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QCE Chemistry Questions Thread
Bri MT:
Hey,
The indicator changes colour when pH = pKa. In this case, we're referring to the pKa of the indicator.
At the half equivalence point, enough titrant (known concentration) has been added to the analyte (unknown concentration) that half of it is protonated and half of it is deprotonated, thus making pKa equal to pH.
These are different pKa s and thus it makes sense that they occur at different pH values.
If you're unclear about ether of these please feel free to ask :)
Edited for clarity
alphabeta:
--- Quote from: Bri MT on February 02, 2020, 10:40:58 pm ---Hey,
The indicator changes colour when pH = pKa. In this case, we're referring to the pKa of the indicator.
At the half equivalence point, enough analyte has been added that half of it is protonated and half of it is deprotonated, thus making pKa equal to pH.
These are different pKa s and thus it makes sense that they occur at different pH values.
If you're unclear about ether of these please feel free to ask :)
--- End quote ---
Thanks Bri MT!
So, just to make sure, in the first instance it is talking about the pKa of the indicator, and in the second instance, it is talking about the pKa of the unknown solution?
I really want to make sure I have a firm grip of the titration concepts, so are there any worksheets with simple and challenging titration problems I can do (preferably with worked solutions, if not I can confirm my answers with you)?
Thanks so much again Bri, appreciate it. :)
Bri MT:
--- Quote from: alphabeta on February 03, 2020, 08:08:13 pm ---Thanks Bri MT!
So, just to make sure, in the first instance it is talking about the pKa of the indicator, and in the second instance, it is talking about the pKa of the unknown solution?
I really want to make sure I have a firm grip of the titration concepts, so are there any worksheets with simple and challenging titration problems I can do (preferably with worked solutions, if not I can confirm my answers with you)?
Thanks so much again Bri, appreciate it. :)
--- End quote ---
I've edited my above post for clarity since titration jargon can be confusing but yeah.
Let's say, for example, that you have NaOH in your burette (titrant, known concentration) and you're trying to figure out what the concentration of some acetic/ethanoic acid (analyte) is. To find the concentration of the acetic acid you want to identify the volume of NaOH required to reach the equivalence point, where the stoichiometric ratio is met. To visually approximate this, an indicator is added which is a weak acid/base that will quickly change which conjugate is present in a greater quantity and, since each conjugate is a different colour, will change colour at that point. When the indicator transitions from one colour to another, there is the same amount of each conjugate present and the pKa (of the indicator) = pH (of the solution). In this example, your indicator might be something like phenolphthalein
Before the equivalence point is reached, there will be a stage where you've added sufficient titrant (in this example NaOH) that half of the analyte (in this case acetic acid) is protonated and half is deprotonated - in other words, half is the conjugate acid and half is the conjugate base. Thus, we are looking at another situation where pKa =pH but this time it's the pKa of our acetic acid since that's what has a 50/50 balance with conjugate base & conjugate acid. For half of our acetic acid to be deprotonated, the volume of NaOH required is half that needed to fully deprotonate our acetic acid in accordance with the equation NaOH + CH3COOH -> H2O + CH3OONa . I.e. the v(NaOH) is half of what v(NaOH) is at the equivalence point.
Remembering that the stoichiometric ratio is met at the equivalence point (which we approximate with the end point, where the indicator changes colour), this is why pH = pKa at the half equivalence point for the acetic acid and is at the equivalence point for the indicator.
Hope this clarifies things!
I don't think I have a worksheet like that on hand but I'll look around and see if I can find any good question sets like that for you. If you haven't already, you might find it useful to do the maths for why pH = pKa when each conjugate is present in the same amount to solidify your understanding.
A.Rose:
Hello
I am a year 12 Chemistry student and I was wondering if you could help me with some questions I have about a Galvanic cell experiment I am doing for my student experiment.
The experiment I decided to do was to change the concentration of the copper sulphate electrolyte in a Daniell cell and measure the voltage produced. The other electrolyte; Zinc nitrate was not changed and kept constant at 0.1M. The copper sulphate concentration increased from 0.25M, 0.5M, 0.75M to 1M. The experiment was performed when the temperature was 24 degrees. The anode was Zinc and the cathode was Copper.
In order to obtain theoretical values - should I use the Nernst equation? Would you be able to tell me how to use the Nernst equation for my experiment?
Also, I understand that voltage increases with concentration, but compared to standard conditions should the voltage produced be higher or lower than the voltage produced at standard conditions and 1M for each electrolyte?
If you can help me that would be absolutely amazing and this is a really important assignment.
Thanks!!
Bri MT:
--- Quote from: A.Rose on February 18, 2020, 04:35:31 pm ---Hello
I am a year 12 Chemistry student and I was wondering if you could help me with some questions I have about a Galvanic cell experiment I am doing for my student experiment.
The experiment I decided to do was to change the concentration of the copper sulphate electrolyte in a Daniell cell and measure the voltage produced. The other electrolyte; Zinc nitrate was not changed and kept constant at 0.1M. The copper sulphate concentration increased from 0.25M, 0.5M, 0.75M to 1M. The experiment was performed when the temperature was 24 degrees. The anode was Zinc and the cathode was Copper.
In order to obtain theoretical values - should I use the Nernst equation? Would you be able to tell me how to use the Nernst equation for my experiment?
Also, I understand that voltage increases with concentration, but compared to standard conditions should the voltage produced be higher or lower than the voltage produced at standard conditions and 1M for each electrolyte?
If you can help me that would be absolutely amazing and this is a really important assignment.
Thanks!!
--- End quote ---
Hey :)
Welcome to the forums!
The Nernst equation is applicable here since you're working with non-standard conditions :). I'm guessing what might be confusing you in the calculation is Q? Q is calculated using the same formula as Kc but we use Q (reaction quotient) rather than Kc (equilibrium constant) if the system isn't at equilibrium. Please also make sure you remember to convert your temperature to Kelvin before plugging it in.
When we apply the Nernst equation to your set up, Q is what is changing between experiments so let's look at that. For Q = [Zn2+]/[Cu2+], we see that Q increases as [Cu2+] decreases. This suggests that your voltage will decrease as [Cu2+] decreases.
Hope this helps, please feel free to ask any follow up questions & best of luck with your assignment :)
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