17.6
Energia libera di Gibbs
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Q1: Why did J. Willard Gibbs introduce a new thermodynamic function?
Gibbs created this function to simplify determining spontaneity. The second law of thermodynamics requires measuring entropy changes for both the system and surroundings, which is difficult. Gibbs's function allows spontaneity to be determined using only the system's enthalpy and entropy, eliminating the need to measure surroundings' entropy.
Q2: How does Gibbs free energy relate to the second law of thermodynamics?
Gibbs free energy (ΔG) is mathematically derived from the second law of thermodynamics direction spontaneous changes. When ΔG is negative, the reaction is spontaneous and the universe's entropy increases. When ΔG is positive, the reaction is nonspontaneous. This relationship makes ΔG a reliable indicator of spontaneity based solely on system properties.
Q3: What does it mean when ΔG equals zero?
When ΔG = 0, the system is at equilibrium. The reactants and products are present in amounts where no net change occurs. At this point, the free energy of the system is neither increasing nor decreasing, indicating the reaction has reached its equilibrium state.
Q4: How is Gibbs free energy similar to mechanical potential energy?
Gibbs free energy is also called chemical potential because it functions like mechanical potential energy. Just as a ball rolls downhill to lower its potential energy, a chemical reaction proceeds spontaneously to lower its chemical potential. Systems naturally move toward states of lower free energy, driving reactions forward.
Q5: What equation defines Gibbs free energy at constant temperature and pressure?
At constant temperature and pressure, Gibbs free energy is defined as ΔG = ΔH − TΔS, where ΔH is enthalpy change, T is absolute temperature, and ΔS is entropy change of the system. This equation shows that spontaneity depends on both the enthalpy and entropy contributions, weighted by temperature.
Q6: Why is Gibbs free energy considered a state function?
Gibbs free energy is a state function because it depends only on the current state of the system, not on the path taken to reach that state. Since both enthalpy and entropy are state functions, and ΔG is defined mathematically from these properties, ΔG is also a state function.
Q7: How does temperature affect the spontaneity of a reaction according to Gibbs free energy?
Temperature directly influences ΔG through the equation ΔG = ΔH − TΔS. The temperature term multiplies entropy change, so increasing temperature amplifies the entropy contribution to spontaneity. This means some reactions that are nonspontaneous at low temperatures may become spontaneous at higher temperatures.