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Topic: Weak Acid Titrated Into Strong Base Issue  (Read 280 times)

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Offline M_Morgo

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Weak Acid Titrated Into Strong Base Issue
« on: November 18, 2020, 06:38:35 PM »
Hello!

I and my chemistry teacher was wondering why the titration of a weak acid to strong base has an inital slow increase? A similiar decrease also occurs for a weak base titrated with a strong acid. I've tried finding an answer, but most websites just state the observation and not explain it.

If anyone knows an explanation, that will make my curiousity amazed,
thanks,
Matt :)
Yr. 12 Student. Matt M.

Offline penta-d

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Re: Weak Acid Titrated Into Strong Base Issue
« Reply #1 on: November 18, 2020, 06:56:37 PM »
I think you are talking about formation of a buffering system. See if the section on weak acids and strong bases on this website helps explain:
https://www.thoughtco.com/acids-and-bases-titration-curves-603656

Offline Borek

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Re: Weak Acid Titrated Into Strong Base Issue
« Reply #2 on: November 18, 2020, 07:01:16 PM »
"Why?" is a dangerous question to ask in science. "How?" is much better one.

If the answer is "if you calculate it from the dissociation equilibrium you will see the effect", does it count?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline M_Morgo

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Re: Weak Acid Titrated Into Strong Base Issue
« Reply #3 on: November 18, 2020, 09:11:28 PM »
Sorry! I am not the greatest at English. My vocabulary and grammatical choices are below average.

I don't understand what you mean by "Why?" is a dangerous question to ask in science, sorry :-\. Can you explain this?

I am curious to why there is an quick increase into the buffering phase for a weak acid titration with a strong base.
Yr. 12 Student. Matt M.

Offline M_Morgo

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Re: Weak Acid Titrated Into Strong Base Issue
« Reply #4 on: November 18, 2020, 09:26:07 PM »
I think you are talking about formation of a buffering system. See if the section on weak acids and strong bases on this website helps explain:
https://www.thoughtco.com/acids-and-bases-titration-curves-603656

I could be misreading, but this didn't help explain why there is a rapid increase into the buffering process. It just made the observation.
Yr. 12 Student. Matt M.

Offline Borek

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Re: Weak Acid Titrated Into Strong Base Issue
« Reply #5 on: November 19, 2020, 03:38:23 AM »
There are several great videos featuring Richard Feynmann in which he explains why "Why?" is a wrong question to ask in science. They are in English though.

I could be misreading, but this didn't help explain why there is a rapid increase into the buffering process. It just made the observation.

That's exactly the problem - we can't answer the question "why?". We can make observations, we can describe them mathematically, we can answer question "how", but it doesn't mean we know "why".

Try it yourself: take any process you think you understand and ask "why it happens?". Once you you answer it, ask again: but why? There is a slight chance you will be able to go deeper. If so, ask again: but why? Each time the question "why" seems to have an answer, but in fact (and at best) it only refers to some deeper phenomena, and at some point the only possible answer is "because that's the way it is".
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline M_Morgo

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Re: Weak Acid Titrated Into Strong Base Issue
« Reply #6 on: November 19, 2020, 06:58:05 AM »
https://www.youtube.com/watch?v=36GT2zI8lVA
I watched this video of Richard Feynman on why, "Why?" is quite a complicated question, however, I would not describe it as "wrong" as such. I don't think it is appropriate for anyone who is not educated enough, it can only be answered to a point.

For my question, I have reworded to:
When it comes to a weak acid titrated with a strong base OR a weak base titrated with a stong acid, how come there is a rapid increase into the buffering process when adding in a small amount of solution?
Yr. 12 Student. Matt M.

Offline AWK

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Re: Weak Acid Titrated Into Strong Base Issue
« Reply #7 on: November 19, 2020, 08:18:03 AM »
Stop fruitless discussions and start calculations.
To 10 cm3 of 0.1 M acetic acid add successively 10 times 1 cm3 of 0.1 M NaOH and for each solution calculate the pH using the Henderson-Hasselbach equation (this is an approximate method, but sufficient to understand the phenomenon).
Use the chart from https://www.chembuddy.com/?left=buffers&right=buffer-capacity. Add an additional vertical volume axis on the graph (next to the buffer capacity 0.01, 0.02, etc., add the volumes of 1 cm3, 2 cm3 ...) and add your points from the calculations (or measurements). The pH value of 0.1 M acetic acid is 2.89, pH value of 0.1 M sodium acetate is 8.89. In this case, a good range of buffer operation is considered to be the range of pH + -1 to the pKa value of the acid, i.e. from 3.77 to 5.77, which for this particular example will mean the amount of added NaOH from 2 to 8 cm3, so at 7 out of 10 titration measurement points we will be observed a pronounced buffering effect.
AWK

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