Nernst Equation Experiment

[studiot]

4 minutes ago, Harry_- said:

As for anode/cathode. I know that for a galvanic cell,  reduction happens at the cathode. Therefore electrons flow to the cathode and is consequently positively charged. In an electrolytic cell i believe it is the other way round.

You have learned your lesson well.

Take a gander at this older thread on batteries

The picture is a clickable link.

 

Good luck with your interviews and sure, post queries here.

[John Cuthber]

9 hours ago, studiot said:

Were you connecting both meters at the same time to the same points ?

Yes

[studiot]

Just to add some more to the original purpose of this thread.

Here are some extracts from (a) Price Physical Chemistry for Biochemists as noted in your other thread (b) an older book Physical Chemistry by Glasstone and

These are all about potentiometric methods, to the accuracy you were looking for.

In the 1960s we had a slide wire potentiometer in high school along with a Weston Standard Cell

This was the method I was thinking of.

 

 

 

 

My apologies for the poor quality scans

Edited September 25, 2019 by studiot

[Harry_-]

Hi,

I know it has been a while, so much has been going on. I have just been writing up my results from this experiment and have come across some trouble with my calculated value for Faraday's constant. It is almost perfecty half what it should be, one of two problems could have occured:

1) In my stupidity I created the wrong concentrations of solution (not very probable as all sorts of anomalous points would be created when I then made my more diluted/concentrated solutions)

2) Something is wrong in my calculations

Could any off you check these calculations to see if there are any problems. I used a Zinc anode and Copper cathode so my value for number of electrons transferred in the balanced ionic equation should be 2. If you require any more info, just say :).

Hope you are all well.

Harry

[Harry_-]

@studiot

[studiot]

Hi Harry,

It says you posted your results on Wednesday, but didn't say which Wednesday.!

Anyway I haven't noticed  these before but will try to look at them. I will be away tomorrow all day so it won't be befor tomorrow evening I'm afraid.

Look then.

Edited November 25, 2019 by studiot

[Harry_-]

Thanks  

[studiot]

Since you have the factor of 2 (electrons transferred) in your calculations I wonder how you measured the voltage, ie what was your zero point since the cell voltage is double the calculated Nernst EMF, as described in the attachments.

[Harry_-]

1 hour ago, studiot said:

Since you have the factor of 2 (electrons transferred) in your calculations I wonder how you measured the voltage, ie what was your zero point since the cell voltage is double the calculated Nernst EMF, as described in the attachments.

I'm not sure what you mean by 'zero point' please could you expand. I used my electrometer to measure the voltage, it was zero-checked before the experiment and then connected to my circuit. What do you believe to be off in the recorded voltages? From my calculations the standard emf should be 1.1v, the value I got experimentally was 1.06 (mainly due to insufficient resistance in the circuit to negate internal lost volts).

[studiot]

15 minutes ago, Harry_- said:

I'm not sure what you mean by 'zero point' please could you expand. I used my electrometer to measure the voltage, it was zero-checked before the experiment and then connected to my circuit. What do you believe to be off in the recorded voltages? From my calculations the standard emf should be 1.1v, the value I got experimentally was 1.06 (mainly due to insufficient resistance in the circuit to negate internal lost volts).

Maybe this answer was too simple, I was just hoping since you are looking for a factor of 2.

 

I highlighted where is says the EMF of the complete cell is the algebraic sum of the two electrodes. But one is positive and one negative so the difference between them is twice their magnitude.

How did you connect your measuring apparatus ?

And what is measured along the bottom scale (x axis) on your graph - it is obscured on your attachment.

Edited November 26, 2019 by studiot

[Harry_-]

18 minutes ago, studiot said:

Maybe this answer was too simple, I was just hoping since you are looking for a factor of 2.

 

I highlighted where is says the EMF of the complete cell is the algebraic sum of the two electrodes. But one is positive and one negative so the difference between them is twice their magnitude.

How did you connect your measuring apparatus ?

And what is measured along the bottom scale (x axis) on your graph - it is obscured on your attachment.

Standard Electrode potentials:

Zinc= -0.76

copper= 0.34

(both of these should account for the reaction using/ taking 2 moles of electrons)

0.34--0.74=1.1v

 

It is weird that I am pretty much exactly a factor of 2 off, which is why it makes me believe that iit is a calculation error rather than a practical one. For example, if I was only using 1 mole of electrons in the balanced ionic equation my value for F would be perfect.

 

Here is the electrometer I used's manual: http://physics-astronomy-manuals.wwu.edu/Keithley 614 Electrometer Manual.pdf

Here is a diagram of the apparatus setup, the electrometer being in series shouldn't be a problem should it? I assumed it had internal parallel circuits to record voltage as it also records current.

