Volume molaire

Résumé

Le volume molaire d'une substance est le volume occupé par une mole de cette substance. Le volume molaire peut être déterminé pour toute substance dans toutes les phases (gaz, liquide, solide). Dans les unités du Système international le volume molaire s'exprime en mètres cubes par mole (), mais il est plus pratique d'utiliser le litre par mole () ou le mètre cube par kilomole (). Il s'agit d'une grandeur molaire. Le volume molaire d'un gaz parfait est de (soit ) dans les conditions normales de température et de pression (CNTP : et ) et de à sous . Le volume molaire est une entité physico-chimique qui augmente généralement avec l'élévation de la température, autrement dit la plupart des corps se dilatent avec une augmentation de température. Il existe des cas dans lesquels le volume molaire diminue avec une augmentation de température, autrement dit des cas de corps se contractant quand la température augmente, ce qui constitue une anomalie dilatométrique : par ex. le volume mo

Source officielle

https://fr.wikipedia.org/wiki/Volume_molaire

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Concepts associés (20)

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Cours associés (21)

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Séances de cours associées (56)

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, , , ,

Ferroelectric orthorhombic lead metaniobate (PbNb2O6) is known to be metastable with respect to the thermodynamically stable nonferroelectric rhombohedral polymorph. The high-temperature tetragonal to low temperature rhombohedral phase transition is reconstructive and thereby sluggish; ferroelectric PbNb2O6 is obtained by quenching from the stable phase field of the tetragonal polymorph. We report on the stabilization of the ferroelectric tungsten bronze polymorphs of PbNb2O6 by minor chemical substitution in the series [(1 - x) PbNb2O6-xBiTiNbO(6)], [(1 - x)PbNb2O6-xNa(0.5)Bi(0.5)Nb(2)O(6)], [(1 - x)PbNb2O6-xK(0.5)Bi(0.5)Nb(2)O(6)], and [(1 - x)PbNb2O6-xCaTiO(3)]. The high- temperature tungsten bronze polymorph is entropy stabilized with respect to the stable rhombohedral polymorph, and we propose that the tungsten bronze is further entropy stabilized by chemical substitution, reducing the transition temperature of the rhombohedral polymorph and further disfavoring the kinetics of the undesired phase transition. Optimized solid-state synthesis and processing to obtain dense ceramics were developed for the solid solutions, and the dielectric, ferroelectric, and piezoelectric properties of the PbNb2O6 solid solutions are reported. Curie temperature is suppressed with chemical substitution in all the systems. Lattice cell parameters display systematic variation with composition, reducing the molar volume, and the lattice parameter ratio 2b/a with increasing degree of substitution, reflecting a suppression of the polarization along the (010) direction due to chemical substitution. The piezoelectric properties improved with increasing substitution level probably due to the ease of poling of the materials with lower T-c. However, some improvements seen with 2% CaTiO3 were not accompanied by T-c decrease.

Wiley-Blackwell2014

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Self excited instabilities or oscillations of a cavitating full load vortex rope occur due to an interaction between the gas volume and the acoustic waves. From the onset of the oscillations, the amplitudes grow until they reach a maximum, called the “limit cycle”. The aim of this paper is to predict and to simulate this full load instability with its corresponding “limit cycle”. The test case is a reduced scale model installed on test rig in the Laboratory for Hydraulic Machines at the EPFL. An advanced hydro acoustic vortex rope model is developed to take into account the energy dissipation due to thermodynamic exchange between the gas and the surrounding liquid. Three key hydro acoustic parameters are set up using both steady CFD simulations and analytical models. First of all, parameters are assumed to be constant and time domain simulation is divergent without reaching the limit cycle. However frequency of instability is well predicted. Then inclusion of nonlinear parameters is found to lead to a limit cycle of finite amplitude. Prediction is compared with results from experiments and is in good agreement. It is shown that nonlinearity of the viscoelastic damping parameter, modelling the energy dissipation, is decisive to reach the limit cycle. Moreover, an energy approach is developed to understand the interaction process between the mass source and the system dissipation to reach the equilibrium at the limit cycle. It brings out that over one period the dissipation can provide energy to the system whereas the mass source dissipates to ensure the equilibrium.

2009

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Hsp70-Hsp40-NEF and possibly Hsp100 are the only known molecular chaperones that can use the energy of ATP to convert stably pre-aggregated polypeptides into natively refolded proteins. However, the kinetic parameters and ATP costs have remained elusive because refolding reactions have only been successful with a molar excess of chaperones over their polypeptide substrates. Here we describe a stable, misfolded luciferase species that can be efficiently renatured by substoichiometric amounts of bacterial Hsp70-Hsp40-NEF. The reactivation rates increased with substrate concentration and followed saturation kinetics, thus allowing the determination of apparent Vmax′ and Km′ values for a chaperone-mediated renaturation reaction for the first time. Under the in vitro conditions used, one Hsp70 molecule consumed five ATPs to effectively unfold a single misfolded protein into an intermediate that, upon chaperone dissociation, spontaneously refolded to the native state, a process with an ATP cost a thousand times lower than expected for protein degradation and resynthesis.

Nature Publishing Group2010

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