Weak acid/base

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Jose Robles 1D
Posts: 100
Joined: Fri Aug 02, 2019 12:15 am

Weak acid/base

Postby Jose Robles 1D » Sun Jan 26, 2020 7:44 pm

What do you look for in a reaction to indicate if there is a weak base or acid involved (rather than strong)?

Chris Tai 1B
Posts: 102
Joined: Sat Aug 24, 2019 12:16 am

Re: Weak acid/base

Postby Chris Tai 1B » Sun Jan 26, 2020 7:47 pm

The best way to determine whether an acid or base is weak is to memorize all the strong acids and bases and see if that acid is strong. If it's not strong, it's a weak acid or base.
The strong acids are HBr, HClO4, H2SO4, HNO3, HCl, and HI.
SImilar logic can be applied to the bases. The strong bases are all the hydroxides and the ones that contain (OH) groups in groups 1 and 2 of the periodic table.

Jacob Puchalski 1G
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Joined: Fri Aug 09, 2019 12:16 am

Re: Weak acid/base

Postby Jacob Puchalski 1G » Sun Jan 26, 2020 7:47 pm

If the acid or base isn't completely dissociated then it's probably weak.

KarineKim2L
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Joined: Fri Aug 30, 2019 12:16 am

Re: Weak acid/base

Postby KarineKim2L » Sun Jan 26, 2020 7:48 pm

We can memorize the common strong acids and bases from the table in the textbook. Also, if the Ka is small, the acid is weak.

Anish Natarajan 4G
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Joined: Thu Jul 25, 2019 12:16 am

Re: Weak acid/base

Postby Anish Natarajan 4G » Sun Jan 26, 2020 8:19 pm

Generally if you're given a Ka/Kb value in a problem it's most likely a weak acid/base but just as a general rule of thumb it has to do with the electronegativity difference

005206171
Posts: 107
Joined: Wed Sep 18, 2019 12:20 am

Re: Weak acid/base

Postby 005206171 » Sun Jan 26, 2020 9:43 pm

Lavelle says to memorize all the strong acids and bases so that anything else you see, you immediately know is weak. Also, as someone mentioned above, having ka or kb as a given means you have a weak acid or base. Remember that strong acids and bases completely disassociate, giving an infinity disassociation constant so they're not considered. There are great tutorials on youtube for quick memorization. Hope this helps!

805422680
Posts: 103
Joined: Sat Sep 14, 2019 12:16 am

Re: Weak acid/base

Postby 805422680 » Sun Jan 26, 2020 9:52 pm

if the Ka value is high, or the pKa value is low, the acid is generally a strong acid and vice versa for a weak acid. this is also true for Kb and bases

stephaniekim2K
Posts: 79
Joined: Fri Aug 09, 2019 12:16 am

Re: Weak acid/base

Postby stephaniekim2K » Sun Jan 26, 2020 9:59 pm

If the acid or base isn't dissociated completely, it is weak. Otherwise, a very low or a very high pH (or a very high or a very low pOH) means it is a strong acid or base.

J_CHEN 4I
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Joined: Tue Nov 14, 2017 3:01 am

Re: Weak acid/base

Postby J_CHEN 4I » Sun Jan 26, 2020 10:06 pm

If the conjugate base of an acid is very stable, the reaction would favor the production of the conjugate base. If the conjugate base of an acid is not very stable, then the production of the conjugate base is not favored. This hints at the acid being weak. Same idea with bases and their conjugate acids. Stability depends on looking at the Lewis structure.

005162902
Posts: 102
Joined: Wed Sep 18, 2019 12:15 am

Re: Weak acid/base

Postby 005162902 » Mon Feb 03, 2020 5:34 pm

Jacob Puchalski 1G wrote:If the acid or base isn't completely dissociated then it's probably weak.


Be careful using this trick because the strong acid H2SO4. It appears that since it has 2 H that it doesn't fully dissociate in water. The other strong acids all only have one H. However, if you memorize the list you find that it does fully dissociate and lose both H protons when in water, making it a very strong acid.
The best way to know the strong acids and bases is to memorize them.

HuyHa_2H
Posts: 100
Joined: Wed Sep 11, 2019 12:15 am

Re: Weak acid/base

Postby HuyHa_2H » Sun Feb 09, 2020 5:28 pm

It's best to memorize them, but something like the pH scale and dissociability can indicate the strengths of acids/bases if given.

Vivianvelazquez_1J
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Re: Weak acid/base

Postby Vivianvelazquez_1J » Sun Feb 09, 2020 5:47 pm

It's easier to memorize the strong bases that are usually used by Lavelle. If you memorize strong acids and bases anything else you see, you immediately know is weak.

Areli C 1L
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Joined: Wed Nov 14, 2018 12:19 am

Re: Weak acid/base

Postby Areli C 1L » Sun Feb 09, 2020 8:21 pm

Honestly, I just try to memorize all the strong acids and bases(there aren't that many) also check if they dissociate completely. :)

Mariana Fuentes 1L
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Joined: Wed Nov 15, 2017 3:00 am

Re: Weak acid/base

Postby Mariana Fuentes 1L » Sun Feb 09, 2020 8:23 pm

Does that mean that Ka and pKa are kind of opposite?

Matthew ILG 1L
Posts: 112
Joined: Sat Aug 17, 2019 12:15 am

Re: Weak acid/base

Postby Matthew ILG 1L » Sun Feb 09, 2020 8:30 pm

Mariana Fuentes 1L wrote:Does that mean that Ka and pKa are kind of opposite?

Yes exactly!! The smaller the value of Ka, the larger the value of pKa, the weaker the acid.


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