Question

If a reaction has positive enthalpy and positive entropy, what will occur? What is the value of Gibbs Free Energy in this situation? What is the exception?

Answer 1

For a reaction with positive enthalpy and entropy, the spontaneity depends on temperature. At low temperatures, the reaction may not be spontaneous, but it can become spontaneous at higher temperatures when the entropy term outweighs the enthalpy term.

Step-by-step explanation:

When a reaction has a positive enthalpy (ΔH) and positive entropy (ΔS), it implies that the process is endothermic and is accompanied by an increase in entropy. The spontaneity of the reaction depends on the temperature and can be understood through the Gibbs free energy (ΔG) equation: ΔG = ΔH - TΔS, where T is the absolute temperature in Kelvin.

If the temperature is low, the enthalpy term may dominate, possibly resulting in a positive Gibbs free energy value (ΔG > 0), and the reaction would be non-spontaneous. However, at higher temperatures, the entropy term (TΔS) becomes more significant. If this term outweighs the positive enthalpy, ΔG can become negative, making the reaction spontaneous. This is the exception to the general rule that a reaction with a positive ΔG is non-spontaneous.

An example of this scenario is the melting of ice. Below 273 K (0ºC), ice melting is non-spontaneous (ΔG > 0), but above this temperature, the reaction becomes spontaneous (ΔG < 0) due to the increasingly dominant TΔS term.