Question 25

[sabha]

On Question 25 I was wondering if I could get a little more explanation. I got it correct, but I'm not sure I had the correct reasoning. Thanks

[admin]

By definition, the anode is where current flows in meaning it is positively charged (anions are actracted to anodes).

Ref:
http://www.av8n.com/physics/anode-cathode.htm
http://www.pcmag.com/encyclopedia_term/0,2542,t=anode&i=37785,00.asp

In the Figure given in this problem, you can clearly see that the larger terminal of the battery (the positive terminal) is on the left meaning that current will flow to the electrode on the left = anode. The solution is irrelevant.

Anode in an electrolytic cell:
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch20/faraday.php

[h99h20baby]

battery therefore soln irrelevant!

A is the positively charged electrode i.e. anode in the diagram so the electrolyte used is irrelevant.



I just want to make sure I understand. Is the anode always positive? Is it because positive charges leave the cathode and travel to the anode? Is that why it does not matter the soln because regardless the anode is always +??
regards

[admin]

The concept that the question is testing is as follows: before getting bogged down in any details, you first assess the diagram. There is a battery. The rule for batteries is that the larger terminal is always positive and the smaller terminal is always negative. Normally that would be the only thing that you would need to memorize regarding this problem but even that is given.

Because the terminal on the left is positive and it is connected by wire to the electrode on the left, A, then means that A has now become positive. Because A is positive, no matter what is in solution, if anything is negative, it will be attracted to A.

On to your other question: is the anode always positive? No, but it is always positive in electrolytic cells (which is the one in this question; driven by a battery). It is negative in a galvanic cell.

Ref: http://en.wikipedia.org/wiki/Anode

[h99h20baby]

so is the galvanic cell when it is a voltaic ceel that is a battery discharging? And if so then is the elctrolytic cell a recharging battery?
I am not sure what to do on the MCAT if it presents a passage that does not directly say GALVANIC/VOLTAIC/ELECTROCHEMICAL or ELECTROLYTIC cell...how do I know??

[admin]

This is very straightforward: electrolytic and galvanic (electrochemical) cells are opposites. Electrolytic cells require energy so you will see it hooked up to a battery. Galvanic cells ARE batteries! They spontaneously create energy (positive net Eo values).

[shaun.w.mc6504]

I was reading the external resource http://www.av8n.com/physics/anode-cathode.htm

So it says that anions, flowing towards the anode make a positive contribution to the current. I understand this in that negative ions will tend to 'pull' current in the direction of the anode more strongly.

Where I get confused is that it states, 'cations flowing towards the cathode will also make a positive contribution towards the current'. I might be looking at this wrong but you consider the inside of the cell the solution with the electrodes, and the outside of the cell the wires with electrons flowing threw it, you essential have a closed system and current into the anode has to flow out the anode and into the cathode at some point. How does this cations flowing towards the cathode in the solution make a positive contribution to this effect, in that the current is positive and so are the cations. Thanks for the help.

[admin]

The discussion from that link underlines the fundamentals which was discussed above and in the Explanation to this problem but then it does discuss some specifics and perspectives that are beyond what you need for the MCAT. I have not comment on those perspectives including the example that you mentioned.

[admin]

Bump.