| An Exothermic Process is any chemical process in which there is a loss of energy. Systems prefer to lose energy. Therefore, systems prefer to undergo exothermic processes. This is a driving force for all chemical processes and can serve as a reason for a chemical process to take place. When a process is exothermic, it will have a change in enthalpy that is negative. |  |
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Gibbs Free Energy is a mathematical expression that combines the two driving forces into a single expression. Frequently, the driving forces do not work together to produce a reaction, or inhibit a reaction. Many times reactions will fulfill one driving force and fail the other. At that point it becomes useful to know how the two forces relate to each other. Gibbs Free Energy accomplishes that factor. The Gibbs Free Energy equation is:  The T term represent the temperature of the process expressed in Kelvin. By inserting information into the equation for enthalpy and entropy along with the value of T, it is possible to produce a value for the change in Gibbs Free Energy. As with the other terms, enthalpy and entropy, the key information is determined by whether the change in Gibbs Free Energy is positive or negative. Studies have shown that if the Gibbs Free Energy term is negative, then the reaction will be spontaneous as written. That means that the driving forces have been fulfilled and the reaction will work. If the term is positive, then the reaction will not work as written. On occasion, if one driving force is fulfilled and the other is not fulfilled, it becomes impossible to determine if the Gibbs Free Energy term is positive or negative. Under those conditions, the temperature of the process becomes an issue. At that time, a reaction may not work at one temperature, but it will work at another temperature. Such processes are referred to as temperature dependent reactions. |
| Hess's Law says that the net change in enthalpy of a reaction will be the sum of the enthalpy changes that occur along the way. Many reactions contain several steps. If so, then each step will have its own enthalpy change. |
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| If all of the changes are added together, then the sum of that addition will be the net change for the system. In other words, ultimately, the only thing that matters is what the net result of a reaction is. Hess's Law is frequently stated as |
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Hess's Law says that the net change in enthalpy of a reaction will be the sum of the enthalpy changes that occur along the way. Many reactions contain several steps. If so, then each step will have its own enthalpy change. If all of the changes are added together, then the sum of that addition will be the net change for the system. In other words, ultimately, the only thing that matters is what the net result of a reaction is. Hess's Law is frequently stated as  |
| A Spontaneous Reaction is a process in which at least one of the driving force requirements has been fulfilled. In such a system, a reason exists for the process to proceed on to completion. Some of these process will require a specific temperature in order for the process to occur. Other processes will occur at any temperature. In any event, the reasons exist for a change to take place. The reaction may not happen automatically. It may be necessary to input energy, or use some other mechanism to get the reaction started, but it will ultimately occur. |
| A Temperature Dependent Process is any process that will not work at one set of temperature values, but will occur at a different set of temperature values. This time of reaction exists when one of the driving forces has been fulfilled and the other has not. Under those conditions, there will be a specific temperature at which the process changes from having a positive Gibbs Free Energy to having a negative Gibbs Free Energy. At that temperature the process changes from being non-spontaneous to being spontaneous or vice-versa. An example of a temperature dependent process would be any melting process. Ice does not melt over a certain range of temperature values, but when heated to a certain point, the ice will start to melt. |
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kdrews@bcpl.net |  |