In chemistry, a steady state is a situation in which all state variables are constant in spite of ongoing processes that strive to change them. For an entire system to be at steady state, i.e. for all state variables of a system to be constant, there must be a flow through the system (compare mass balance). A simple example of such a system is the case of a bathtub with the tap running but with the drain unplugged: after a certain time, the water flows in and out at the same rate, so the water level (the state variable Volume) stabilizes and the system is in a steady state.
The steady state concept is different from chemical equilibrium. Although both may create a situation where a concentration does not change, in a system at chemical equilibrium, the net reaction rate is zero (products transform into reactants at the same rate as reactants transform into products), while no such limitation exists in the steady state concept. Indeed, there does not have to be a reaction at all for a steady state to develop.
The term steady state is also used to describe a situation where some, but not all, of the state variables of a system are constant. For such a steady state to develop, the system does not have to be a flow system. Therefore, such a steady state can develop in a closed system where a series of chemical reactions take place. Literature in chemical kinetics usually refers to this case, calling it steady state approximation.
In simple systems the steady state is approached by state variables gradually decreasing or increasing until they reach their steady state value. In more complex systems state variables might fluctuate around the theoretical steady state either forever (a limit cycle) or gradually coming closer and closer. It theoretically takes an infinite time to reach steady state, just as it takes an infinite time to reach chemical equilibrium.
Both concepts are, however, frequently used approximations because of the substantial mathematical simplifications these concepts offer.
Cette page est générée automatiquement et peut contenir des informations qui ne sont pas correctes, complètes, à jour ou pertinentes par rapport à votre recherche. Il en va de même pour toutes les autres pages de ce site. Veillez à vérifier les informations auprès des sources officielles de l'EPFL.
In chemistry, the rate law or rate equation for a chemical reaction is a mathematical equation that links the rate of forward reaction with the concentrations or pressures of the reactants and constant parameters (normally rate coefficients and partial reaction orders). For many reactions, the initial rate is given by a power law such as where [\mathrm{A}] and [\mathrm{B}] express the concentration of the species \mathrm{A} and \mathrm{B}, usually in moles per liter (molarity, M).
In physics, a mass balance, also called a material balance, is an application of conservation of mass to the analysis of physical systems. By accounting for material entering and leaving a system, mass flows can be identified which might have been unknown, or difficult to measure without this technique. The exact conservation law used in the analysis of the system depends on the context of the problem, but all revolve around mass conservation, i.e., that matter cannot disappear or be created spontaneously.
In chemistry, a stepwise reaction (also called an overall reaction, complex reaction, and multistep reaction, among others) is a chemical reaction with one or more reaction intermediates, which by definition involves at least two consecutive elementary reactions. In a stepwise reaction, not all bonds are broken and formed at the same time. Hence, intermediates appear in the reaction pathway going from the reactants to the products.
Les étudiants intègrent les notions de potentiels électriques, de niveau de Fermi de l'électron et appliquent l'équation de Nernst. Ils comprennent la structure d'une interface électrifiée. Les généra
Introduction to Chemical Engineering is an introductory course that provides a basic overview of the chemical engineering field. It addresses the formulation and solution of material and energy balanc
Ce cours fournit aux étudiants l'expérience pratique avec les "opérations unitaires" simples basées sur le transfert de chaleur et de masse. Les étudiants développent la capacité d'augmenter l'échelle
Encapsulation techniques open up new possibilities to control the kinetics and location of the release of active ingredients. Despite the progresses achieved to obtain a better control over the dimensions and composition of the capsules, the encapsulation ...
EPFL2022
,
Nanowire (NW) arrays interfaced with biological cells have been demonstrated to be potent tools for advanced applications such as sensing, stimulation, or drug delivery. Many implementations, however, have so far only been studied with rather robust basic ...
WILEY2022
, ,
Increasing the protein production capacity of the PURE cell-free transcription-translation (TX-TL) system will be key to implementing complex synthetic biological circuits, and to establishing a fully self-regenerating system as a basis for the development ...