Question 24

[mango]

hi, I understand in an electrolytic cell, the anode is the positive electrode . And in the voltaic cell, the anode is the negative electrode.

How do you determine glycine movement toward the cathode but not anode? thank you so much.

[Guest]

Whether in an electrolytic cell or galvanic cell, the flow of e's is always anode to cathode. I checked with diagrams in my MCAT book.

Zwitterion just means dipolar (having a positive charge and a negative charge). At pKa of 2.3, it is acidic, so that means in the solution there are lots of H+ and the carboxyl group half of them are going to be protonated. For the amino group all will be protonated. If the amino group is fully protonated, that means a positive charge. So if the question was what was the amino acid at pKa 2.3, we would look for one that had a 1+ charge.

Now lets consider the other far spectrum. Let's say the question asked for pKa of 9.7, that means solution is basic and going to pluck a lot of protons. So at that pKa half of the amino groups are deprotonated, and all of the carboxyl group is deprotonated. If COOH is deprotonated, you get COO-, and -NH3+ amino group deprotonated, you get neutral -NH2, so the sum of the charges is 1-.

Now for the problem is asking! It gives a pKa that is between 2.3 and 9.7 not either or. So instead of being either 1+ sum or 1- sum, it has a combo of both 1+ and 1-, so in the solution the amino acid is going to have a -COO- (1-) and an -NH3+ (1+). I wanted to explain the above first before the bottom made sense why if it is between the pKas it is a zwitterion (dipolar).

[mcat_premed3832]

From: http://en.wikipedia.org/wiki/Cathode

A cathode is an electrode through which electric current flows out of a polarised electrical device. Mnemonic: CCD (Cathode Current Departs).

It follows from this universal definition that in a galvanic cell, shown as an illustrative example, the cathode is the positive electrode, where conventional current flows outwards. This outwards current is carried internally by positive ions leaving the electrolyte. It is continued externally by electrons moving inwards, negative charge moving one way amounting to positive current flowing the other way. In an electrolytic cell, the cathode is the negative terminal, which sends current back to the external generator.

Because we are separating a chemical in solution, we are talking about an electrolytic cell so the cathode is negative.

[mcat_premed3832]

More mnemonics!

I had this professor for biology whose name was Professor GANS and this lady was a really big PANE. It was weird that HER daughters name was LEO

GANS- Galvanic Anode Negative Spontaneous
PANE- Positive Anode Nonspontaneous Electrolytic

Volatic = Galvanic = Discharging
Electrolytic = Charging

For discharging systems (like batteries), the anode is negative and the cathode is positive.


For charging systems (like capacitors in electrophoresis), the anode is positive and the cathode is negative.

No matter what type of electrochemical cell, oxidation occurs at the anode, reduction occurs at the cathode, and electrons flow from the anode (where they are lost via oxidation) to the cathode (where they are gained in reduction).

[admin]

Yikes! Hmmm, High Electrons Reduction will have to do!

Actually, the most common mnemonic is the following:

LEO is a GERC

Loss of Electrons is Oxidation is Anode

Gain Electrons Reduction Cathode

[mcat_premed3832]

Electrolytic and galvanic cells are considered to be opposites. The reason is that a galvanic cell (= voltaic = a battery) produces current based on a spontaneous reaction. An electrolytic cell, on the other hand, has an external current applied to the the system. Because the question states that an electric field is applied, the cell can only be electrolytic.

[acebshaw2450]

A battery uses a voltaic cell, so if we were applying the current using a battery, then the positive amino acid would flow to the negative anode. Without the question telling us how the electric field is applied, it is impossible to know. Bad question.

[mohsinalam5969]

@ALL
if we carefully look at the answers explanation we will come to know that it states negative electrode....
whatever the cell is (volatic or galvanic) glycine will always move towards negative electrode...in electrolytic cell it will move towards cathode(negative electrode) while in case of galvanic cell it will move towards anode(negative electrode)
hope u understand this.....
Now for the problem is asking! It gives a pKa that is between 2.3 and 9.7 not either or. So instead of being either 1+ sum or 1- sum, it has a combo of both 1+ and 1-, so in the solution the amino acid is going to have a -COO- (1-) and an -NH3+ (1+). I wanted to explain the above first before the bottom made sense why if it is between the pKas it is a zwitterion (dipolar).

[mcat_premed3832]

[neens20043391]

I am sorry but I am still confused

Someone please CLARIFY..
so since the pH = 2 and its lower than pka1 = 2.3, the solution is acidic so it will donate a proton to the glycine. This will make the glycine + charged. So it will flow toward the negative electrode.

Galvanic cell = Anode is negative and cathode positive
Electrolytic cell = Anode is positive and Cathode negative....
Now how do you know which cell the question is talking about? because only depending on that we can answer whether the glycine will flow toward the anode or the cathode.
ALSO, isn't there a RULE that anions always flow toward the anode and the cations flow toward the cathode. If thats the case, then the answer would be B. Am I thinking the right way?

HELP! ...