📚 Gibbs Free Energy: The Basics
Gibbs Free Energy ($G$) is a thermodynamic potential that can be used to predict the spontaneity of a process at constant temperature and pressure. It combines enthalpy ($H$) and entropy ($S$) to determine if a reaction or process will occur spontaneously.
- 🌡️ The formula for Gibbs Free Energy is: $G = H - TS$, where $T$ is the temperature in Kelvin.
- 🔍 $\Delta G = \Delta H - T\Delta S$: This equation tells us the change in Gibbs Free Energy for a process.
- ✅ If $\Delta G < 0$: The process is spontaneous (favored).
- ❌ If $\Delta G > 0$: The process is non-spontaneous (requires energy input).
- ⚖️ If $\Delta G = 0$: The system is at equilibrium.
🧊 Phase Transitions Explained
Phase transitions involve changes in the physical state of a substance (e.g., solid, liquid, gas). Gibbs Free Energy helps determine the conditions under which these transitions occur.
- 💧Melting (Solid to Liquid): At the melting point, the Gibbs Free Energy of the solid and liquid phases are equal. Increasing the temperature favors the liquid phase because of the higher entropy.
- ♨️Boiling (Liquid to Gas): At the boiling point, the Gibbs Free Energy of the liquid and gas phases are equal. Increasing the temperature favors the gas phase due to its significantly higher entropy.
- ❄️Sublimation (Solid to Gas): Similar to melting and boiling, sublimation occurs when the Gibbs Free Energy of the solid and gas phases are equal.
- 📝 The temperature dependence of Gibbs Free Energy is crucial. As temperature increases, the $-TS$ term becomes more significant, favoring phases with higher entropy (typically gases).
🌡️ Temperature's Role in Phase Transitions
Temperature plays a critical role in determining the spontaneity of phase transitions. The Gibbs Free Energy equation explicitly includes temperature, highlighting its influence.
- 📈Increasing Temperature: Generally favors phases with higher entropy (e.g., gas over liquid or solid).
- 📉Decreasing Temperature: Generally favors phases with lower entropy (e.g., solid over liquid or gas).
- 📊 At specific temperatures (transition temperatures), the Gibbs Free Energy change for the transition is zero, indicating equilibrium between the phases.
🧮 Calculating Gibbs Free Energy Change
To determine whether a phase transition will occur spontaneously at a given temperature, calculate $\Delta G$ using the equation $\Delta G = \Delta H - T\Delta S$.
- 🧪 Determine the enthalpy change ($\Delta H$) and entropy change ($\Delta S$) for the transition. These values are often available in thermodynamic tables.
- 🔢 Convert the temperature to Kelvin (K).
- 💻 Plug the values into the equation and calculate $\Delta G$.
- ✅ If $\Delta G$ is negative, the transition is spontaneous at that temperature.
💡 Practical Example: Water
Consider the phase transition of water from liquid to gas (boiling). At 100°C (373.15 K) and 1 atm pressure, the Gibbs Free Energy change is zero, indicating equilibrium. Below 100°C, $\Delta G$ is positive, meaning boiling is non-spontaneous. Above 100°C, $\Delta G$ is negative, and boiling is spontaneous.
📝 Practice Quiz
Test your knowledge with these questions:
- ❓ What is Gibbs Free Energy, and why is it important?
- ❓ Explain how Gibbs Free Energy relates to the spontaneity of a process.
- ❓ How does temperature affect the Gibbs Free Energy of a system?
- ❓ Describe the relationship between Gibbs Free Energy and phase transitions.
- ❓ Explain why ice melts above 0°C using Gibbs Free Energy concepts.
- ❓ How can you calculate the Gibbs Free Energy change for a reaction or process?
- ❓ What does a negative, positive, and zero value of $\Delta G$ indicate?
