Roles of Energy and Entropy in Phase Equilibria

consider the enthalpy of the system. Since we have an ideal gas, there is no intermolecular potential energy in the gas phase, just molecular kinetic energy, which is 

Now consider a much lower temperature. Since entropy is multiplied by T, the effect of randomness (entropy) cannot compete with the minimization of enthalpy in this case, the equilibrium state (minimum g) occurs when the system exists as a pure solid with no a in vapor. Hence all the vapor crystallizes. Conversely, at very high temperatures, the effect of entropy dominates and only vapor exists. This behavior is consistent with our experience. Solids exist at low temperature and as the temperature is increased, they sublime (or melt). 

The second law states that entropy goes to a maximum yet if our system were at maximum entropy, the solid phase would not exist. How do you resolve this paradox [Hint How is the temperature being kept constant ] 

EXAMPLE 6.1 We have seen that the Gibbs energy determines whether a process can occur spontaneously. 

ROI0 of Gibbs concept can be applied to understand aspects of biological systems. Use the Gibbs energy 

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