difficulties recognizing weak acids and bases

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Samantha Hoegl Roy 2C
Posts: 81
Joined: Fri Sep 28, 2018 12:15 am

difficulties recognizing weak acids and bases

Postby Samantha Hoegl Roy 2C » Wed Jan 16, 2019 11:50 pm

I still have difficulties figuring out if a substance is a weak acid or base; for example, NH3. Does anyone have any advice and tips to make it easier?

David Effio 1H
Posts: 38
Joined: Wed Feb 14, 2018 3:01 am

Re: difficulties recognizing weak acids and bases

Postby David Effio 1H » Thu Jan 17, 2019 12:11 am

So one dead giveaway of a wake acid or base is the presence of acidity or basicity constant (Ka or Kb respectively). Only a weak acid/base would have one of these. A chemical reaction involving a weak acid/base would have a double arrow, as some of the reactants would form back from the product through the reverse reaction

A giveaway for strong acids or bases would be that a strong acid/base is completely dissociated, meaning all of the acid or base will become its conjugate base or acid, leaving a concentration of 0 of the reactant. This can be determined by looking at the formula and seeing that the arrow is only pointing in the right direction, no double arrows.
Note, this is also why strong acids and bases do not have a Ka or Kb value, as dividing the resulting product concentration by the reactant concentration of 0 is not possible.

Strong acids usually have a pH of 0-1, as the resulting H30+ concentration is usually very large, while weak acids usually have a pH a little higher, as not all the acid dissociated into its conjugate base and a proton.

I hope that helps!

mcredi
Posts: 63
Joined: Fri Sep 28, 2018 12:16 am

Re: difficulties recognizing weak acids and bases

Postby mcredi » Thu Jan 17, 2019 9:20 am

Honestly, it is helpful to memorize them so the strong acids are hydrochloric acid, nitric acid, sulfuric acid, hydrobromic acid, hydroiodic acid, perchloric acid, and chloric acid. The only weak acid formed by the reaction between hydrogen and a halogen is hydrofluoric acid.

Henry Dudley 1G
Posts: 61
Joined: Fri Sep 28, 2018 12:16 am

Re: difficulties recognizing weak acids and bases

Postby Henry Dudley 1G » Sat Jan 19, 2019 4:33 pm

David Effio 1H wrote:So one dead giveaway of a wake acid or base is the presence of acidity or basicity constant (Ka or Kb respectively). Only a weak acid/base would have one of these. A chemical reaction involving a weak acid/base would have a double arrow, as some of the reactants would form back from the product through the reverse reaction

A giveaway for strong acids or bases would be that a strong acid/base is completely dissociated, meaning all of the acid or base will become its conjugate base or acid, leaving a concentration of 0 of the reactant. This can be determined by looking at the formula and seeing that the arrow is only pointing in the right direction, no double arrows.
Note, this is also why strong acids and bases do not have a Ka or Kb value, as dividing the resulting product concentration by the reactant concentration of 0 is not possible.

Strong acids usually have a pH of 0-1, as the resulting H30+ concentration is usually very large, while weak acids usually have a pH a little higher, as not all the acid dissociated into its conjugate base and a proton.

I hope that helps!


So for all of the problems with a given Ka or Kb value we are dealing with a weak acid or weak base?

Swetha Ampabathina1I
Posts: 64
Joined: Fri Sep 28, 2018 12:18 am

Re: difficulties recognizing weak acids and bases

Postby Swetha Ampabathina1I » Sat Jan 19, 2019 4:44 pm

Most likely yes you will be dealing with weak acids and bases.

Andrew Bennecke
Posts: 62
Joined: Fri Sep 28, 2018 12:15 am

Re: difficulties recognizing weak acids and bases

Postby Andrew Bennecke » Mon Jan 21, 2019 1:34 pm

If a substance can donate or accept H+ atoms or lone e- pairs, it is most likely a weak acid or base. The only exceptions would be the strong acids (HCl, HF,HBr, etc.) and the strong bases (Group 1 or 2 elements + OH- ion).

Charles Hood Disc 1C
Posts: 39
Joined: Fri Sep 28, 2018 12:19 am

Re: difficulties recognizing weak acids and bases

Postby Charles Hood Disc 1C » Mon Jan 21, 2019 1:48 pm

It can help to draw a Lewis structure and look for any lone pairs, or any hydrogens that could be donated.

005199302
Posts: 108
Joined: Fri Sep 28, 2018 12:15 am

Re: difficulties recognizing weak acids and bases

Postby 005199302 » Mon Jan 21, 2019 6:38 pm

Just a note: it is helpful to remember that nitrogen with a lone pair means it's a weak base

daniella_knight1I
Posts: 57
Joined: Fri Sep 28, 2018 12:18 am

Re: difficulties recognizing weak acids and bases

Postby daniella_knight1I » Sun Jan 27, 2019 4:18 pm

If you see a Ka or Kb constant that's a giveaway. If not you'll have to draw out the lewis structure and look at the lone pairs to determine if it's a weak acid or base.


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