volts electrochemistry question

[arc5005]

What problem can occur when a voltage greater than 1.86 volts in applied to an aqueous metal halide salt solution over a long duration of time?

A. Hydrolysis of water to yield H3O+ (aq) and OH- (aq) occurs once the metal cation is completely reduced.
B. Electrolysis of water to yield H2 (g) and O2 (g) occurs once the metal cation is completely reduced.
C. The metal is further reduced into metallic anions.
D. The halide is further oxidized into halogen cations.

B) Electrolysis of water to yield H2 (g) and O2 (g) occurs once the metal cation is completely reduced.
The addition of voltage does not form hydronium and hydroxide ions, because both oxygen and hydrogen carry the same oxidation states as they do in water. This means that no redox chemistry transpired, so voltage was not necessary. Choice A eliminated. The further reduction of metals to anions and halogens to cations is not realistic, so eliminate choice C & D. The addition of 1.86 volts can cause the reduction of protons to hydrogen gas and the reduction of oxygen within water to oxygen gas.

Can anyone explain this in another way, please?

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[Doogie.Howser]

Background:
You have to ask yourself "What happens when I apply a voltage to a solution?" What is the question regarding? And you nailed it, it's electrochem. As you know, electrochem works by stripping off or adding electrons to molecules present in a solution. Remember making batteries in gen chem lab or lecture? A current is induced in a wire because you are allowing for a flow of electrons to go through the wire. This current can only be induced by a flow of electrons, which comes from the voltage applied to a solution. Voltage allows for this flow of electrons by the reduction and oxidation of molecules present.

Answer:
Now. After a long duration of time, your batteries stop working. Why? Because the metal cannot be further reduced into metallic anions (answer C) and the halide can't be further oxidized into halogen cations (answer D). In order to keep the energizer bunny going, that flow of electrons needs to continue. The only way for the flow of electrons to continue is by the electrolysis of water (choice B). But, you also have to know that 1.86 V is enough energy to split water (read metabolism if you don't remember).
Choice A cannot be the answer because if water is split in that way, there isn't a flow of electrons created. The voltage allows for oxidation and reduction of molecules.

Hope this helps! If you're still confused, ask here or send me a message.

 

[arc5005]

Background:
You have to ask yourself "What happens when I apply a voltage to a solution?" What is the question regarding? And you nailed it, it's electrochem. As you know, electrochem works by stripping off or adding electrons to molecules present in a solution. Remember making batteries in gen chem lab or lecture? A current is induced in a wire because you are allowing for a flow of electrons to go through the wire. This current can only be induced by a flow of electrons, which comes from the voltage applied to a solution. Voltage allows for this flow of electrons by the reduction and oxidation of molecules present.

Answer:
Now. After a long duration of time, your batteries stop working. Why? Because the metal cannot be further reduced into metallic anions (answer C) and the halide can't be further oxidized into halogen cations (answer D). In order to keep the energizer bunny going, that flow of electrons needs to continue. The only way for the flow of electrons to continue is by the electrolysis of water (choice B). But, you also have to know that 1.86 V is enough energy to split water (read metabolism if you don't remember).
Choice A cannot be the answer because if water is split in that way, there isn't a flow of electrons created. The voltage allows for oxidation and reduction of molecules.

Hope this helps! If you're still confused, ask here or send me a message.

thank you! and actually, i took gen chem like 8 years ago, so I don't hahahaha, but MCAT review has been a good refresher

 

[Doogie.Howser]

Hahah oh damn! Good luck with the review!

 

[rabbott1971]

Choice A cannot be the answer because if water is split in that way, there isn't a flow of electrons created. The voltage allows for oxidation and reduction of molecules.

Thanks for your explanation. I didn't understand this last part. Why wouldn't these ions (H+ and OH-) allow for the flow of electrons? Or conversely, why would the flow of electrons be better facilitated by creating H2 and O2?

 

[Doogie.Howser]

Thanks for your explanation. I didn't understand this last part. Why wouldn't these ions (H+ and OH-) allow for the flow of electrons? Or conversely, why would the flow of electrons be better facilitated by creating H2 and O2?

H+ and OH- production doesn't allow for a flow of electrons because when water is split into H3O+ and OH-, there isn't a change in the oxidation states. It is not an oxidation-reduction reaction. The significance of electrochem is created by redox reactions. What is the oxidation state of H and O in water, and what are their oxidation states in hydronium and hydroxide? (Actually figure it out). Once you do that, check the oxidation state of diatomic hydrogen and diatomic oxygen and compare it to water; it changes. Therefore, answer B is redox, while answer A isn't redox.

Does this help?

 

[rabbott1971]

An electrolytic solution has conductivity, meaning flow of electrons. Would these ions not be conductive? The gases would not. I think this is where I'm not tracking your explanation about flow of electrons. I can see there is no redox with the ionic solution, and I understand that electrolysis of water produces the gas products. I just don't follow your statement about electron flow.