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March 29, 2024, 06:36:11 am

Author Topic: QCE Chemistry Questions Thread  (Read 31192 times)

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Phytoplankton

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Re: QCE Chemistry Questions Thread
« Reply #105 on: May 07, 2022, 06:42:28 pm »
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hi people,
For chemistry I did the galvanic cell prac. My group used magnesium sulphate (with aluminum electrode) and copper sulphate (copper electrode). We tested different concentrations and had so many issues with getting the voltage but, we eventually got our results.

The results are pretty all over the place (doesn't show any correlation- the number are like {it is in order}; 0.08, 0.05, 0.14, 0.53, 0.15)  and now i'm stuck trying to figure out how to write for the rest of the report and the kind of analysis required to get some good analysis done.
I would like some help with what I can actually write.

Hey priya22,

Yeah, those results do seem pretty all over the place. So, there are a few recommendations I would give you.

The first one would be depending on your due date. If you have a bit of time, I would recommend redoing the practical. My guess as to what would be wrong is the voltmeter so I would consider switching out the voltmeter for another one if you were redoing it.

If redoing the practical is not an option, then I would go and ask your teacher what you should do in this case and maybe even ask if she has sample/past-practical data you can use for the report. If she does give data to you, then hooray, you get to analyse some good data. If she doesn't, then she'll give you a way to tackle the problem and more insight on how to work with it.

If in the very unlikely case that asking your teacher doesn't work out at all, then I would analyse the data you have right now (while trying to make some sense with it in terms of log, linear, polynomial graphs etc.) and attribute the lack of valid and reliable data to the countless errors in a school laboratory such as variation in lab conditions, equipment uncertainty, experimenter errors etc.

TLDR: ask your teacher.

Anyways, hope this gave you even the slightest idea what to do but feel free to ask any more questions.
QCE Class of 2021: Physics | Chemistry | Biology | Specialist Mathematics | Mathematical Methods | General English

Uni 2022-2028: BMedSci and MD

priya22

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Re: QCE Chemistry Questions Thread
« Reply #106 on: May 09, 2022, 11:15:15 am »
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Hey priya22,

Yeah, those results do seem pretty all over the place. So, there are a few recommendations I would give you.

The first one would be depending on your due date. If you have a bit of time, I would recommend redoing the practical. My guess as to what would be wrong is the voltmeter so I would consider switching out the voltmeter for another one if you were redoing it.

If redoing the practical is not an option, then I would go and ask your teacher what you should do in this case and maybe even ask if she has sample/past-practical data you can use for the report. If she does give data to you, then hooray, you get to analyse some good data. If she doesn't, then she'll give you a way to tackle the problem and more insight on how to work with it.

If in the very unlikely case that asking your teacher doesn't work out at all, then I would analyse the data you have right now (while trying to make some sense with it in terms of log, linear, polynomial graphs etc.) and attribute the lack of valid and reliable data to the countless errors in a school laboratory such as variation in lab conditions, equipment uncertainty, experimenter errors etc.

TLDR: ask your teacher.

Anyways, hope this gave you even the slightest idea what to do but feel free to ask any more questions.
Thank you very much!
It's too late to ask the teacher, so I'll go with the last option.
I have a few more questions: for the experiment I used a aluminium electrode in magnesium sulphate in the half-cell that was used to change the concentration (cause there was no magnesium), is this correct, what are the effects of not using a magnesium electrode (in the MgSO4), and how would I talk about this in the report?

Plus, I had trouble with getting the voltage to show up on the voltage meter, i suspect it's due to the use of aluminium instead of magnesium, is this correct?

« Last Edit: May 09, 2022, 04:38:39 pm by priya22 »

Phytoplankton

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Re: QCE Chemistry Questions Thread
« Reply #107 on: May 10, 2022, 05:46:15 pm »
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Thank you very much!
It's too late to ask the teacher, so I'll go with the last option.
I have a few more questions: for the experiment I used a aluminium electrode in magnesium sulphate in the half-cell that was used to change the concentration (cause there was no magnesium), is this correct, what are the effects of not using a magnesium electrode (in the MgSO4), and how would I talk about this in the report?

Plus, I had trouble with getting the voltage to show up on the voltage meter, i suspect it's due to the use of aluminium instead of magnesium, is this correct?



Happy to help!

Yes, you are correct about the voltage. Your voltmeter won't show any voltage because your redox reaction in that half-cell simply cannot, spontaneously, occur. This is because the two species at play in that half cell here are Al solid metal and Mg2+ (which is the MgSO4 as the electrolyte). As seen in the electrochemical series, Mg is more reactive than Al (in other words, more likely to oxidise or lose electrons). So in order for your galvanic cell to work, the Mg would have to take in electrons (reduce) but it won't allow that because Mg is stronger than Al to do what it wants (so Mg2+ likes its ion state). You can also see this in the standard electrode potential table where the value for Mg reduction is much lower than Al suggesting that Mg does not like to be reduced.

To answer your first question, to make a galvanic cell work, the electrode is supposed to be either an inert metal or the metal of the solution. So, in your case, your aluminium electrode is neither inert nor (obviously) Mg (as MgSO4 is your electrolyte solution). If you place a metal (Mg) into its own metal solution (MgSO4) and keep your Cu electrode in your CuSO4 solution, then you will certainly get a voltage. This is because, now the Mg electrode will freely break down into its ions (as it likes to be oxidised) and the Cu2+ ions will deposit as copper solid onto the copper electrode (as it likes to be reduced). This movement of electrons (and ions) will produce a voltage equal to the difference of the electrode potentials in the table.

