September 30, 2022, 03:47:00 PM
Forum Rules: Read This Before Posting

### Topic: Activity vs. Concentration  (Read 19911 times)

0 Members and 1 Guest are viewing this topic.

#### eddieq

• Guest
##### Activity vs. Concentration
« on: March 29, 2005, 10:19:47 AM »
why is activity preffered over concentration in electrochemistry? :biggrin2:
« Last Edit: March 29, 2005, 12:19:33 PM by eddieq »

#### Borek

• Mr. pH
• Deity Member
• Posts: 27182
• Mole Snacks: +1765/-406
• Gender:
• I am known to be occasionally wrong.
##### Re:Activity vs. Concentration
« Reply #1 on: March 30, 2005, 02:41:25 AM »
In general, activity is what is important when doing any equlibrium calculations. In all possible places - dissociation constants, solubility products, complexation constants, Nernst equation and so on - it is activity that is important, not the concentration.

In diluted solutions that makes no difference, the more concentrated the solution, the worse the situation.

ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### eddieq

• Guest
##### Re:Activity vs. Concentration
« Reply #2 on: March 30, 2005, 03:26:52 AM »
Does that mean it is more accurate if activity is used ? what scientific reason is there to back it up like ... does it have anything to do with ideal and non ideal situations???

#### Borek

• Mr. pH
• Deity Member
• Posts: 27182
• Mole Snacks: +1765/-406
• Gender:
• I am known to be occasionally wrong.
##### Re:Activity vs. Concentration
« Reply #3 on: March 30, 2005, 04:31:08 AM »
For the diluted solutions there is no problem, but once the solutions get more concentrated discrepancy between calculated and observed concentrations in equlibrium rises. That's why the activity concept was introduced. Look for Debye-Huckel theory, activity coefficients and ionic strength of the solution. Formulas for the calculations are presented on my web site (try info on pH calculation), but for the explanation you have to either find a book or google the web.
« Last Edit: September 16, 2005, 08:03:27 PM by Borek »
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### ssssss

• Guest
##### Re:Activity vs. Concentration
« Reply #4 on: March 30, 2005, 10:50:33 AM »
If you see your book you will find a table that tells relation between conductivity of the electrolyte with conc. you can see the sharp difference.

In case of strong electrolytes such as HCL when the conc is high the charge transfer through the ions is quite difficult because of strong electrostatic force between them but this is not equally liable with weak electrolytes such as acetic acid.In fact weak electrolytes work better with lower conc.[ostwald law states conductivity is inversely prprtional to square root of conc. for weak electrolytes].

#### michaelbing

• Guest
##### Re:Activity vs. Concentration
« Reply #5 on: April 17, 2005, 10:07:47 PM »
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

• Oilfield Consulting Chemist
• Retired Staff
• Full Member
• Posts: 658
• Mole Snacks: +88/-2
• Gender:
• My desk agrees with the law of entropy
##### Re:Activity vs. Concentration
« Reply #6 on: April 18, 2005, 12:54:05 AM »
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.
There are 10 kinds of people in this world: Those who understand binary, and those that do not.

#### Juan R.

• Chemist
• Full Member
• Posts: 148
• Mole Snacks: +24/-3
• Gender:
##### Re:Activity vs. Concentration
« Reply #7 on: May 08, 2005, 05:45:04 AM »
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  .

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.
The first canonical scientist.

#### Borek

• Mr. pH
• Deity Member
• Posts: 27182
• Mole Snacks: +1765/-406
• Gender:
• I am known to be occasionally wrong.
##### Re:Activity vs. Concentration
« Reply #8 on: May 08, 2005, 06:16:29 AM »
basically because concentration is an approximate concept.

and

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

Interesting approach. Never thought about it this way.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### Juan R.

• Chemist
• Full Member
• Posts: 148
• Mole Snacks: +24/-3
• Gender:
##### Re:Activity vs. Concentration
« Reply #9 on: May 08, 2005, 07:52:55 AM »
I said "because concentration is an approximate concept."

