We can rewrite Equation in terms of the new function: In fact this, will become our most favorite criteria for equilibrium at constant and. Note that there are several equivalent ways to write the Gibbs free energy in terms of the other thermodynamic functions that we have introduced. Some of the relations for the molar Gibbs free energy are:.
Note that if we had considered a phase change that takes place at constant volume:. Figure Illustration of how the entropy of the universe was calculated for a phase transformation occurring for a body that is kept at constant pressure and temperature.
At constant pressure, the heat of transformation is provided by the enthalpy change in the material undergoing a transformation. The same argument can be made with regards to the gaseous phase. Recalling that the average molar volume is. Because the temperature is above the triple point, the free energy follows a continuous path even though it is not everywhere differentiable from gas to liquid to solid. This, again, indicates, the unusual property of water that its solid phase is less dense than its liquid phase in the coexistence region.
There is a liquid curve on this plot that is completely disconnected from the gas-solid curve, suggesting that, below the triple point, the liquid state can exist metastably if at all. AT the triple point, the solid can transition into the liquid or gas phases depending on the value of the free energy. Near the critical temperature, we see the liquid-gas transition line, while the solid line is disconnected.
Above the critical temperature, the system exists as a supercritical fluid, which is shown on the lower line, and this line now shows derivative discontinuity. As discussed before there are many other forms of work possible, such as electrical work, magnetic work or elastic work. These they are commonly incorporated in the formalism of thermodynamics by adding other terms, e. After we address these reaction issues, we can look at phase change.
Add a comment. Active Oldest Votes. Improve this answer. Chet Miller Chet Miller In this case, how the liquid is reforming? If yes, so the system is unchanged, how there's a change in the entropy of the system? The liquid is not reforming. We are removing heat until all the liquid has changed to ice.
Look back at the initial and final states that we identified. Whenever you are trying to determine changes in thermodynamic functions, it is important to precisely identify the initial and final states that you are considering. The system is not unchanged. Only the free energy is unchanged.
This reaction as written, is entropically favorable , and enthalpically unfavorable; it is entropically driven. The change in Gibbs energy is equal to the maximum amount of work that a system can perform on the surroundings while undergoing a spontaneous change at constant temperature and pressure :.
We can rearrange this equation as follows:. Only if the process occurs infinitely slowly in a perfectly reversible manner will the entropy of the universe be unchanged. Because no real system is perfectly reversible, the entropy of the universe increases during all processes that produce energy.
One of the major challenges facing engineers is to maximize the efficiency of converting stored energy to useful work or converting one form of energy to another. Because enthalpy is one of the components of Gibbs free energy, we are consequently unable to measure absolute free energies; we can measure only changes in free energy. Is the reaction spontaneous as written? By definition, the standard free energy of formation of an element in its standard state is zero at One mole of Cl 2 gas at To make this determination, we need to evaluate the kinetics of the reaction.
Use the following data:. From the calculated value, determine whether the reaction is spontaneous as written. Use these data:.
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