(82) Gibbs Free Energy

Introduction to Gibbs Free Energy

• Mr. Andersen discusses the challenges students face in understanding Gibbs Free Energy.

• Quote from Willard Gibbs highlights the complexity of entropy and thermodynamics.

• Aim: Simplify the concept of Gibbs Free Energy (Delta G) and its significance in biology.

Understanding Free Energy

• Free Energy is often misinterpreted; better understood as "available energy."

• Delta G represents energy available for work, not just an abstract concept.

• The confusion often arises from the equation containing terms like enthalpy (H) and entropy (S).

Spontaneous Reactions

• Spontaneous reactions require an initial push and then occur without further energy input.

• These reactions typically release energy and increase surrounding energy levels.

• Examples of spontaneous reactions:

  • Rolling ball down a slide

  • Diffusion of particles

  • Cherry bomb explosion

Example 1: Ball Rolling Down a Slide

• A ball at the top of a slide represents potential energy at its maximum.

• As the ball rolls down, total energy of the system (enthalpy) decreases:

  • High potential energy at top converts to lower energy at the bottom (H decreases).

  • The reaction is spontaneous; energy is released.

Example 2: Diffusion

• Molecules within a container spread out when a wall is removed, illustrating diffusion.

• The entropy (S), or disorder of the system, increases:

  • Original order (molecules on one side) shifts to a more disordered state (molecules spread out).

  • Entropy increases during diffusion, indicating a spontaneous reaction.

Example 3: Cherry Bomb

• A cherry bomb does not explode initially due to low temperature.

• Adding heat (increasing temperature) makes the reaction more spontaneous:

  • Higher temperature promotes molecular motion, increasing likelihood of explosion.

Applying Gibbs Free Energy Concept

• Relationship between enthalpy (H), entropy (S), and temperature on Gibbs Free Energy:

• If Delta G (ΔG) decreases, the spontaneity of a reaction increases:

  • Spontaneous reactions have ΔG < 0; defined as exergonic reactions (energy-releasing).

  • Non-spontaneous reactions have ΔG > 0; termed endergonic reactions (energy-storing).

Summary of Gibbs Free Energy Calculations

• A decrease in H (enthalpy) leads to a decrease in ΔG.

• An increase in S (entropy) also contributes to a decrease in ΔG.

• Increasing temperature further decreases ΔG to favor spontaneity.

Significance in Biological Processes

Cellular Respiration

• A prime example of a spontaneous reaction (exergonic) in biology:

  • Glucose energy is released by converting it to carbon dioxide and water.

  • ΔG for cellular respiration is -686 kcal/mol (exergonic reaction).

• Energy is released during the breakdown of glucose: spontaneous reaction with ΔG less than zero.

• Activation energy is necessary to initiate the reaction.

Photosynthesis

• Opposite process to cellular respiration; an endergonic reaction:

  • Requires sunlight, carbon dioxide, and water.

  • Energy from sunlight is stored in glucose, leading to a positive ΔG.

• Energy transformations illustrate the relationship between glucose and ATP.

ATP as Energy Currency

• ATP (adenosine triphosphate) serves as the primary energy currency in cells:

  • ATP breaks down into ADP (adenosine diphosphate), releasing energy (ΔG < 0).

  • ATP can be regenerated from ADP, requiring energy input (ΔG > 0).

• Energy derived from respiration is converted into ATP, which powers cellular activity.

Conclusion

• Life depends on energy transformations through Gibbs Free Energy:

  • Sun's energy is captured during photosynthesis (endothermic process), energy is stored in glucose.

  • During cellular respiration, stored energy is released for biological functions (exothermic process).

  • The overall transformation results in increased disorder and energy distribution in the system, illustrating the concept of Gibbs Free Energy.