What is the difference between gibbs free energy and enthalpy

Free Energy is not energy : A much more serious difficulty with the Gibbs function, particularly in the context of chemistry, is that although G has the units of energy joules, or in its intensive form, J mol —1 , it lacks one of the most important attributes of energy in that it is not conserved.

Thus although the free energy always falls when a gas expands or a chemical reaction takes place spontaneously, there need be no compensating increase in energy anywhere else. Referring to G as an energy also reinforces the false but widespread notion that a fall in energy must accompany any change. But if we accept that energy is conserved, it is apparent that the only necessary condition for change whether the dropping of a weight, expansion of a gas, or a chemical reaction is the redistribution of energy.

Free Energy is not even "real" : G differs from the thermodynamic quantities H and S in another significant way: it has no physical reality as a property of matter, whereas H and S can be related to the quantity and distribution of energy in a collection of molecules e. The free energy is simply a useful construct that serves as a criterion for change and makes calculations easier.

References Chang, Raymond. Physical Chemistry for the Biosciences. Sansalito, CA: University Sciences, Atkins, Peter and de Paula, Julio. For example, water has a greater entropy than ice because energy is more spread out in water than in ice. Some reactions are spontaneous eg. A spontaneous process happens by itself without any energy added to the system apart from the activation energy. For reactions in solution, the equilibrium constant that comes from the calculation is based on concentrations K c.

Use the following standard-state free energy of formation data to calculate the acid-dissociation equilibrium constant K a at for formic acid:. HCO 2 aq HCO 2 - aq Click here to check your answer to Practice Problem Click here to see a solution to Practice Problem The Temperature Dependence of Equilibrium Constants. Equilibrium constants are not strictly constant because they change with temperature.

We are now ready to understand why. The standard-state free energy of reaction is a measure of how far the standard-state is from equilibrium. But the magnitude of G o depends on the temperature of the reaction.

As a result, the equilibrium constant must depend on the temperature of the reaction. A good example of this phenomenon is the reaction in which NO 2 dimerizes to form N 2 O 4.

This reaction is favored by enthalpy because it forms a new bond, which makes the system more stable. The reaction is not favored by entropy because it leads to a decrease in the disorder of the system. NO 2 is a brown gas and N 2 O 4 is colorless. We can therefore monitor the extent to which NO 2 dimerizes to form N 2 O 4 by examining the intensity of the brown color in a sealed tube of this gas. What should happen to the equilibrium between NO 2 and N 2 O 4 as the temperature is lowered?

For the sake of argument, let's assume that there is no significant change in either H o or S o as the system is cooled.

The contribution to the free energy of the reaction from the enthalpy term is therefore constant, but the contribution from the entropy term becomes smaller as the temperature is lowered. As the tube is cooled, and the entropy term becomes less important, the net effect is a shift in the equilibrium toward the right. The figure below shows what happens to the intensity of the brown color when a sealed tube containing NO 2 gas is immersed in liquid nitrogen.

There is a drastic decrease in the amount of NO 2 in the tube as it is cooled to o C. Use values of H o and S o for the following reaction at 25C to estimate the equilibrium constant for this reaction at the temperature of boiling water C , ice 0C , a dry ice-acetone bath C , and liquid nitrogen C :. The value of G for a reaction at any moment in time tells us two things. The sign of G tells us in what direction the reaction has to shift to reach equilibrium.

The magnitude of G tells us how far the reaction is from equilibrium at that moment. The potential of an electrochemical cell is a measure of how far an oxidation-reduction reaction is from equilibrium. The Nernst equation describes the relationship between the cell potential at any moment in time and the standard-state cell potential.

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Free Energy vs Enthalpy. Free energy gives the total energy available to perform thermodynamic work. Enthalpy gives the total energy of a system that can be converted to heat. Free energy gives the energy that can be converted to mechanical work of the system. Enthalpy gives the energy that can be converted to non-mechanical work of the system.