Chemistry Forums for Students > Physical Chemistry Forum

Activity vs. Concentration

<< < (2/3) > >>

michaelbing:
I desparately need to talk w/ someone pertaining to Chem 1212 (solubility / equivalence point) questions...If you will call asap @ 478-994-6007 COLLECT!!! and offer help - - I will Western Union you $$$ for your efforts; I am currently pulling an A in class, have worked all weekend on homework, and am unsure about answers....I need someone to talk to - - TONIGHT --- Thanks

eugenedakin:
I have sent you an e-mail to contact me via MSN messenger (no long distance charges) ... I will be on the net unit about 11:15 pm tonight... I'll help you with some of your questions.

Eugene Dakin Ph.D., P.Chem.

Juan R.:
basically because concentration is an approximate concept.

cB is the number of molecules of specie B. In phase space (ideal gas), there is no quantum correlations between molecules and each molecule is a well-defined specie. In real phase (gas, and specially solutions and solids) there is not real molecules. There is a global system formed by N particles. The situation is even worse in electrochemical systems because there is charges and electrostatic Coulomb forces are large correlation ones.

Therefore it is difficult to define cB in a system where B (in the usual chemical sense) really does not exist  :o.

However in some sense "B" is inside the total system of N particles and therefore it is natural to believe in some substitute for cB: e.g. activity aB

Some books attempt to claim that one may always substitute cB by aB in non ideal phases. This is not true.

In fact, that ad hoc rule was source (and continue to be) of confusion in the past. E.g. if one follows that "standard rule" one obtains from chemical kinetics

B + D ---> products

part aB / part t = - kaBaD                            (wrong)

but this equation does not work. The correct is

part cB / part t = - kaBaD and, therefore, some textbooks do

part cB / part t = - k'cBcD and introduces nonideal effects (e.g. strengh of an external electric field directly in the rate constant).

From macroscopic canonical science one obtains the correct answers to these questions without adtitional asumptions or wrong reasoning. Moreover, at equilibrium the rate part "collapses" and survives only the activity part (right part) and by this reason in equilibrium formulas it appears the activity instead of concentration. For instance the equlibrium constant is computed from both products and products-1 of activities instead of concentrations.

Moreover, from microscopic canonical science, one obtain the details of why. There is no real molecules in the ideal gas sense, therefore there is not typical concentration.

Perhaps some chemists are perplexed of that molecules (in the usual "chemical" sense of individual entities) do not exist, but let me remember that usual quantum chemistry computations are only valid for idealized phases, where each individual molecule is a well-defined entity.

In fact, my profesor of quantum mechanics said us an anecdote (i believe is real one)of an inorganic chemist that using the Gaussian computed an entalpy, apply it, and find a sound discrepancy. He just forgot that computed "Gaussian entalpy" was valid only for ideal gas of that molecule. In condensed matter situations, one may introduce solvent effects and in more sophisticated approaches simply ignore the language of wavefunctions, Schrödinger equation, and all that.

Borek:

--- Quote from: Juan R. on May 08, 2005, 05:45:04 AM ---basically because concentration is an approximate concept.
--- End quote ---

and


--- Quote ---Therefore it is difficult to define cB in a system where B (in the usual chemical sense) really does not exist  :o.
--- End quote ---

Interesting approach. Never thought about it this way.

Juan R.:
I said "because concentration is an approximate concept."

It read ambigous. As said, the chemical specie is really therein, Of course ;D

I mean that the use of concentration in the typical chemical of a "molecule forever" sense like a "source" for several dynamics properties (including equilibrium ones) is incorrect.

I wait now it is more clear my previous post. Thanks!!

Navigation

[0] Message Index

[#] Next page

[*] Previous page