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Equilibrium Constants Determination


Chemical equilibrium is the state which indicates that the concentrations of reactants and products no longer change in a reaction. Aiming to quantitatively analyze the equilibrium state of a reaction, scientists use the equilibrium constant, Keq, or KC, which is a function of the molar concentrations of reactants and products. The equilibrium constant determines the number of products relative to reactants at equilibrium. It has been shown that all chemical reactions that are at equilibrium can be described utilizing the equilibrium constant.

Consider the following general chemical equation:

general chemical equation

In this equation, letters a, b, c, d indicate the coefficients used to balance the chemical equation while the letters A, B, C, D represent the substances involved in a reaction.

The equilibrium constant for the reaction above can be written as:

equilibrium constant

Energy diagram for a reaction and the spin-unrestricted LC-BLYP/(aug-)cc-pVDZ level of computation was adopted with solvent effect for discussing the equilibrium between 2 and 5.Figure 1. Energy diagram for a reaction and the spin-unrestricted LC-BLYP/(aug-)cc-pVDZ level of computation was adopted with solvent effect for discussing the equilibrium between 2 and 5. (Kobayashi, M.; et al. 2020)

Our Services

At Alfa Chemistry, we apply density functional theory (DFT) calculation and Rice-Ramsperger-Kassel-Marcus (RRKM) theory to study the equilibrium process of chemical reactions, and determine the equilibrium constant. According to the well-designed calculation formulas, we provide the following equilibrium constant determination services:

  • DFT calculation

1. We have developed a DFT method to investigate the origin of the regioselectivities of chemical reactions, studying the equilibrium state of them.

2. Our teams perform DFT calculations at the MN15/def2-SVP level of theory using the SMD solvent model to calculate the equilibrium and rate constants.

3. The determination of the appropriate solvent to separate tautomers is of vital importance since the reaction rate and equilibrium constant of this reaction vary greatly in different solvents. Our team use the DFT level of B3LYP and the 6-311++G** basis set to identify the proper solvent for different reactant, and perform accurate equilibrium constants calculation.

4. The solvent effect is also taken into consideration in the calculation utilizing the polarizable continuum model PCM, in which the transition state of the tautomeric reaction is determined using the quadratic synchronous transit (QST2) method.

5. Combined with molecular dynamics simulation and DFT method which is based on the best functional/basis set combinations of CAM-B3LYP and 6–311++g(d,p) to analyze the reversible reaction process, including the calculation of equilibrium constants.

  • RRKM method

The RRKM-steady state approximation method is widely applied to calculate the equilibrium constants and branching ratios of the main products. The obtained overall equilibrium constant is in agreement with the available reported experimental data over the wide range of temperature from 300 to 3000 K. Our groups have developed and validated RRKM method to calculate the equilibrium constant of single molecule reaction.

Our Advantages

  • At Alfa Chemistry, the calculated reaction Gibbs energies and activation Gibbs energies is in line with the results of experimental observation, which indicate that our calculation is accurate and reliable.
  • In addition, we also simulate some molecular parameters such as solubility parameters and free volume fractions, which have a high correlation with equilibrium constants.

Equilibrium constant determination provides an effective way to optimize the chemical process. Our equilibrium constant determination services remarkably reduce the cost, promote further experiments, and enhance the understanding of chemical process for customers worldwide. Our personalized and all-around services will satisfy your innovative study demands. If you are interested in our services, please don't hesitate to contact us. We are glad to cooperate with you and witness your success!


  • Kobayashi, M.; et al. Theoretical and Experimental Studies on the Near‐Infrared Photoreaction Mechanism of a Silicon Phthalocyanine Photoimmunotherapy Dye: Photoinduced Hydrolysis by Radical Anion Generation. ChemPlusChem. 2020, 85(9).

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