brightsky's Chem Thread

[brightsky]

okay another dilemma:

Volta's pile was a crude form of the modern day dry cell and consisted of alternating zinc and copper discs separated by paper disks soaked in ammonium salt solution. However, the device stopped producing electricity as hydrogen gas produced at the cathode polarised the electrode.

Write the balanced half equation for the reaction occurring at the cathode.

surely Cu is being produced at the cathode? so...Cu2+(aq) + 2e- --> Cu(s).

now the stuff in bold is perplexing. the question did not mention anything about H+ ions (self-ionisation of H2O produces some H+ but surely the amount of H+ produced here is negligible). and how can hydrogen gas possibly be produced, even if there were H+ ions in solution? Cu2+ ion is a stronger oxidant than H+, so surely it will grab electrons more readily and get reduced? perhaps after a while, when the concentration of Cu2+ decreases, some H2 may be formed, but the argument seems to me unconvincing. any help appreciated!

[lzxnl]

okay another dilemma:

Volta's pile was a crude form of the modern day dry cell and consisted of alternating zinc and copper discs separated by paper disks soaked in ammonium salt solution. However, the device stopped producing electricity as hydrogen gas produced at the cathode polarised the electrode.

Write the balanced half equation for the reaction occurring at the cathode.

surely Cu is being produced at the cathode? so...Cu2+(aq) + 2e- --> Cu(s).

now the stuff in bold is perplexing. the question did not mention anything about H+ ions (self-ionisation of H2O produces some H+ but surely the amount of H+ produced here is negligible). and how can hydrogen gas possibly be produced, even if there were H+ ions in solution? Cu2+ ion is a stronger oxidant than H+, so surely it will grab electrons more readily and get reduced? perhaps after a while, when the concentration of Cu2+ decreases, some H2 may be formed, but the argument seems to me unconvincing. any help appreciated!

Erm...ammonium salt solution?

[brightsky]

Erm...ammonium salt solution?

okay so NH4+ can act as an acid and donate protons to the solution. but that still doesn't mean that H+ will get reduced, does it? I think i'm missing something critical...

also another question:

The lead-acid accumulator uses a redox reaction to produce a steady voltage of 12 volts. The battery is composed of six cells that are each made up of a lead electrode grid plate and a lead (IV) oxide electrode, PbO2, in a H2SO4 solution. The overall equation occurring in the cell can be written as

Pb(s) + PbO2(s) + 2H2SO4(aq) --> 2PbSO4(s) + 2H2O (l)

Which ions in the electrolyte of the battery would have a net migration towards the positive electrode?

so i'm assuming the accumulator is discharging, in which case positive electrode = cathode. cations migrate towards cathode. so the answer should be H+ ions. but the answer is in fact SO4 2-(aq). why is that?

[lzxnl]

okay another dilemma:

Volta's pile was a crude form of the modern day dry cell and consisted of alternating zinc and copper discs separated by paper disks soaked in ammonium salt solution. However, the device stopped producing electricity as hydrogen gas produced at the cathode polarised the electrode.

Write the balanced half equation for the reaction occurring at the cathode.

surely Cu is being produced at the cathode? so...Cu2+(aq) + 2e- --> Cu(s).

now the stuff in bold is perplexing. the question did not mention anything about H+ ions (self-ionisation of H2O produces some H+ but surely the amount of H+ produced here is negligible). and how can hydrogen gas possibly be produced, even if there were H+ ions in solution? Cu2+ ion is a stronger oxidant than H+, so surely it will grab electrons more readily and get reduced? perhaps after a while, when the concentration of Cu2+ decreases, some H2 may be formed, but the argument seems to me unconvincing. any help appreciated!

The question didn't mention copper ions...just copper metal as far as I've seen...did it ever say copper ions?

okay so NH4+ can act as an acid and donate protons to the solution. but that still doesn't mean that H+ will get reduced, does it? I think i'm missing something critical...

also another question:

The lead-acid accumulator uses a redox reaction to produce a steady voltage of 12 volts. The battery is composed of six cells that are each made up of a lead electrode grid plate and a lead (IV) oxide electrode, PbO2, in a H2SO4 solution. The overall equation occurring in the cell can be written as

Pb(s) + PbO2(s) + 2H2SO4(aq) --> 2PbSO4(s) + 2H2O (l)

Which ions in the electrolyte of the battery would have a net migration towards the positive electrode?

so i'm assuming the accumulator is discharging, in which case positive electrode = cathode. cations migrate towards cathode. so the answer should be H+ ions. but the answer is in fact SO4 2-(aq). why is that?

