Driving Forces
         Driving Forces are those factors which are responsible for causing a reaction to proceed on to completion. All chemical processes need to have some reason for taking place. Those reasons are the Driving Forces. There are two factors which seem to drive a reaction, causing it to occur.

- All systems strive to release energy: In other words, all chemical processes would prefer to undergo an exothermic process.
- All systems strive to increase entropy: in other words, all chemical precesses would prefer to undergo a change that leads to a state of greater disorder.

        Ideally, chemical processes will accomplish both of these ideas. In reality, many systems are only able to achieve one of them. Under those conditions, the resulting reaction will be dependent upon the temperature of the system.

Enthalpy
        Enthalpy is another term that is basically the same as energy. It refers to the energy content of a system. Symbolized with H, it is most frequently found expressed as H. Chemists find it more useful to examine the change in Enthalpy than to know the Enthalpy. The change in the Enthalpy of a system is usually measured as the difference between the final enthalpy and the initial enthalpy of a process.

        Based on that idea, it is clear that if the enthalpy at the end of a reaction is less than what was initially present, then the value of the enthalpy change is a negative number. Because of this, exothermic reactions will always be identified as having negative H values.

Entropy
        Unlike enthalpy, absolute values of entropy can be measured. Entropy involves thermal motion. Absolute zero, or 0 K, is the temperature at which all motion in solids ceases and the entropy is considered to be zero. Changes in entropy can be measured in comparison with this zero baseline, and chemistry handbooks contain tables of entropy values for elements and compounds. Entropy has units of J/K.

Free energy
         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 to two forces relate to each other. Free Energy accomplishes that factor.

        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 Free Energy. As with the other terms, enthalpy and entropy, the key information is whether the change in Free Energy is positive or negative. Studies have shown that if the 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 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.

  How does energy affect change?

  What are your energy needs?