Fifty years in physical chemistry: homage to mentors, methods, and molecules

D Herschbach1

  • 1Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA. hbach@chemistry.harvard.edu

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Expressing Solution Concentration02:48

Expressing Solution Concentration

A solute is a component of a solution that is typically present at a much lower concentration than the solvent. Solute concentrations are often described with qualitative terms such as dilute (of relatively low concentration) and concentrated (of relatively high concentration).
Concentrations may be quantitatively assessed using a wide variety of measurement units, each convenient for particular applications. Molarity (M) is a useful concentration unit for many applications in chemistry.
Calculating Equilibrium Concentrations02:05

Calculating Equilibrium Concentrations

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...
The Small x Assumption02:20

The Small x Assumption

If a reaction has a small equilibrium constant, the equilibrium position favors the reactants. In such reactions, a negligible change in concentration may occur if the initial concentrations of reactants are high and the Kc value is small. In such circumstances, the equilibrium concentration is approximately equal to its initial concentration. This estimation can be used to simplify the equilibrium calculations by assuming that some equilibrium concentrations are equal to the initial...
Energy Diagrams, Transition States, and Intermediates02:13

Energy Diagrams, Transition States, and Intermediates

Free-energy diagrams, or reaction coordinate diagrams, are graphs showing the energy changes that occur during a chemical reaction. The reaction coordinate represented on the horizontal axis shows how far the reaction has progressed structurally. Positions along the x-axis close to the reactants have structures resembling the reactants, while positions close to the products resemble the products.  Peaks on the energy diagram represent stable structures with measurable lifetimes, while other...
Electrophiles02:28

Electrophiles

This lesson explains the definition, classification, and characteristic features of an electrophile that are key features of nucleophilic substitution reactions. An analysis of their charge and orbital picture helps understand their reactivity for seeking electrons. Electrophiles can be classified into positive and neutral species. Other classes include free radicals and polar functional groups.
While a positive electrophile, like a proton, reacts due to its vacant, low-energy 1s orbital, the...
What is Physical Chemistry?01:23

What is Physical Chemistry?

Physical chemistry is a branch of chemistry that studies the principles from physics underlying chemical reactions. It provides deep insights into the behaviors of molecules, the forces they experience, and their interactions and chemical reactions.The term "physical chemistry" was introduced by Mikhail Lomonosov in 1752. Since then, it has seen significant contributions from notable scientists such as Josiah Willard Gibbs, Wilhelm Ostwald, Jacobus Henricus van't Hoff, and Linus Pauling.Key...