molecule is less stable than the simpler molecules into which it can be
split
, • Equilibrium, the point at which forward and reverse reactions occur at the same rate,
describes a state of maximum stability
• Systems never spontaneously move away from equilibrium
• A process is spontaneous and can perform work only when it is moving toward
equilibrium
Free Energy and Metabolism
• The concept of free energy can be applied to the chemistry of life’s processes
Exergonic and Endergonic Reactions in Metabolism
• Chemical reactions can be classified based on their free-energy changes
– An exergonic reaction (“energy outward”) proceeds with a net release of free
energy to the surroundings
– An endergonic reaction (“energy inward”) absorbs free energy from the
surroundings
