spontaneous reactions are those that occur without the input of matter or energy into the system and are characterized by an increase in entropy. In order to determine whether a given reaction is spontaneous, we must consider both the enthalpy and entropy changes that occur for the system.
A spontaneous reaction occurs when the product formation is favored at the conditions of the reaction, and the enthalpy and entropy changes satisfy certain criteria. Specifically, for a reaction to be spontaneous, ΔG must be negative. ΔG is the difference between the enthalpy of the products (ΔHfinal) and the enthalpy of the reactants (ΔHinitial), taking into account any changes in enthalpy during the reaction. Additionally, ΔS must be positive, indicating an increase in entropy after the reaction proceeds.
When both ΔH and ΔS are positive, the sign of ΔG is negative, and the reaction is spontaneous at all temperatures. This corresponds to both driving forces being in favor of product formation. On the other hand, when ΔH is positive and ΔS is negative, the sign of ΔG is positive, and the reaction cannot be spontaneous.
It's important to note that ΔG is temperature-dependent. At low temperatures, an exothermic reaction (ΔH < 0) with a positive ΔS may become spontaneous, while at high temperatures, an endothermic reaction (ΔH > 0) with a negative ΔS may become spontaneous.
An example provided in the document is the combustion of ethylene, which is an exothermic reaction with a positive ΔS. The ΔG value for this reaction at standard conditions is negative, indicating spontaneous燃烧反应。At low temperatures, the positive TΔS term can overcome the negative ΔH term, leading to a negative ΔG and thus spontaneous combustion.
In contrast, the reaction of nitrogen and oxygen to form nitric oxide is nonspontaneous at normal temperatures and pressures. It is highly endothermic with a slightly positive ΔS. While nitric oxide can be produced at high temperatures, this reaction is not spontaneous at ordinary temperatures.
Therefore, the spontaneous reactions are those that release free energy as they react. These include combustion of fossil fuels, photosynthesis in plants, and any other redox reactions that result in the production of stable products. The occurrence of spontaneous reactions is affected by the temperature of the system and the enthalpy and entropy changes that occur during the reaction.