I think they're trying to say "anions move towards the positive electrode due to attraction". To be honest, this is a bit of a grey area for me because the electrode with higher electric potential should always attract the anions, but the rule states that cations move to the cathode.

[brightsky]

The question didn't mention copper ions...just copper metal as far as I've seen...did it ever say copper ions?

oh fudge...true. i'm jumping ahead of myself. thanks!

I think they're trying to say "anions move towards the positive electrode due to attraction". To be honest, this is a bit of a grey area for me because the electrode with higher electric potential should always attract the anions, but the rule states that cations move to the cathode.

erghh...so what are we supposed to write? I've always been a bit iffy about this section of the course as well. in galvanic cell, electrons move from negative electrode to positive electrode, which sort of makes sense because negative moves towards positive. but then once electrons get to the positive electrode, they make that solution negative, and so cations from electrolyte must move in to balance the charge. is the electrode itself positively charged, or do we superimpose the polarity onto the electrode? it seems dubious that the positive electrode should attract electrons AND cations...

[brightsky]

And also, is there a way of intuitively working out the relative strength of bonds? For instance, without reference to any data booklets, etc., how do we work out which of H2 gas, CO2 gas, and H2O gas has the stronger bonds? Thanks!

[lzxnl]

oh fudge...true. i'm jumping ahead of myself. thanks!

erghh...so what are we supposed to write? I've always been a bit iffy about this section of the course as well. in galvanic cell, electrons move from negative electrode to positive electrode, which sort of makes sense because negative moves towards positive. but then once electrons get to the positive electrode, they make that solution negative, and so cations from electrolyte must move in to balance the charge. is the electrode itself positively charged, or do we superimpose the polarity onto the electrode? it seems dubious that the positive electrode should attract electrons AND cations...

Did you read my thread on electrochemistry? "Polarity" as such doesn't exist. Both electrodes are neutral.

As an example, the negative anode attracts negative charges in the sense that negative charges are required to balance the circuit.
Likewise, the positive cathode attracts positive charges for the circuit to be balanced.

And also, is there a way of intuitively working out the relative strength of bonds? For instance, without reference to any data booklets, etc., how do we work out which of H2 gas, CO2 gas, and H2O gas has the stronger bonds? Thanks!

It's actually really hard to work it out intuitively. There are arguments in favour of CO2 over H2 and H2O and vice versa.
Hydrogens are small, so the bond length is tiny.
However, H2O is polar, so there is an electrostatic attraction as well. http://www.cem.msu.edu/~reusch/OrgPage/bndenrgy.htm shows both O-H and H-H bonds are similar in strength.
As for C=O, it is a double bond, and there is a similarly polar bond, so you'd expect C=O to be strong. In fact, here it is the strongest out of the three. Note that C-O is weaker than both of the alternatives due to carbon's large size.

[brightsky]

to what depth do we need to know about cracking? do we need for instance to know about catalytic and thermal cracking, and be able to write cracking equations?

[thushan]

Nothing.

[lzxnl]

That's part of the production of ethene, not part of the main course. The question can give you equations from cracking, but you won't have to know them from memory. Nor will you have to actually know anything about the production of ethene, but if you've looked at it before, you'll be better placed if a question on ethene does arise.

[brightsky]

with regard to the following question:

"It is incorrect to state that glucose reacts:
A. with oxygen exothermically to produce carbon dioxide and water
B. endothermically to produce cellulose and water
C. exothermically to produce carbon dioxide and ethanol
D. exothermically to produce starch and water"

are we expected to know the delta H of the reactions above?

[SocialRhubarb]

I don't know what you 'should' know, but cellulose and starch are polymers of glucose, and so reacting glucose units to form cellulose or starch should be endothermic, hence D should be the answer.

[brightsky]

wait so all condensation reactions are endothermic? how did you work that one out? thanks!

also, if we were asked to put aspartic acid into zwitterionic form, should the carboxyl group in the Z-group be deprotonated as well, or is it only the 'main' carboxyl group that gets deprotonated?

[Patches]

According to my teacher we shouldn't, because a zwitterion has an overall neutral charge (which obviously it wouldn't if the residue was protonated/deprotonated). Obviously not certain though.

[SocialRhubarb]

I think just generally reactions which combine simple molecules to make more complex molecules are endothermic. I don't think it's always true, but it works at least for most biological polymers.

It may just be because I do biology as well, since it covers a bit of biochemistry in a simplified format.