Spontaneous and Nonspontaneous Processes An Introduction

Another fuel with great potential is hydrogen. Its most attractive features are that 

An interesting aspect of all physical and chemical changes that we have not yet emphasized is that they all occur naturally or spontaneously in one direction. For example, a shiny iron (Fe) pipe will eventually acquire a coating of rust (Fe20g). An equation representing the formation of rust is 

Once the rust begins to form, the formation of rust continues without any additional action or external influence. The reverse process, the conversion of the rust coating to give iron metal and oxygen, does not happen naturally or spontaneously under ambient conditions. Before we consider possible explanations for these observations, we should explore the scientific meaning of the terms spontaneous and nonspontaneous. 

A spontaneous process is a process that occurs in a system left to itself once started, no action from outside the system (no external action) is necessary to make the process continue. Conversely, a nonspontaneous process will not occur unless some external action is continuously applied. 

In the 1870s, Pierre Marcellin Berthelot and Julius Thomsen independently proposed that the direction of spontaneous change is the direction in which the enthalpy of a system decreases. In a system in which enthalpy decreases, heat is given off by the system to the surroundings. Berthelot and Thomsen developed the hypothesis that exothermic reactions should be spontaneous. Let us return to the example of fhe rusting of iron, and a few other examples, to test this hypothesis. 

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