question about amphoteric compounds

[ramzax]

For one of my questions :
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23. When placed in an electric field, which of the following compounds will migrate toward the cathode at pH 7.0?

H2NCH2COOH
H2NCH2CHNH2COOH
HOOCCH2CHNH2COOH
HOOCCHNH2CH2-S-S-CH2-CHNH2COOH


For question 23, the correct answer is choice B. This question deals with the amphoteric properties of amino acids. At neutral or alkaline pH values, the COOH group of an amino acid dissociates to become a negatively-charged carboxylate. At a neutral or acidic pH, the NH2 group is protonated and becomes positively charged. Thus at neutral pH an amino acid is a dipolar ion. The overall charge of an amino acid depends on the ratio of positively charged amino groups to negatively charged carboxylates in the molecule. In order for a molecule to migrate to the cathode, its overall charge must be positive. Of the choices given, choice B is the only one in which the positively charged amino groups would outnumber the negatively charged carboxylate groups. Therefore, only choice B would be positively charged at pH 7. Again then, the correct answer is B.



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I'm confused about why a positive particle would migrate to a cathode. What determines the charges at a cathode or anode. I would think that cathodes are positive, thus deterring positive molecules from migrating towards it.


Thanks,
Me

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[spyderracing32]

My thinking was wrong, sleepy is right. Very good description.

 

[futuredoctor10]

RED CAT, AN OX is a great mnemonic also

 

[sleepy425]

ummmmm, ok. here's the thing, the best way to think about this is to sort of think of an electrolytic cell:

to set up the electric field, imagine two strips of wire that are separated in space (or separated in some solution). anyway, to generate an electric field, you need to apply current between the two strips of wire to develop a charge difference between the two strips. this will drive electrons to one strip and away from the other strip. so one strip will be oxidized, that's the anode, and the other will be reduced, that's the cathode. the cathode, because it has an excess of electrons, is negatively charged. the anode, which is electron-deficient, is positively charged.

so at pH 7.0, the most positively charged molecule will migrate towards the cathode, which is negatively charged.

NH3 is NOT oxidized when it is converted to NH4+. It's just not, it's an acid/base reaction. you change formal charge, but you don't change oxidation state of the atoms. the reason you get migration is the same as why DNA migrates towards the positively charged anode in agarose gel electrophoresis. DNA is negatively charged and the anode is positively charged, so it migrates that way.

 

[friyaz]

ummmmm, ok. here's the thing, the best way to think about this is to sort of think of an electrolytic cell:

to set up the electric field, imagine two strips of wire that are separated in space (or separated in some solution). anyway, to generate an electric field, you need to apply current between the two strips of wire to develop a charge difference between the two strips. this will drive electrons to one strip and away from the other strip. so one strip will be oxidized, that's the anode, and the other will be reduced, that's the cathode. the cathode, because it has an excess of electrons, is negatively charged. the anode, which is electron-deficient, is positively charged.

so at pH 7.0, the most positively charged molecule will migrate towards the cathode, which is negatively charged.

NH3 is NOT oxidized when it is converted to NH4+. It's just not, it's an acid/base reaction. you change formal charge, but you don't change oxidation state of the atoms. the reason you get migration is the same as why DNA migrates towards the positively charged anode in agarose gel electrophoresis. DNA is negatively charged and the anode is positively charged, so it migrates that way.

you are like the king of chemistry