Gibbs Free Energy What is free energy? Amount of available energy - - PowerPoint PPT Presentation

Gibbs Free Energy

What is free energy?

• Amount of available energy to do work
• Equation for Gibbs free energy:

∆G = ∆H - T∆S

∆G = ∆H - T∆S

• ∆G – change in Gibbs free energy
• ∆H – change in enthalpy (total energy of a

system)

• T – temperature in Kelvin (°C + 273)
• ∆S – change in entropy (measure of disorder
• f a system)

Spontaneous Reaction

• A “spontaneous” reaction means that the

reaction is thermodynamically capable of happening (reacting)

• It DOES NOT mean “quickly”

What does a change in Gibbs free energy (∆G ) mean?

• If ∆G is less than 0, then the reaction will

release energy (exergonic) – reactions occur spontaneously

• If ∆G is greater than 0, then the reaction

absorbs energy (endergonic) – reactions do not occur spontaneously

• If ∆G is equal to 0, then the system is at

equilibrium

Coupled Reactions

(Watch this 6 minute Bozeman Video on Coupled Reactions)

• One reaction is used to power another

reaction OR

• The transfer of energy from an exergonic

process to an endergonic process

Cellular Respiration

• Exergonic reaction
• ∆G = - 686 kcal/mol of energy
• ∆G < 0, the reaction releases energy when

glucose is metabolized

Photosynthesis

• Endergonic reaction
• ∆G = 686 kcal/mol of energy
• ∆G > 0, the reaction absorbs energy from the

sun and stores the energy in the chemical bonds of glucose

Synthesis of ATP

• ADP + Pi → ATP
• Endergonic
• ∆G = 7.3 kcal/mol of energy
• ∆G > 0, the reaction absorbs energy when ADP

is bonded to inorganic phosphate to produce ATP

Hydrolysis of ATP

• ATP → ADP + Pi
• Exergonic
• ∆G = - 7.3 kcal/mol of energy
• ∆G < 0, the reaction releases energy when the

terminal (last) inorganic phosphate is removed from ATP to produce ADP

ADP + Pi ATP