Hoff, van

Broekhof van der Gen Red. Trav. Chim. Pays-Bas 1984, 103, 305 Brockhof van Elburg Hoff van der Gen Reel. Trav. Chim. Pays-Bas 1984,103, 317. 

The enthalpy changes due to dimerization are determined from the van t Hoff relation. For a dimerization reaction between species i and j... 

The measurement techniques most frequently used are derived from Raoult s and Van t Hoff s laws applied to cryometry, ebulliometry, osmometry, etc. They are not very accurate with errors on the order of ten per cent. Consequently, the molecular weight is often replaced by correlated properties. The mean average temperature or viscosity can thus replace molecular weight in methods derived from ndM. 

A connnon approach has been to measure the equilibrium constant, K, for these reactions as a fiinction of temperature with the use of a variable temperature high pressure ion source (see section (Bl.7.2)1. The ion concentrations are approximated by their abundance in the mass spectrum, while the neutral concentrations are known from the sample mlet pressure. A van t Hoff plot of In K versus /T should yield a straight Ime with slope equal to the reaction enthalpy (figure B1.7.11). Combining the PA with a value for basicityG at one temperature yields a value for A.S for the half-reaction involving addition of a proton to a species. While quadnipoles have been tire instruments of choice for many of these studies, other mass spectrometers can act as suitable detectors [19, 20]. 

Figure Bl.7.11. Van t HofF plot for equilibrium data obtained for die reaetion of isobutene with anuuonia in a high pressure ion soiiree (reprodueed from data in [19]).

Weber G 1995 van t Hoff revisited enthalpy of association of protein subunits J. Rhys. Chem. 99 1052-9... 

Naghibi H, Tamura A and Sturtevant J M 1995 Significant discrepancies between van t Hoff and calorimetric enthalpies Proc. Natl Acad. Sc/. USA 92 5597-9... 

Chirality (handedness) is older than life on tliis planet. Still it was not until 1848 when Pasteur manually separated enantiomeric crystals that chirality in chemistry was first appreciated ". The independent work of Van t Hoff and Le Bel revealed the molecirlar origin behind this phenomenon. 

The enthalpies of complexation of 3.8c to the copper(lf) - amino acid ligand complexes have been calculated from the values of at 20 C, 25 1C, 30 1C, 40 1C and 50 1C using the van t Hoff equation. Complexation entropies have been calculated from the corresponding Gibbs energies and enhalpies. 

Stereochemistry refers to chemistry in three dimensions Its foundations were laid by Jacobus van t Hoff and Joseph Achille Le Bel m 1874 Van t Hoff and Le Bel mde pendently proposed that the four bonds to carbon were directed toward the corners of a tetrahedron One consequence of a tetrahedral arrangement of bonds to carbon is that two compounds may be different because the arrangement of their atoms m space IS different Isomers that have the same constitution but differ m the spatial arrangement of their atoms are called stereoisomers We have already had considerable experience with certain types of stereoisomers—those involving cis and trans substitution patterns m alkenes and m cycloalkanes... 

Van t Hoff was the recipient of the first Nobel Prize in chemistry in 1901 for his work in chemical dynam ICS and osmotic pressure—two topics far removed from stereochemistry... 

The experimental facts that led van t Hoff and Le Bel to propose that molecules having the same constitution could differ m the arrangement of their atoms m space concerned the physical property of optical activity Optical activity is the ability of a chiral sub stance to rotate the plane of plane polarized light and is measured using an instrument called a polarimeter (Figure 7 5)... 

Although Pasteur was unable to provide a structural explanation—that had to wait for van t Hoff and Le Bel a quarter of a century later—he correctly deduced that the enantiomeric quality of the crystals was the result of enantiomeric molecules The rare form of tartanc acid was optically inactive because it contained equal amounts of (+) tartaric acid and (—) tartaric acid It had earlier been called racemic acid (from Latin racemus meaning a bunch of grapes ) a name that subsequently gave rise to our pres ent term for an equal mixture of enantiomers... 

The molecular weight analysis presented above is a purely thermodynamic result and is independent of any model. The procedure requires dilute solutions, but is not based on the assumption of ideality, even though Eq. (8.88) is a variation of the van t Hoff equation. 

The solute molecular weight enters the van t Hoff equation as the factor of proportionality between the number of solute particles that the osmotic pressure counts and the mass of solute which is known from the preparation of the solution. The molecular weight that is obtained from measurements on poly disperse systems is a number average quantity. 

When B = 0, the solution behaves ideally, at least through second-order effects. This means that deviations from ideality might be observed at still higher concentrations, but that the van t Hoff equation applies at least in dilute solutions for systems with B = 0. 

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