Exergonic

Chemical reactions are referred to in terms of whether the free enthalpy G of the components involved in the reaction decreases or increases, as exergonic or endergonic reactions. These terms are not to be confused with exothermic and endothermic ( see below and deferrals).

• 2.1 Thermodynamics of chemical reaction
• 2.2 Exothermic and endothermic reaction

Exergonic and endergonic reactions

Reactions that are thermodynamically favorable, are called exergonic or exergonic. A reaction is exergonic then, if in the course of the free enthalpy G of the reactants decreases, so if the reaction has a negative sign. Ender Gone reactions, however, are thermodynamically unfavorable, has a positive sign. Both exergonic and endergonic reactions occur in principle "voluntarily" from, provided that the reaction kinetics permits. In exergonic reactions, the equilibrium is just ahead on the side of the products than in the endergonic. It should be noted here that on closer inspection, each reaction is an equilibrium reaction.

An example of an endergonic process is the formation of a protein in an aqueous solution of amino acids. This reaction can only be realized if it is coupled to other exergonic processes, so that the sum of the sign is negative; in biological systems, this is usually achieved by the hydrolysis of ATP. Without this coupling the reaction would run off though, but only to a marginal extent, which would be totally inadequate for dealing with the biochemical task. Since the back reaction of the endergonic reaction is always exergonic (and vice versa ), proteins were supposed to spontaneously decay back into their amino acids. However, the rate of decomposition reaction is so small that it can be ignored under physiological conditions, that is, the peptide bonds are kinetically stable in this case. Here, then decides an argument from the reaction kinetics.

Systems strive always to the equilibrium state, because here the free enthalpy G assumes the minimum value. Has reached its equilibrium system, the concentrations of the reactants do not change anymore because G can be further reduced on any way, and it is valid.

Important distinctions

• G is the only hypothetical free total enthalpy (since the Gibbs free energy has no absolute zero, this is a hypothetical quantity )
• Is generally the change in Gibbs free energy in a process
• Describes the change in the free energy at complete reaction to occur

The complete passage of a reaction is hypothetical, since each reaction proceeds only until the chemical equilibrium. Nevertheless, it is an important parameter, because it allows the equilibrium constant can be calculated:

Determination of the free reaction enthalpy

Is given by the following relationship ( often called Gibbs -Helmholtz equation):

• = Free reaction
• = Enthalpy of reaction (ie, change in enthalpy of the substances in the reaction to occur )
• = Reaction entropy (ie the entropy change of the substances in the reaction to occur )

Can help with tabulated values ​​( Standardreaktionsentropien and Standardreaktionsenthalpien ) are calculated for standard conditions. This is called Free standard reaction. Conversion to other temperatures can be done using the van ' t Hoff equation.

Interpretation of the Gibbs - Helmholtz equation

Thermodynamics of chemical reaction

Driving force for the passage of a chemical reaction is the increase in entropy S in the universe (see second law of thermodynamics ).

If we consider a system that can not exchange heat with the surroundings ( adiabatic system ), this is the only condition that must be submitted to a spontaneously occurring process. The key is not present if the individual reaction exergonic or endergonic is, but that the system is not yet in equilibrium.

Exothermic and endothermic reaction

Allowing the system to heat exchange with the environment ( diabatisches system ), the entropy change must be considered in the environment in addition. This can be detected via the change in enthalpy of the system:

• , thermal energy is dissipated to the environment as a negative contribution when the reaction is exothermic, and in this manner the entropy of the environment increases,
• As a positive contribution, if the reaction is endothermic and the entropy in the environment decreases, because thermal energy is concentrated in the reaction vessel.

The system aims to conditions with the minimum free energy G, because this is the state of maximum entropy.