Thermochemistry Synthlet

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On the Colorado State University Department Chemical & Biological Engineering there are a set of online software tools for calculating various thermodynamic and kinetic properties of ideal gas mixtures.

One of these is a Chemical Equilibrium Calculator that used a much deeper theoretical model compared to the one used on in the Chemogenesis web book. For example, their calculation of the Haber ammonia synthesis:

 

On the Colorado State University Department Chemical & Biological Engineering there are a set of online software tools for calculating thermodynamic and kinetic properties of ideal gas mixtures., including a Chemical Equilibrium Calculator that operates at a much deeper theoretical model level that the used on this website. For example, their calculation of the Haber ammonia synthesis:

The Gibbs function is:          

DG
=
DH
T
DS
General form of the Gibbs' Eqn
DrG
=
DrH
T298
DrS
Reaction, standard pressure & temperature
DrG°
=
DrH
T
DrS
Standard pressure, non-standard termperature
  • D (delta) means "change in".
  • DH is the change in enthalpy or heat equivalent.
  • T is the thermodynamic temperature, the temperature in Kelvin.
  • DS is the change in entropy or "dispersion of energy", where gases are more dispersed and have greater entropy than liquids, etc.
  • Reactions can be exothermic, –DH, or endothermic, +DH.
  • Reactions [the local system] can decrease in entropy, –DS, or they can increase in entropy, +DS.
  • For a reaction to occur the Gibbs free energy must be negative: DG < 0.00
  • DH, DS & DG refer to standard pressure (1.0 atm) and temperature (298K).
  • DH°, D & D refer to 1.0 atm pressure, but non-standard temperatures.

Due to the form of the Gibbs function there are four possibilities. Reactions can have:

+ +
Positive Enthalpy
&
Positive Entropy
+ –
Positive Enthalpy
&
Negative Entropy
– +
Negative Enthalpy
&
Negative Entropy
– –
Negative Enthalpy
&
Positive Entropy

 

Sub select:

 

 

  • The reaction is endothermic: DH°rxn is positive (Drxn > 0.00).
  • Entropy increases: DS°rxn > 0.00 (positive), and the reaction system becomes more dispersed (disordered).

The temperature at which the reaction is at equilibrium, ie where DG°rxn = 0.00, is given by T = DH°rxn/DS°rxn = 0K (0°C). Above this temperature the reactions's equilibrium position moves to the right. Below this temperature the reactions's equilibrium position moves to the left.

At low temperature the magnitude of the reaction enthalpy term, DH°rxn, is greater than the magnitude of the temperature times entropy term, |DH°rxn| > |T x DS°rxn|, and the Gibbs free energy, DG°rxn, is positive.

Low Temp: Equilibrium position to the left, but with a slow rate of reaction.

At high temperature the rate of reaction will be higher, equilibrium will be reached quickly, and the temperature times entropy term, T x DS°rxn, is greater in magnitude than the enthalpy term, |T x DS°rxn| > |DH°rxn|. The Gibbs free energy, DGrxn, is negative and the reaction will be 'spontaneous', where the term spontaneous means the reaction is energetically down-hill and feasible. However, an increase in temperature may be necessary to overcome the reaction's activation energy and/or make the reaction occur in a reasonable time.

High Temp: Equilibrium position lies to the right and with a fast rate of reaction.

At 298K (25°C) the equilibrium constant, Keq =

+ –

  • The reaction is endothermic: DH°rxn is positive (DH°rxn > 0.00).
  • Entropy decreases: DS°rxn < 0.00 (negative), and the system becomes less dispersed (more ordered).

It follows that the Gibbs free energy, DG°rxn, is positive at all temperatures. The direct forward reaction, as shown from left to right, cannot proceed thermally under any circumstances.

Thus, the reverse (right-to-left) reaction is thermodynamically spontaneous at all temperatures, where the term spontaneous means the reaction is energetically down-hill and feasible. However, an increase in temperature may be necessary to overcome the reaction's activation energy and/or make the reaction occur in a reasonable time.

At 298K (25°C) the equilibrium constant, Keq = .

[[Note that there must be a photochemical or other multi-step synthetic route available to make the material shown as the product, or the substance could not exist!]] (Is a single product a special case?)

– +

  • The reaction is exothermic: DH°rxn is negative (Drxn < 0.00).
  • Entropy increases: Drxn > 0.00, so the local system becomes more dispersed (disordered) as the reaction proceeds.

The Gibbs free energy: DG°rxn, is negative at all temperatures and so the reaction is spontaneous at all temperatures, where the term spontaneous means the reaction is energetically down-hill and feasible. However, an increase in temperature may be necessary to overcome the reaction's activation energy and/or make the reaction occur in a reasonable time.

At 298K (25°C) the equilibrium constant, Keq = .

– –

  • The reaction is exothermic: DH°rxn is negative (Drxn < 0.00).
  • Entropy decreases: DS°rxn < 0.00 (negative), and the reaction system becomes less dispersed (more ordered).

The temperature at which the reaction is at equilibrium, ie where DG°rxn = 0.00, is given by T = DH°rxn/DS°rxn = K (°C). Below this temperature the reactions's equilibrium position moves to the right, and above this temperature the reactions's equilibrium position moves to the left.

At low temperature the magnitude of the reaction enthalpy term, DH°rxn, is greater than the magnitude of the temperature times entropy term, |DH°rxn| > |T x DS°rxn|. The Gibbs free energy, DG°rxn, is negative and the reaction will be 'spontaneous', where the term spontaneous means the reaction is energetically down-hill and feasible. However, an increase in temperature may be necessary to overcome the reaction's activation energy and/or make the reaction occur in a reasonable time.

Low Temp: Equilibrium position to the right, but the rate of reaction may be low.

At high temperature the rate of reaction will be higher and equilibrium will be reached quickly, but the temperature times entropy term, T x DS°rxn, is greater in magnitude than the enthalpy term, |T x DS°rxn| > |DH°rxn|. The equilibrium position will lie to the left hand side, Keq < 1.00, and the formation of reactants will be favoured over products.

High Temp: The rate of reaction is fast, but the equilibrium lies to the left.

At 298K (25°C) the equilibrium constant, Keq = .

 


Why Reactions Occur
Timeline of Structural Theory

© Mark R. Leach 1999-


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