Final answer:
A spontaneous reaction is characterized by a negative change in Gibbs free energy (ΔG), which can occur under various conditions of enthalpy (ΔH) and entropy (ΔS) changes, implying that it's not necessarily instantaneous or always heat releasing.
Step-by-step explanation:
The true statement about spontaneous reactions is that they are indicated by a negative change in Gibbs free energy (ΔG). This is because a spontaneous reaction is one that releases free energy and moves to a more stable state. The change in Gibbs free energy (ΔG) can be calculated from the equation ΔG = ΔH - TΔS, where ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy.
Based on this equation:
- When ΔH is negative (exothermic reaction) and ΔS is positive (increase in disorder), ΔG will always be negative, and the reaction will be spontaneous at all temperatures.
- When ΔH is positive (endothermic reaction) and ΔS is negative (decrease in disorder), ΔG will always be positive, and the reaction can never be spontaneous.
- When ΔH is positive and ΔS is also positive, the reaction may be spontaneous at high temperatures.
- When ΔH is negative and ΔS is also negative, the reaction may be spontaneous at low temperatures.
Therefore, spontaneous reactions are not necessarily instantaneous, nor do they always release heat or have a positive value of ΔS.