Second Deprotonations

[Mrudula Akkinepally]

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]

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]

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]

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.

[SavannahScriven_1F]

Specifically, the second deprotonation (with K_a2) does not affect the pH and can be ignored when K_a2 is less than K_a1*10^-3. Usually though K_a2 is much smaller than K_a1 so it's pretty obvious. Hope this helps!

[Katie Phan 1K]

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

[vanessanguyen3I]

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