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

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!!
The first canonical scientist.

#### Borek

• Mr. pH
• Deity Member
• Posts: 27182
• Mole Snacks: +1765/-406
• Gender:
• I am known to be occasionally wrong.
##### Re:Activity vs. Concentration
« Reply #10 on: May 08, 2005, 09:08:59 AM »
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.

That's the way i read it from the very beginning

That's a new approach for me, but I have already long discussion in eighties on the solutions and concentrations and diffusion with my professor. I was working in electrochemistry field at my uni, trying to simulate numerically diffusion at microelectrodes. It occured to me that when the electrode is really very small (and we were using ones with the radius measured in Angstroms), when the concentration of reacting electrolyte is small, and when there is very fast potential sweep (measured in kV or even in MV per second) diffusion is no longer continuous process. In fact it never is, but usually we have enough particles so that statistics is on our side.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### Juan R.

• Chemist
• Full Member
• Posts: 148
• Mole Snacks: +24/-3
• Gender:
##### Re:Activity vs. Concentration
« Reply #11 on: May 09, 2005, 04:33:55 AM »

It occured to me that when the electrode is really very small (and we were using ones with the radius measured in Angstroms), when the concentration of reacting electrolyte is small, and when there is very fast potential sweep (measured in kV or even in MV per second) diffusion is no longer continuous process. In fact it never is, but usually we have enough particles so that statistics is on our side.

Whow!! Really difficult problem.

When the system is small there are superfitial and quantum effects to be studied. Chemical thermodynamics (and kinetics) are only valid for macroscopic bodies. In my work in nanothermodynamics I show that there are addittional thermal effects of the order of (1/N).

When the potential is very fast you are basically outside of the limit of application of linear-slow approximation (i.e. chemical kinetics and thermodynamics) and you need more general formulations (basically nonlinear ones). Canonical thermodynamics is nonlinear and study those effects. A special formulation rather developed (it was formulated the last century) is extended thermodynamics. The basic idea is that the fluxes are also thermodynamical variables to be introduced in the fundamental equation. Extended thermodynamics would permit you to compute correction terms for very, very fast processes.

From extended thermodynamics (canonical thermodynamics is more general still!!) one can derive generalizations to the usual law of diffusion. In fact, it is usually ignored ("hidden") in physical chemistry textbooks that usual laws of transport are valid only in very constrained situations (linear, average, slow processes, macroscopic bodies, etc.).

E.g. Fourier law for heat transport violates relativistic theory and it is only valid for small temperature gradients.
The first canonical scientist.

#### Donaldson Tan

• Editor, New Asia Republic
• Retired Staff
• Sr. Member
• Posts: 3177
• Mole Snacks: +261/-13
• Gender:
##### Re:Activity vs. Concentration
« Reply #12 on: May 13, 2005, 07:09:52 PM »
i haven't seen any rate equation takes in account of activity. must we do so for chemical reactions taking place in highly concentrated solutions? how about taking in account of activity due to formation of ion-pairs in gas phase reaction system?
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

#### Borek

• Mr. pH
• Deity Member
• Posts: 27182
• Mole Snacks: +1765/-406
• Gender:
• I am known to be occasionally wrong.
##### Re:Activity vs. Concentration
« Reply #13 on: May 13, 2005, 07:39:57 PM »
i haven't seen any rate equation takes in account of activity. must we do so for chemical reactions taking place in highly concentrated solutions?

Every equilibrium is in fact dynamic and kinetically controlled.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### Donaldson Tan

• Editor, New Asia Republic
• Retired Staff
• Sr. Member
• Posts: 3177
• Mole Snacks: +261/-13
• Gender:
##### Re:Activity vs. Concentration
« Reply #14 on: May 13, 2005, 09:28:02 PM »
Every equilibrium is in fact dynamic and kinetically controlled.

I dont understand what you meant. please explain. thanks
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006