What is the significance of Gibbs free energy?

Gibbs free energy?

• What is the physical significance of gibbs freeenergy

• Answer:

  Some reactions are spontaneous because they give off energy in the form of heat (deltaH < 0). Others are spontaneous because they lead to an increase in the disorder of the system (deltaS > 0). Calculations of deltaH and deltaS can be used to probe the driving force behind a particular reaction. What happens when one of the potential driving forces behind a chemical reaction is favorable and the other is not? We can answer this question by defining a new quantity known as the Gibbs free energy (G) of the system, which reflects the balance between these forces. The Gibbs free energy of a system at any moment in time is defined as the enthalpy of the system minus the product of the temperature times the entropy of the system. G = H - TS The Gibbs free energy of the system is a state function because it is defined in terms of thermodynamic properties that are state functions. The change in the Gibbs free energy of the system that occurs during a reaction is therefore equal to the change in the enthalpy of the system minus the change in the product of the temperature times the entropy of the system. deltaG = deltaH - delta(TS) If the reaction is run at constant temperature, this equation can be written as follows. deltaG = deltaH - TdeltaS The change in the free energy of a system that occurs during a reaction can be measured under any set of conditions. If the data are collected under standard-state conditions, the result is the standard-state free energy of reaction (deltaGo). deltaGo = deltaHo - TdeltaSo The beauty of the equation defining the free energy of a system is its ability to determine the relative importance of the enthalpy and entropy terms as driving forces behind a particular reaction. The change in the free energy of the system that occurs during a reaction measures the balance between the two driving forces that determine whether a reaction is spontaneous. As we have seen, the enthalpy and entropy terms have different sign conventions. The entropy term is therefore subtracted from the enthalpy term when calculating deltaGo for a reaction. Because of the way the free energy of the system is defined, deltaGo is negative for any reaction for which deltaHo is negative and deltaSo is positive. deltaGo is therefore negative for any reaction that is favored by both the enthalpy and entropy terms. We can therefore conclude that any reaction for which deltaGo is negative should be favorable, or spontaneous. Favorable, or spontaneous reactions: deltaGo < 0 Conversely, deltaGo is positive for any reaction for which deltaHo is positive and deltaSo is negative. Any reaction for which deltaGo is positive is therefore unfavorable. Unfavorable, or non-spontaneous reactions: deltaGo > 0 Reactions are classified as either exothermic (deltaH < 0) or endothermic (deltaH > 0) on the basis of whether they give off or absorb heat. Reactions can also be classified as exergonic (deltaG < 0) or endergonic (deltaG > 0) on the basis of whether the free energy of the system decreases or increases during the reaction. When a reaction is favored by both enthalpy (deltaHo < 0) and entropy (deltaSo > 0), there is no need to calculate the value of deltaGo to decide whether the reaction should proceed. The same can be said for reactions favored by neither enthalpy (deltaHo > 0) nor entropy (deltaSo < 0). Free energy calculations become important for reactions favored by only one of these factors.