riamu
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why is there an increase in successive pka values for a tripotic acid ?eg;citric acid
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Blackhart
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For each successive pKa value there is a proton is being removed from a more and more negatively charged species. That should be enough to get you started .
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riamu
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(Original post by Blackhart)
For each successive pKa value there is a proton is being removed from a more and more negatively charged species. That should be enough to get you started .
nope,i still didnt get it

if you could explain it scientifically,please
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Blackhart
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For a triprotic acid there are three succecssive pKa values, one for each removal of H+ (a proton) which is positively charged. For the first pKa value you are taking a positive species from a neutral one.

For the next pKa value the proton is now being taken a species with a more negative charge (because we already removed a positive proton). Remember positve and negative attract so it will take more energy to remove this proton due to those attractive forces. This higher energy to remove means the Ka will be lower as the amount of dissociation will be lower so there will be higher conc. of [HA] and lower conc of [H+][A-]. Therefore pKa will be higher for each successive value.
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riamu
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(Original post by Blackhart)
For a triprotic acid there are three succecssive pKa values, one for each removal of H+ (a proton) which is positively charged. For the first pKa value you are taking a positive species from a neutral one.

For the next pKa value the proton is now being taken a species with a more negative charge (because we already removed a positive proton). Remember positve and negative attract so it will take more energy to remove this proton due to those attractive forces. This higher energy to remove means the Ka will be lower as the amount of dissociation will be lower so there will be higher conc. of [HA] and lower conc of [H+][A-]. Therefore pKa will be higher for each successive value.
okay assuming one of the terminal -COOH group hydrogen is removed so now there is negative charge on that end but how will it affect the removal of hydrogen at the other end?
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charco
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(Original post by riamu)
okay assuming one of the terminal -COOH group hydrogen is removed so now there is negative charge on that end but how will it affect the removal of hydrogen at the other end?
1. Try to think of electrons not as point charges but rather more as a flux of negativeness. Molecules are closer to lumps than the structures that represent them.
2. All equilibria are affected by the stability of the species on either side of the equilibrium.

3. Carboxylic acids, in general, are much more stable than the corresponding conjugate bases (dissociation of ethanoic acid is about 1%)

If a negative charge develops on one part of the molecule it affects the electron density of all of the remaining molecule. Effectively, there is a larger amount of charge (negativeness) to spread around the whole molecule.

It is going to be harder to remove a hydrogen ion held by a slightly greater amount of negative charge, so the O-H bond is harder to break and the species formed, which would have a double negative charge, is less thermodynamically stable, pulling the equilibrium back to the LHS.

The same arguments apply to AH2- <==> A3- + H+
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