Self assessment chemistry

[Meredith92]

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Hey guys,
I dont want to post the passage because I dont think I'm allowed to, but i could really use help understanding the set up described in the passage. its about coulometric determination of oxygen concentration. I dont understand this concept. oxygen is bubbled over an electrode of Ag but i dont get where the electrons are coming from if BOTH oxygen and Ag are getting reduced ( so clearly the Ag is not giving electrons to the O2). do the electrons from the Cd just get released in the water and it reduces both Ag+ and O2? If so, why is the Ag even necessary? In addition, how is this even a circuit? there are no wires attaching the two electrodes. Do the electrons just float into the water?
Thanks for your help!

 

[MedGrl@2022]

Hey guys,
I dont want to post the passage because I dont think I'm allowed to, but i could really use help understanding the set up described in the passage. its about coulometric determination of oxygen concentration. I dont understand this concept. oxygen is bubbled over an electrode of Ag but i dont get where the electrons are coming from if BOTH oxygen and Ag are getting reduced ( so clearly the Ag is not giving electrons to the O2). do the electrons from the Cd just get released in the water and it reduces both Ag+ and O2? If so, why is the Ag even necessary? In addition, how is this even a circuit? there are no wires attaching the two electrodes. Do the electrons just float into the water?
Thanks for your help!

Hi Meredith.... Did you ever figure this out? I just took the self assessment yesterday and this was one of the passages that I had the most trouble with. Now I see that both Ag+ and O2 are reduced. Which makes the location of the silver electrode the cathode because reduction occurs there? However, wouldn't there have to be a reducing agent that becomes oxidized?

Now I understand the answers but your right it is a little confusing.

Did you already take your MCAT? How do you think these self-assessments reflect your actual practice MCAT scores? I thought most of the self-assessments were easy.

 

[Meredith92]

i literally spent so long on this one passage, and i think what i "took" from it was that oxidation ALWAYS occurs at the anode and reduction ALWAYS at the cathode.. and from that you can answer those questions. but honestly the set up of that problem is still SO confusing to me. but ive never seen anything else like it so i woudlnt worry about it. i think i got high 80's for physics and chem self assessments and i got 12-14's on all my practice exams for physical sciences. i pray that i got in that range for my actual exam, but well see!!

 

[adrakdavra]

Electrons don't actually flow through water by themselves. The need to be "escorted" through the water by ions. For example, a H2O water moecule can give up one of its H atoms but keep its electron, and become OH-
(the "-" means it's negatively charged). The other H2O molecule then becomes H3O+. The OH- and H3O+ can both move through the water and carry electrical charges. If you put two electrodes in water and attached them to a battery, the electrical current would be carried by the OH- and H3O+... very slowly.

 

[MedGrl@2022]

i literally spent so long on this one passage, and i think what i "took" from it was that oxidation ALWAYS occurs at the anode and reduction ALWAYS at the cathode.. and from that you can answer those questions. but honestly the set up of that problem is still SO confusing to me. but ive never seen anything else like it so i woudlnt worry about it. i think i got high 80's for physics and chem self assessments and i got 12-14's on all my practice exams for physical sciences. i pray that i got in that range for my actual exam, but well see!!

congratulations! I bet you did awesome! 🙂

 

[HotHamH2O]

without reading the passage and knowing anymore details I think the key to this is remembering that molecular oxygen (O2) has an oxidation number of 0. So oxygen goes from 0 to -2 which is reduction.