Learn basic and advanced concepts of Debye Huckel Onsager Equation to clear IIT JEE Main, Advanced & BITSAT exam at Embibe, prepared by ✓ IIT Faculty. The verification of the Debye-Huckel-Onsager equation is more difficult for in the derivation of the Onsager equation holds good only for ions in dilute solution. Notes on Debye-Hückel Theory. We seek: µi = µi o + RT ln simple expression for ψi. One of the fundamental laws of electrostatics (Maxwell’s first equation) is.

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To calculate this energy two steps are needed.

In this situation the mean activity coefficient is deruvation to the square root of the ionic strength. This is the potential energy of a single ion in a solution.

Activity coefficients of single ions cannot be measured experimentally because an electrolyte solution must contain both positively charged ions and negatively charged ions.

The solvent pale blue is shown as a uniform medium, without structure. Similarly each anion is surrounded by a cloud with net positive charge. Activity, ais proportional to concentration, c.

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For very low values of the ionic strength the value of the denominator in the expression above becomes nearly equal to one. The ions are shown as spheres with unit electrical charge. However, this equation only applies to very dilute solutions and has been largely superseded by other equations due to Fuoss and Onsager, and and later.

It was found to give excellent agreement for “dilute” solutions.


Debye–Hückel theory – Wikipedia

In general, the mean activity coefficient of a fully dissociated electrolyte of formula A n B m is given by [4]. Limiting here means “at the limit of the infinite dilution”. An Introduction to Aqueous Electrolyte Solutions.

Solution of this equation is far from straightforward. In addition it was assumed that the electric field causes the charge cloud to be distorted away from spherical symmetry. This equation applies to electrolytes with equal numbers of ions of each charge. Activity coefficients are themselves functions of concentration as the amount of inter-ionic interaction increases as the concentration of the electrolyte increases.

To apply this formula it is essential that the cloud has spherical symmetry, that is, the charge density is a function only of distance from the central ion as this allows the Poisson equation to be cast in terms of spherical coordinates with no angular dependence. The first step is to specify the electrostatic potential for ion j by means of Poisson’s equation.

A snapshot of a 2-dimensional section of an idealized electrolyte solution is shown in the picture. Real solutions show departures from this kind of ideality at all but the very lowest concentrations see, for example, Raoult’s law. Extensions of the theory [11]. Non-ideality of more concentrated solutions arises principally but not exclusively because ions of opposite charge attract each other due to electrostatic forces, while ions of the same charge repel each other.

For example, with the electrolyte NaCl. Moreover, ionic radius is assumed to be negligible, but at higher concentrations, the ionic radius becomes comparable to the radius of the ionic atmosphere.

Lowering of freezing point and related phenomena” PDF. Retrieved from ” https: Thermodynamic models Electrochemistry Equilibrium chemistry Peter Debye. In consequence ions are not randomly distributed throughout the solution, as they would be in an ideal solution. All the postulates of the original theory were retained. In order to accommodate these effects in the thermodynamics of solutions, the concept of activity was introduced: Nevertheless, the two equations can be combined to produce the Poisson—Boltzmann hhuckel.


The mean activity coefficient is given by the logarithm of this quantity as follows see also: The result has the form of the Helmholtz equation [10].

By calculating the mean activity coefficients from them the theory could be tested against experimental data. In the chemistry of electrolyte solutions, an ideal solution is a solution whose colligative properties are proportional to the concentration of derivatioj solute. The second step is to calculate the charge density by means of a Boltzmann distribution. For symmetrical electrolytes, this reduces to the modified spherical Bessel equation.

Fo Read Edit View history. By using this site, you agree to the Terms of Use and Privacy Policy. The main extensions are the Davies equationPitzer equations and specific ion interaction theory. Freezing point depression measurements has been used to this purpose.

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The most significant aspect of this result is the prediction that the mean activity coefficient is a function of ionic strength rather than the electrolyte concentration.