Second Deprotonations

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Mrudula Akkinepally
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Second Deprotonations

Postby Mrudula Akkinepally » Fri Jan 22, 2021 11:47 pm

Hi everyone!

I am doing practice problems from outline 2 and question 6E.3 says to only ignore second deprotonations when the approximation is justified.

I was wondering what that meant and how we know when the approximation is justified?

Thank you :)

Lillian Ma 1I
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Re: Second Deprotonations

Postby Lillian Ma 1I » Sat Jan 23, 2021 2:34 am

You can ignore the 2nd deprotonation when the Ka value for that deprotonation is much smaller than the first one, but I'm not sure if there is an exact cutoff for what is considered much smaller.

Mikayla Kwok 3K
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Re: Second Deprotonations

Postby Mikayla Kwok 3K » Sat Jan 23, 2021 12:51 pm

I think you can typically ignore second deprotonations if the acid is a weak polyprotic acid, because the Ka2 would be extremely small, but for H2O4, which is a strong polyprotic acid, we should account for the second deprotonation in calculations.

Pranav Daggubati 3C
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Re: Second Deprotonations

Postby Pranav Daggubati 3C » Sat Jan 23, 2021 1:51 pm

The approximation is justified when weak bases/acids are present and they don't disassociate very much, causing a relatively much lower value of x compared to the initial concentration. In this case, the second deprotonation rarely happens due to the weak base barely deprotonating even once. That is why we can ignore second deprotonations here.

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Re: Second Deprotonations

Postby SavannahScriven_1F » Sat Jan 23, 2021 2:04 pm

Specifically, the second deprotonation (with Ka2) does not affect the pH and can be ignored when Ka2 is less than Ka1*10-3. Usually though Ka2 is much smaller than Ka1 so it's pretty obvious. Hope this helps!

Katie Phan 1K
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Re: Second Deprotonations

Postby Katie Phan 1K » Sat Jan 23, 2021 7:06 pm

2nd protonation is usually much smaller than 1st anyway.

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Re: Second Deprotonations

Postby vanessanguyen3I » Sat Jan 23, 2021 8:57 pm

I think you can ignore the second deprotonation as long as Ka1 > Ka2. This ensures that the second protonation will not change the pH by a significant amount. I think the textbook says that the only exception is sulfuric acid. Hope this helps

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