Final answer:
A chemical reaction has a negative ΔG when it is spontaneous, characterized by an exothermic reaction with increased entropy or an endothermic reaction with increased entropy at higher temperatures. Positive ΔG indicates non-spontaneity and is characterized by an endothermic reaction with decreased entropy or exothermic reaction with decreased entropy not significant for spontaneity at lower temperatures.
Step-by-step explanation:
In understanding whether a chemical reaction has a negative or positive ΔG, it is crucial to know the signs of both the entropy (ΔS) and enthalpy (ΔH) changes involved in the reaction. A negative change in Gibbs free energy (ΔG) indicates a spontaneous reaction under constant temperature and pressure. This occurs when the reaction is exothermic (ΔH < 0) and/or the entropy increases (ΔS > 0). However, if the reaction is endothermic (ΔH > 0) but also has an increase in entropy (ΔS > 0), the sign of ΔG can be negative or positive depending on the magnitude of temperature (T) since the ΔG = ΔH - TΔS. If the entropy decrease (ΔS < 0) is greater than the enthalpy decrease (ΔH < 0), ΔG can be positive, even when the reaction is exothermic, indicating non-spontaneity at a given temperature.
So, to sort items from the answer bank: a negative ΔG would be characterized by either an exothermic reaction with a decrease in entropy (ΔH < 0 and ΔS < 0) or an endothermic reaction with an increase in entropy (ΔH > 0 and ΔS > 0, especially at higher temperatures). For a positive ΔG, the reaction would either be endothermic with a decrease in entropy (ΔH > 0 and ΔS < 0) or exothermic with a decrease in entropy that is not significant enough to drive the reaction spontaneously (ΔH < 0 and ΔS < 0, especially at lower temperatures).