The best parameter for determining the spontaneity of a process depends on the conditions under which the process occurs.

If the process occurs at constant temperature and pressure, the change in Gibbs free energy (ΔG) is the best parameter for determining spontaneity. A negative ΔG indicates that the process is spontaneous under these conditions, while a positive ΔG indicates that the process is non-spontaneous.

If the process occurs at constant temperature and variable pressure, the standard Gibbs free energy change (ΔG°) is the best parameter for determining spontaneity. A negative ΔG° indicates that the process is spontaneous under standard conditions, while a positive ΔG° indicates that the process is non-spontaneous.

If the process occurs at variable temperature and pressure, the change in entropy (ΔS) is the best parameter for determining spontaneity. A positive ΔS indicates that the process is spontaneous, while a negative ΔS indicates that the process is non-spontaneous. However, the value of ΔS alone cannot determine the spontaneity of a process, as the sign of ΔG also depends on the enthalpy change (ΔH) of the process. Therefore, in such cases, the sign of ΔG should be considered along with the value of ΔS.

In summary, the best parameter for determining spontaneity depends on the conditions under which the process occurs. ΔG or ΔG° is the best parameter for processes occurring at constant temperature and pressure or variable pressure, while ΔS is the best parameter for processes occurring at variable temperature and pressure.

If the process occurs at constant temperature and pressure, the change in Gibbs free energy (ΔG) is the best parameter for determining spontaneity. A negative ΔG indicates that the process is spontaneous under these conditions, while a positive ΔG indicates that the process is non-spontaneous.

If the process occurs at constant temperature and variable pressure, the standard Gibbs free energy change (ΔG°) is the best parameter for determining spontaneity. A negative ΔG° indicates that the process is spontaneous under standard conditions, while a positive ΔG° indicates that the process is non-spontaneous.

If the process occurs at variable temperature and pressure, the change in entropy (ΔS) is the best parameter for determining spontaneity. A positive ΔS indicates that the process is spontaneous, while a negative ΔS indicates that the process is non-spontaneous. However, the value of ΔS alone cannot determine the spontaneity of a process, as the sign of ΔG also depends on the enthalpy change (ΔH) of the process. Therefore, in such cases, the sign of ΔG should be considered along with the value of ΔS.

In summary, the best parameter for determining spontaneity depends on the conditions under which the process occurs. ΔG or ΔG° is the best parameter for processes occurring at constant temperature and pressure or variable pressure, while ΔS is the best parameter for processes occurring at variable temperature and pressure.