Exothermic processes spontaneous reactions

Exothermic process or reaction (A// < 0) with A5 > 0 —> Enthalpy and entropy favor spontaneity. 

The formation of rust on iron is an exothermic and spontaneous reaction. The reverse reaction is endothermic and nonspontaneous. You might conclude that all exothermic processes are spontaneous and all endothermic processes are nonspontaneous. But remember that ice melting at room temperature is a spontaneous, endothermic process. Something other than AH plays a role in determining whether a chemical process occurs spontaneously under a given set of conditions. That something is called entropy. 

In everyday life, we say the diffusion of a dye just happens but, as scientists, we say the process is spontaneous. In years past, it was thought that all spontaneous reactions were exothermic, with non-spontaneous reactions being endothermic. There are now many exceptions to this overly simplistic rule thus, we can confidently say that the sign of AH does not dictate whether the reaction is spontaneous or not, so we need a more sophisticated way of looking at the problem of spontaneity. 

A process occurring in a system is spontaneous if AG is negative, and it is not spontaneous if AG is positive, regardless of the sign of A5(system). The size of AG (which is negative) is maximized for those processes and reactions for which A 5 is positive and which are exothermic, with a negative value of AH. 

The reaction is homogeneously catalyzed by NO. Although the oxidation process is exothermic and spontaneous, the reaction is very slow without a catalyst. The mechanism of the reaction is as follows ... 

The equilibria of all reactions under such conditions are displaced toward exothermic processes, even those that lead to the formation of highly ordered systems. Furthermore, one should bear in mind the possibility of a kind of autoregulation of the predominant direction of such spontaneous reactions processes with a relatively small heat release (closer to resonance processes ) could proceed with higher probability and, as the complexity of the molecules formed increases, the probability of the dissipation of the evolved energy among the intramolecular degrees of freedom becomes more pronounced. Therefore it seems possible that at very low temperatures under the conditions of initiation by cosmic rays, even most complex molecules can be formed with a small, but still measurable, rate, and that slow exothermic low-temperature reactions can play some part in the processes of chemical and biological evolution. 

B All reactions that are both exothermic and cause an increase in entropy will be spontaneous, but the converse (choice A) is not true. Some spontaneous reactions are exothermic but decrease entropy and some are endothermic and increase entropy. Choice B is correct. The reverse reaction of a non-spontaneous reaction (choice C) will be spontaneous. Melting snow (choice D) requires heat. Therefore it is an endothermic process... 

Chemical reactions, like the metathesis reaction that produces lead (II) iodide, can occur spontaneously, just like physical processes. It was once believed that only exothermic processes occurred spontaneously however, it has been shown that many endothermic reactions can occur spontaneously as well. Another factor that must be considered when determining the spontaneity of a reaction is entropy. 

Do you notice a correlation Iron rusting and methane burning are exothermic and spontaneous. The reverse reactions are endothermic and nonspontaneous. Based upon reactions such as these, some nineteenth-century scientists concluded that all exothermic processes are spontaneous and all endothermic processes are nonspontaneous. However, you need not look far for evidence that this conclusion is incorrect. For example, you know that ice melts at room temperature. That s a spontaneous, endothermic process. 

In the mid-19 century, some thought that the sign of the enthalpy change (AH), the heat added or removed at constant pressure (qp), was the criterion for spontaneity. They thought that exothermic processes (AH < 0) were spontaneous and endothermic ones (AH > 0) were nonspontaneous. This hypothesis had a lot of support from observation after all, many spontaneous processes are exothermic. All combustion reactions, such as methane burning, are spontaneous and exothermic ... 

The history of chemistry records numerous attempts to deal with this problem of the tendency of a process to occur spontaneously. It was noticed very early that most spontaneous reactions are exothermic, and it was originally thought that the heat evolved was the "driving force of a reaction. This idea was put forward in particular... 

The fact that the system has lost mass indicates that the process is exothermic. All spontaneous nuclear reactions are exothermic. 

The low-temperature oxidation of coal is an exothermic process and (as noted earlier) the rate of reaction increases markedly with temperature. Thus, under the appropriate conditions, the oxidation sequence can (and often does) lead to the spontaneous ignition of the coal. 

The exothermicity of this reaction varies from A r = -58.1 kcal mol" for X = F to A/7r = -9.7 kcal moF for X = Br. The radical CH2O2 decomposes at low pressure spontaneously to yield predominantly CO2 [27]. It appears that CH2O2 formed chemically activated - as in ozonolysis - follows a different decomposition pattern than CH2O2 formed kinetically hot in the abstraction process (8). 

---