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Related Concept Videos

Calculating the Equilibrium Constant02:46

Calculating the Equilibrium Constant

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The equilibrium constant for a reaction is calculated from the equilibrium concentrations (or pressures) of its reactants and products. If these concentrations are known, the calculation simply involves their substitution into the Kc expression.
For example, gaseous nitrogen dioxide forms dinitrogen tetroxide according to this equation:
38.3K
Conjugate Addition (1,4-Addition) vs Direct Addition (1,2-Addition)01:27

Conjugate Addition (1,4-Addition) vs Direct Addition (1,2-Addition)

4.4K
α,β-Unsaturated carbonyl compounds with two electrophilic sites, the carbonyl carbon, and the β carbon, are susceptible to nucleophilic attack via two modes: conjugate or 1,4-addition and direct or 1,2-addition.
Conjugate addition results in a thermodynamically stable product. The reaction retains the stronger C=O bond at the expense of the weaker C=C π bond. The process is slow as the β carbon is less electrophilic than the carbonyl carbon.
Direct addition products are...
4.4K
Calculating Standard Free Energy Changes02:49

Calculating Standard Free Energy Changes

25.7K
The free energy change for a reaction that occurs under the standard conditions of 1 bar pressure and at 298 K is called the standard free energy change. Since free energy is a state function, its value depends only on the conditions of the initial and final states of the system. A convenient and common approach to the calculation of free energy changes for physical and chemical reactions is by use of widely available compilations of standard state thermodynamic data. One method involves the...
25.7K
Titration Calculations: Strong Acid - Strong Base02:28

Titration Calculations: Strong Acid - Strong Base

34.1K
Calculating pH for Titration Solutions: Strong Acid/Strong Base
A titration is carried out for 25.00 mL of 0.100 M HCl (strong acid) with 0.100 M of a strong base NaOH. The pH at different volumes of added base solution can be calculated as follows:
(a) Titrant volume = 0 mL. The solution pH is due to the acid ionization of HCl. Because this is a strong acid, the ionization is complete and the hydronium ion molarity is 0.100 M. The pH of the solution is then:
34.1K
Calculating Equilibrium Concentrations02:05

Calculating Equilibrium Concentrations

53.8K
Being able to calculate equilibrium concentrations is essential to many areas of science and technology—for example, in the formulation and dosing of pharmaceutical products. After a drug is ingested or injected, it is typically involved in several chemical equilibria that affect its ultimate concentration in the body system of interest. Knowledge of the quantitative aspects of these equilibria is required to compute a dosage amount that will solicit the desired therapeutic effect.
A more...
53.8K
Calculating pH Changes in a Buffer Solution02:45

Calculating pH Changes in a Buffer Solution

58.9K
A buffer can prevent a sudden drop or increase in the pH of a solution after the addition of a strong acid or base up to its buffering capacity; however, such addition of a strong acid or base does result in the slight pH change of the solution. The small pH change can be calculated by determining the resulting change in the concentration of buffer components, i.e., a weak acid and its conjugate base or vice versa. The concentrations obtained using these stoichiometric calculations can be used...
58.9K

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Related Experiment Video

Updated: Feb 11, 2026

Isotopic Effect in Double Proton Transfer Process of Porphycene Investigated by Enhanced QM/MM Method
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On the Difference Between Additive and Subtractive QM/MM Calculations.

Lili Cao1, Ulf Ryde1

  • 1Department of Theoretical Chemistry, Chemical Centre, Lund University, Lund, Sweden.

Frontiers in Chemistry
|April 19, 2018
PubMed
Summary

The combined quantum mechanical/molecular mechanical (QM/MM) approach offers flexibility in studying biochemical reactions. This study clarifies additive and subtractive schemes, finding both viable with appropriate link-atom corrections.

Keywords:
QM/MMadditive QM/MMelectrostatic embeddinghaem oxygenasemechanical embeddingsubtractive QM/MMsulfite oxidase

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Area of Science:

  • Computational Chemistry
  • Biochemistry
  • Molecular Modeling

Background:

  • Combined quantum mechanical/molecular mechanical (QM/MM) methods are essential for studying reactions in biochemical macromolecules.
  • Implementation details, particularly the treatment of the QM/MM interface, vary significantly.

Purpose of the Study:

  • To clarify the similarities and differences between additive and subtractive QM/MM schemes.
  • To evaluate the impact of different link-atom corrections in the subtractive scheme.

Main Methods:

  • Comparative analysis of additive and subtractive QM/MM schemes.
  • Testing of three link-atom correction types: van der Waals, electrostatic, and bonded interactions.

Main Results:

  • Additive and subtractive QM/MM schemes are fundamentally similar and require identical parameters.
  • Electrostatic and bonded link-atom corrections can introduce geometric and energetic issues.
  • Van der Waals link-atom corrections yield results comparable to the additive scheme.

Conclusions:

  • Both additive and subtractive QM/MM schemes are recommended for studying biochemical reactions.
  • Careful consideration of link-atom corrections, especially for van der Waals interactions, is important for accurate results.