As for what you would do in the report, I recommend you talk about this theory I talked about above (with more elaboration) and thus, explain why you didn't get a voltage with your respective electrodes. The issue is that, assuming you didn't get any voltage values at all for your experiment, you wouldn't have any graphs/tables to do any analysis in which you may lose marks on the two criteria below:
  • "appropriate application of algorithms, visual and graphical representations of data about chemical equilibrium systems or oxidation and reduction demonstrated by correct and relevant processing of data"
  • ­"thorough identification of relevant trends, patterns or relationships"

In the end, you want to discuss all of these issues in your evaluation area where you talk about your errors, however, I really don't have any advice to help you in the two criteria above as you don't have any graphs/tables to process and analyse (really sorry about that). Remember, you will still have raw data and it will be all 0 as you didn't get any voltages.

I hope this helped but try the best you can to analyse and relate it to your theory/rationale. Again, feel free to ask more questions.
QCE Class of 2021: Physics | Chemistry | Biology | Specialist Mathematics | Mathematical Methods | General English

Uni 2022-2028: BMedSci and MD

priya22

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Re: QCE Chemistry Questions Thread
« Reply #108 on: May 14, 2022, 10:11:21 am »
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Happy to help!

Yes, you are correct about the voltage. Your voltmeter won't show any voltage because your redox reaction in that half-cell simply cannot, spontaneously, occur. This is because the two species at play in that half cell here are Al solid metal and Mg2+ (which is the MgSO4 as the electrolyte). As seen in the electrochemical series, Mg is more reactive than Al (in other words, more likely to oxidise or lose electrons). So in order for your galvanic cell to work, the Mg would have to take in electrons (reduce) but it won't allow that because Mg is stronger than Al to do what it wants (so Mg2+ likes its ion state). You can also see this in the standard electrode potential table where the value for Mg reduction is much lower than Al suggesting that Mg does not like to be reduced.

To answer your first question, to make a galvanic cell work, the electrode is supposed to be either an inert metal or the metal of the solution. So, in your case, your aluminium electrode is neither inert nor (obviously) Mg (as MgSO4 is your electrolyte solution). If you place a metal (Mg) into its own metal solution (MgSO4) and keep your Cu electrode in your CuSO4 solution, then you will certainly get a voltage. This is because, now the Mg electrode will freely break down into its ions (as it likes to be oxidised) and the Cu2+ ions will deposit as copper solid onto the copper electrode (as it likes to be reduced). This movement of electrons (and ions) will produce a voltage equal to the difference of the electrode potentials in the table.

As for what you would do in the report, I recommend you talk about this theory I talked about above (with more elaboration) and thus, explain why you didn't get a voltage with your respective electrodes. The issue is that, assuming you didn't get any voltage values at all for your experiment, you wouldn't have any graphs/tables to do any analysis in which you may lose marks on the two criteria below:
  • "appropriate application of algorithms, visual and graphical representations of data about chemical equilibrium systems or oxidation and reduction demonstrated by correct and relevant processing of data"
  • ­"thorough identification of relevant trends, patterns or relationships"

In the end, you want to discuss all of these issues in your evaluation area where you talk about your errors, however, I really don't have any advice to help you in the two criteria above as you don't have any graphs/tables to process and analyse (really sorry about that). Remember, you will still have raw data and it will be all 0 as you didn't get any voltages.

I hope this helped but try the best you can to analyse and relate it to your theory/rationale. Again, feel free to ask more questions.

Thank you. but now my teacher told me to work on another group's results. I am not sure how Zinc nitrate and Copper sulphate voltaic cells (the other group's one) works could you explain?

and their data is decreasing over an increasing concentration, I am not sure why?


Phytoplankton

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Re: QCE Chemistry Questions Thread
« Reply #109 on: May 17, 2022, 07:32:37 am »
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Thank you. but now my teacher told me to work on another group's results. I am not sure how Zinc nitrate and Copper sulphate voltaic cells (the other group's one) works could you explain?

and their data is decreasing over an increasing concentration, I am not sure why?

Hey priya22,

Ok that sounds a bit better now. Copper sulfate and zinc nitrate is one of the most common galvanic cells school's work on and I'm assuming its a copper electrode in the CuSO4 and a zinc electrode in Zn(NO3)2. All you have to do is apply the same theory I talked about before to this as well. So, take a look at either the electrochemical series or the reactivity series. You can see that zinc is more reactive meaning it would like to oxidise (lose electrons) - this can be seen as it is higher in the reactivity series and shows a lower Eo in the electrochemical series. This means that the zinc electrode will happily oxidise into its ions and thus, the copper ions in the electrolyte will happily deposit solid copper onto the electrode. This will produce a movement of electrons (with help from the salt bridge as well), creating electricity. Ensure you discuss the movement of ions and electrons in the solutions and the salt bridge.

As for the data you got, the voltage should be increasing as you increase the concentration and assuming you mean voltage when you say "data", that shouldn't be right. Increasing concentration means you increase the ions in the electrolyte which means it carries more of an electrical current, yielding a higher voltage. If the decrease of your voltage is small, I think you should be able to explain it through errors or otherwise, your other group may have accidentally wrote the data the wrong way so you can just fix that. That's really all I can say about that.

It was good that you got another group's data as well as half cells that make sense. Good luck and again, feel free to ask questions!
QCE Class of 2021: Physics | Chemistry | Biology | Specialist Mathematics | Mathematical Methods | General English

Uni 2022-2028: BMedSci and MD