 

 

Finally, my x-axis was ln([Oxidising Agent] / [Reducing Agent])

[studiot]

On 11/26/2019 at 7:31 PM, Harry_- said:

It is weird that I am pretty much exactly a factor of 2 off, which is why it makes me believe that iit is a calculation error rather than a practical one. For example, if I was only using 1 mole of electrons in the balanced ionic equation my value for F would be perfect.

Thanks for the data so far, what were the concentrations you used?

I have been working through your figures.
No you have not made an arithmetical error.

But the slope of your graph is 0.026 and it should be 0.012

I haven't figured out why yet, but I'm working on it.
 

Meanwhile here is some data from someone (Petrucci - General Chemistry) who has done your experiment.

Note that he changes from naperian logs to base 10 logs and also has a much wide range of concentrations.
Also example 19.5 in the middle of the page is nothing to do with the rest of the page.

[Harry_-]

8 minutes ago, studiot said:

Thanks for the data so far, what were the concentrations you used?

I have been working through your figures.
No you have not made an arithmetical error.

But the slope of your graph is 0.026 and it should be 0.012

I haven't figured out why yet, but I'm working on it.
 

Meanwhile here is some data from someone (Petrucci - General Chemistry) who has done your experiment.

Note that he changes from naperian logs to base 10 logs and also has a much wide range of concentrations.
Also example 19.5 in the middle of the page is nothing to do with the rest of the page.

My concentrations that I wanted to use started at molarity 2 and were tested against the other electrolyte of molarity 1.5 and 1.

I had an issue with creating 2 molarity solutions and the solutions were saturated, so I doubled the volume to make molarity 1 solutions as a start.

I could be possible that I made up the solutions wrongly as I was working very fast

[studiot]

Maybe, but I think there is something deeper.

See what you make of Petrucci's figures.

[studiot]

On 11/26/2019 at 7:31 PM, Harry_- said:

Here is a diagram of the apparatus setup, the electrometer being in series shouldn't be a problem should it? I assumed it had internal parallel circuits to record voltage as it also records current.

 

Just spotted this, which may be the problem.

 

I don't know what your resistor(s) were in series with the electrometer but I think the input resistance to the meter might have been to high.

5x10¹³ Ohms is huge.

Can you repeat with something like 10 to 100 MΏ in parallel with the meter?

I assume you had selected voltmeter mode?

[Harry_-]

23 hours ago, studiot said:

Just spotted this, which may be the problem.

 

I don't know what your resistor(s) were in series with the electrometer but I think the input resistance to the meter might have been to high.

5x10¹³ Ohms is huge.

Can you repeat with something like 10 to 100 MΏ in parallel with the meter?

I assume you had selected voltmeter mode?

Unfortunately I do not have the equipment anymore. My circuit drawing is setup for measuring current and not voltage, It has been a while since I did this experiment so I'm not sure if I put the electrometer in series with the resistors or if I set it up the correct way. Its embarrassing I know. However, even if my electrometer was in series with the resistor (100M Ohm) the voltmeter's impedance is 5x10¹³. Compared with 100x10⁶ of my resistor in series, shouldn't this mean that my results should still be accurate to the degree that I have.

[studiot]

13 minutes ago, Harry_- said:

Unfortunately I do not have the equipment anymore. My circuit drawing is setup for measuring current and not voltage, It has been a while since I did this experiment so I'm not sure if I put the electrometer in series with the resistors or if I set it up the correct way. Its embarrassing I know. However, even if my electrometer was in series with the resistor (100M Ohm) the voltmeter's impedance is 5x10¹³. Compared with 100x10⁶ of my resistor in series, shouldn't this mean that my results should still be accurate to the degree that I have.

V = IR

Since V was of the order of 1 volt, what value of current does that make I ?

I don't see any good reason why the equal concentration voltage was not 1.1 volts either.

But hey, never mind, you have learned a lot and had some fun.

[Harry_-]

5 minutes ago, studiot said:

V = IR

Since V was of the order of 1 volt, what value of current does that make I ?

I don't see any good reason why the equal concentration voltage was not 1.1 volts either.

But hey, never mind, you have learned a lot and had some fun.

This is also weird to me as if the electrometer was in parallel, I would expect a slightly lower value for standard emf (due to the lost volts not completely being cancelled out by the resistor) however if the electrometer was in series I would have expected a very precise value of E⁰.

What if the ZnSO₄ or my CuSO₄ wasn't perfectly pure therefore each of my subsequent solutions were slightly off so even my standard emf (if [reducing agent] was larger than [oxidising agent]) would be lower than expected. As I added water to my solutions to make them more dilute this could have changed the shape of my graph.

I'm also having a hard time believing I would put a voltmeter in series 😂

[studiot]

29 minutes ago, Harry_- said:

This is also weird to me as if the electrometer was in parallel, I would expect a slightly lower value for standard emf (due to the lost volts not completely being cancelled out by the resistor) however if the electrometer was in series I would have expected a very precise value of E⁰.

