Greater than unity definition11/7/2023 It should be obvious that there is no arrangement of matter- coils of wire, magnets, weights, gears, liquid or gas systems etc.- which can extract energy from NOTHING merely as a result of this clever arrangement. However, that hasn’t stopped people from trying- even long after we finally understood the clear theoretical underpinnings of why all such past attempts were failures, and future attempts a complete waste of time.ĭevices which attempt to do this take myriad forms as limitless as the human imagination: Perpetual Motion Machines of the First Kind What is my personal standard of exceptional proof? A report in a credible, public, refereed journal, followed up by a replication of the result by a totally unrelated team who also publish in the same journal. A violation of any of these laws would require exceptional proof to be considered credible. That said, they are most certainly not mere guidelines you can set aside any time you see something that you feel cannot be explained within their limits. The laws of thermodynamics are, as all scientific theories, subject to change if we find observations that call them in to question. They are not untested hypotheses, nor are they in question for macroscopic systems in the physics community. They have a sound theoretical, mathematical and and mechanistic underpinning. For electrolyte solutions at higher concentrations, Debye–Hückel theory needs to be extended and replaced, e.g.While it's correct to state that these laws are, in scientific terms, "theories", they also fit all observed data for macroscopic systems and are considered universal, i.e. Where ν = ν + + ν – represent the stoichiometric coefficients involved in the ionic dissociation processĮven though γ + and γ – cannot be determined separately, γ ± is a measurable quantity that can also be predicted for sufficiently dilute systems using Debye–Hückel theory. (One cannot add cations without putting in anions at the same time). Ca 2+) isn't measurable because it is experimentally impossible to independently measure the electrochemical potential of an ion in solution. In a liquid solution the activity coefficient of a given ion (e.g. One can define activities for the cations and anions separately ( a + and a –). When the solute undergoes ionic dissociation in solution (for example a salt), the system becomes decidedly non-ideal and we need to take the dissociation process into consideration. The relative activity of a species i, denoted a i, is defined as:Ī i = e μ i − μ i ⊖ R T Ionic solutions For hydrochloric acid solutions, the minimum is around 0.4 M. Although at low ionic strength (< 0.1 M) the activity coefficient approaches unity, this coefficient can actually increase with ionic strength in a high ionic strength regime. When a 0.1 M hydrochloric acid solution containing methyl green indicator is added to a 5 M solution of magnesium chloride, the color of the indicator changes from green to yellow-indicating increasing acidity-when in fact the acid has been diluted.In a solution of potassium hydrogen iodate KH(IO 3) 2 at 0.02 M the activity is 40% lower than the calculated hydrogen ion concentration, resulting in a much higher pH than expected.Two examples serve to illustrate this point: However, there are circumstances where the activity and the concentration are significantly different and, as such, it is not valid to approximate with concentrations where activities are required. The same is often true of equations for reaction rates. The activity of an ion is particularly influenced by its surroundings.Įquilibrium constants should be defined by activities but, in practice, are often defined by concentrations instead. The difference between activity and other measures of concentration arises because the interactions between different types of molecules in non-ideal gases or solutions are different from interactions between the same types of molecules. For gases, the activity is the effective partial pressure, and is usually referred to as fugacity. Activity depends on temperature, pressure and composition of the mixture, among other things. The activity of pure substances in condensed phases (solid or liquids) is normally taken as unity (the number 1). īy convention, activity is treated as a dimensionless quantity, although its value depends on customary choices of standard state for the species. The term "activity" in this sense was coined by the American chemist Gilbert N. In chemical thermodynamics, activity (symbol a) is a measure of the "effective concentration" of a species in a mixture, in the sense that the species' chemical potential depends on the activity of a real solution in the same way that it would depend on concentration for an ideal solution. Measure of the effective concentration of a species in a mixture
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