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Author Topic: Can pH for any compounds be calculated with an equation?  (Read 3275 times)

Offline taregg

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like strong or weak acid and base in percent ....can you show me for some exsample with equation...no standard values.
« Last Edit: 09/05/2013 15:40:51 by chris »


 

Offline Bored chemist

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Practically speaking, you can't.

The strength of acids and bases are measured. They are not calculated from an equation.
But, once you know the strength you can use that to calculate the pH of mixtures.
http://en.wikipedia.org/wiki/Henderson%E2%80%93Hasselbalch_equation
 

Offline Lmnre

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Re: Can pH for any compounds be calculated with an equation?
« Reply #2 on: 10/05/2013 15:56:28 »
Does "calculated with an equation" include evaluating the formula and trying to determine its [approximate] pH? For example, could evaluating the formula H2SO4 determine its approximate pH? Could comparing multiple formulas, such as HCl, HF and HBr, determine their relative pH to one another? Is there a general "philosophy" regarding their pH that can be understood from these formulas that could be applied to other R-Cl, R-F and R-Br formulas?
 

Offline damocles

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Re: Can pH for any compounds be calculated with an equation?
« Reply #3 on: 11/05/2013 00:33:13 »
Does "calculated with an equation" include evaluating the formula and trying to determine its [approximate] pH? For example, could evaluating the formula H2SO4 determine its approximate pH? Could comparing multiple formulas, such as HCl, HF and HBr, determine their relative pH to one another? Is there a general "philosophy" regarding their pH that can be understood from these formulas that could be applied to other R-Cl, R-F and R-Br formulas?

That is a very interesting point lmnre. The first thing I would say is that there is no equation as such -- chemists do not think in terms of equations -- but there are numerous "rules of thumb", and rationalizations to back them up (mostly "after the fact" rationalizations). One of them is that acids where the acidic hydrogen is bonded directly to another element become a little stronger as one proceeds down a group of the periodic table, and a lot weaker as one shifts back to the left on the periodic table. Another is that for equivalent oxo-acids (R-OH) , they become weaker as one proceeds down a group of the periodic table, and stronger as the number of oxygen atoms bonded directly to the central atom increases. The rationalization offered to explain these facts concerns an examination of the structure of the respective anions, and a consideration of how they might delocalize the negative charge they would accumulate.

HI>HBr>HCl>HF>>H2Se>H2S>H2O>>AsH3>PH3>NH3
HOI<HOBr<HOCl
HOClO3>HOClO2>HOClO>HOCl
 

Offline Bored chemist

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Re: Can pH for any compounds be calculated with an equation?
« Reply #4 on: 11/05/2013 09:53:44 »

The first thing I would say is that there is no equation as such -- chemists do not think in terms of equations --

Here are the equations we "don't use"
http://en.wikipedia.org/wiki/Hammett_equation
They are a subset of these ones
http://en.wikipedia.org/wiki/Free-energy_relationship

But as I said, fundamentally, they are based on measurements.
 


Offline damocles

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Re: Can pH for any compounds be calculated with an equation?
« Reply #6 on: 12/05/2013 06:14:06 »
From Bored Chemist:
Quote
Here are the equations we "don't use"
http://en.wikipedia.org/wiki/Hammett_equation
They are a subset of these ones
http://en.wikipedia.org/wiki/Free-energy_relationship


Thankyou BC! These two wikipedia articles point out exactly what I was saying about chemists' use of equations.

(1) They are rules of thumb rather than universal in scope. In particular note that
    They do not follow from any thermodynamic treatment, though they are the result of thermodynamic intuition.
    They are not universal in scope, neither for all substituents nor for all reactions; However they have a wide enough scope to have been very useful to working chemists.
    They are only expressing correlations of around 95%. This might not be clear to naive readers of this forum, but the constants are obtained from regression lines of best fit, so the 3-figure accuracy to which they are expressed is perhaps a little misleading. However it is also true that most kinetic work is only carried out to about 5% accuracy, with perhaps a few of the better ones getting down to 2%

(2) There is a lot of "after the fact" rationalization involved
    In determining substituent constants
    in deciding whether the treatment will be applicable to a particular reaction series
    In determining reaction constants
 

Offline syhprum

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Re: Can pH for any compounds be calculated with an equation?
« Reply #7 on: 15/05/2013 13:23:29 »
With the aid of super computers it is possible to compute the mass of particles such as Protons from first principles so I would have thought it would be possible to compute the ph value of compounds although I cannot see any good reason for doing it.
 

Offline damocles

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Re: Can pH for any compounds be calculated with an equation?
« Reply #8 on: 15/05/2013 22:35:10 »
With the aid of super computers it is possible to compute the mass of particles such as Protons from first principles so I would have thought it would be possible to compute the ph value of compounds although I cannot see any good reason for doing it.

That post only makes it obvious that you are a physicist by instinct, Wal. Chemists have good reasons for doing it -- for example: will it be worth going to the trouble of synthesizing a new (or expensive) acid to achieve a desired pH as well as other desired properties. And even simple chemical systems are a long way beyond the complexity of the systems that physicists mostly deal with, and usually quite intractable (from first principles).

Check out wikipedia on "molecular orbital theory" as an example.
 

Offline Bored chemist

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Re: Can pH for any compounds be calculated with an equation?
« Reply #9 on: 16/05/2013 19:31:36 »
"With the aid of super computers it is possible to compute the mass of particles such as Protons from first principles so I would have thought it would be possible to compute the ph value of compounds ."
I wouldn't.
It takes,, as you say, a hell of a lot of computing power to sort out something simple like the mass of a proton.
Now imagine sorting out, from first principles, the interactions among the elementary particles in a simple acid like acetic (not to mention the behaviour of all the water molecules round it because gas phase acidities are often very different from those measured in solution.)
Seriously, how could it be easier to work out the properties of acetic acid (2 carbons 2 oxygens and  4 hydrogens i.e. 12+16+4 =32 protons,  30 electrons and roughly as many neutrons ) than to work out the properties of just one of those particles?

On the other hand, it is, as has been pointed out, very useful to be able to give a reasonable estimate of the strength of the acidity of a material.
« Last Edit: 16/05/2013 19:33:25 by Bored chemist »
 

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Re: Can pH for any compounds be calculated with an equation?
« Reply #9 on: 16/05/2013 19:31:36 »

 

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