Driving Forces

  Following is a selection of concepts and terms that are associated with Driving Forces.

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.

Endothermic Process:

 An Endothermic Process is a process in which there is an increase in the energy of the system. Such a process will have a greater level of internal energy after the system has undergone a change. Endothermic processes are not the preferred types of changes. Endothermic reactions will fail the Enthalpy driving force.  

 

Energy Level Diagram:

 An Energy Level Diagram is a graph that measures the energy content of a system. Many energy level diagrams are a single vertical line. The bottom of the line corresponds to a system with zero energy. The farther up the line a system is located, the greater the energy content of the system. In other words, chemical systems will strive be located as far down the energy level diagram as possible. To do so, will achieve the desired condition of reduced energy content. Some energy level diagrams will also contain a horizontal axis. This usually corresponds to a time variable. Such a graph would monitor the change in the energy content of a system over a period of time.  

   Many chemical processes consist of multiple steps. Such processes can have complex energy level diagrams. The graphs will show the gain and loss that is associated with each change of energy that takes place during the net process.

 

Enthalpy:

Enthalpy is traditionally defined as the heat content of a system at constant pressure. However, for most common systems, and purposes of simplification, Enthalpy is approximately equal to energy of a system. While it is not true to do so, at higher levels of science, at this point it is safe to treat Enthalpy and energy as if they are the same thing. In other words, enthalpy refers to the energy content of a system. Symbolized with H, it is most frequently found expressed as change in 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. That is,

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:

 Entropy is the term used to refer to the disorder, or disorganization, in a system. It is symbolized as S, but is more frequently seen as a change in S. Chemists find it more useful to know the change in the entropy of a system than to know the individual entropy values. The change in entropy is calculated from the equation

Based on that equation, it should be clear that if the final entropy is greater than the initial entropy, then the system will have a positive change in entropy. Since chemical processes strive to increase entropy, the preferred values for the change in S will be positive.

 Questions and comments should be sent to :
   kdrews@bcpl.net  

Updated September 1, 2000