A spontaneous reaction at 298 K (25°C) is a reaction that occurs without any external energy input, such as heat or light. To determine if a reaction is spontaneous, we look at the Gibbs Free Energy change (ΔG):

* ΔG < 0: The reaction is spontaneous (exergonic)

* ΔG > 0: The reaction is non-spontaneous (endergonic)

* ΔG = 0: The reaction is at equilibrium

The Gibbs Free Energy change is related to enthalpy change (ΔH) and entropy change (ΔS) by the equation:

ΔG = ΔH - TΔS

where:

* ΔH is the change in enthalpy (heat released or absorbed)

* T is the temperature in Kelvin

* ΔS is the change in entropy (disorder or randomness)

Therefore, a spontaneous reaction at 298 K can occur in the following scenarios:

1. Exothermic reaction (ΔH < 0) with an increase in entropy (ΔS > 0): This scenario always results in a negative ΔG, making the reaction spontaneous.

2. Exothermic reaction (ΔH < 0) with a small decrease in entropy (ΔS < 0): If the enthalpy change is significantly negative, it can overcome a small decrease in entropy, leading to a negative ΔG and a spontaneous reaction.

3. Endothermic reaction (ΔH > 0) with a large increase in entropy (ΔS > 0): If the entropy increase is significant enough to outweigh the positive enthalpy change, the reaction will have a negative ΔG and be spontaneous.

Examples of spontaneous reactions at 298 K:

* Combustion of fuels: Exothermic with a large increase in entropy.

* Dissolving of table salt in water: Endothermic with a large increase in entropy.

* Neutralization reaction of a strong acid with a strong base: Exothermic with a small increase in entropy.

Note: The spontaneity of a reaction can also be influenced by factors such as concentration, pressure, and the presence of catalysts.