What if the ZnSO₄ or my CuSO₄ wasn't perfectly pure therefore each of my subsequent solutions were slightly off so even my standard emf (if [reducing agent] was larger than [oxidising agent]) would be lower than expected. As I added water to my solutions to make them more dilute this could have changed the shape of my graph.

I'm also having a hard time believing I would put a voltmeter in series 😂

A thought about solutions occurs to me.

Did you take water of crystallisation into account in your weighing out of the chemicals?

http://www.docbrown.info/page01/AqueousChem/AqueousChem5.htm

They give 5 for copper sulphate and 7 for magnesium, which is the same as zinc.

[Harry_-]

3 minutes ago, studiot said:

A thought about solutions occurs to me.

Did you take water of crystallisation into account in your weighing out of the chemicals?

http://www.docbrown.info/page01/AqueousChem/AqueousChem5.htm

They give 5 for copper sulphate and 7 for magnesium, which is the same as zinc.

Yes, I did take these into account, I can't remember what the exact values were for each compound but I used the value on the bottle. However if the bottles were contaminated/ not pure (very possible as I was using school chemicals which students in practicals can just scoop out what they need directly from the bottle using a spatula) wouldn't this cause my entire gradient to be off as in dilution I wouldn't be making the concentrations of the solutions I thought I was making. 

I also was only using a 2 d.p scale (only one I have) so I cannot expect each solution to be the exact same concentration.

[John Cuthber]

On 11/29/2019 at 4:55 PM, studiot said:

Just spotted this, which may be the problem.

 

I don't know what your resistor(s) were in series with the electrometer but I think the input resistance to the meter might have been to high.

5x10¹³ Ohms is huge.

Can you repeat with something like 10 to 100 MΏ in parallel with the meter?

I assume you had selected voltmeter mode?

Ideally, the input resistance of a voltmeter is infinite.
10^13 ohms will be close enough.

Putting a resistor in parallel will slightly reduce the accuracy of the measurement.

[studiot]

On 11/30/2019 at 6:24 PM, John Cuthber said:

Ideally, the input resistance of a voltmeter is infinite.
10^13 ohms will be close enough.

Putting a resistor in parallel will slightly reduce the accuracy of the measurement.

Yes you are correct an I would be interested in your ideas as to how the experimental results might have come about.

The results, though sparse, do show a creditable straight line but at double the correct slope.

Further the equal concentration EMF is off the expected value by an unusually large amount (0.04 volts), but does lie on the straight line.
 

These two  facts about the data seem to mitigate against overvoltage/polarisation effects.

But I can also draw a very creditable S shape through the results.

I also found a website where a crank was trying to prove his Daniell cell was giving an inexhaustable supply of electrical energy.
Strangely his equal concentration voltage was also 1.06 volts.

Edited December 2, 2019 by studiot

[John Cuthber]

On 12/2/2019 at 2:37 PM, studiot said:

Yes you are correct an I would be interested in your ideas as to how the experimental results might have come about.

Let's face it. I'm not shy.

If I had any meaningful suggestions for an explanation, I'd put them forward.

 

I will make one point.

It's not a difficult experiment to set up.

Anyone with any "theory" about what went wrong could repeat the experiment.

 

I found this on the web. Are their data concordant with yours?
https://pennyroyalresearch.files.wordpress.com/2017/01/the-effect-of-temperature-and-concentration-on-galvanic-cells.pdf

Edited December 4, 2019 by John Cuthber

[studiot]

1 hour ago, John Cuthber said:

Let's face it. I'm not shy.

If I had any meaningful suggestions for an explanation, I'd put them forward.

 

I will make one point.

It's not a difficult experiment to set up.

Anyone with any "theory" about what went wrong could repeat the experiment.

 

I found this on the web. Are their data concordant with yours?
https://pennyroyalresearch.files.wordpress.com/2017/01/the-effect-of-temperature-and-concentration-on-galvanic-cells.pdf

 

Thank you , John. I did find that article when I was looking and although I think the author did a very creditable job for year 12 (not sue what that is in Oz) I also think he was reading too much into too little information.

 

Was it concordant?

Well all the data was pointed at temperature variation as opposed to concentration variation and I haven't extracted the concentration slope at 25^oC,
I note that the voltage readings were generally above those observations made by Harry.

I thought there was not enough comparison to be worth offering here, but but I will look harder over the next couple of days.

[studiot]

OK so here is some quick figuring.

Harry slope =   0.0415

Joel slope   =    0.0015

Nernst equn = 0.012

As you can see Joel only has two points on a much wider concentration range  + / -   10 : 1 as opposed to Harry + / -   2 : 1                         1

 

Edited December 4, 2019 by studiot