Acid Strength

Act 1 : meeting someone new : Acid

An acid is something…

that can *donate H+ ions*

for anybody to be an acid, it has to have…


and the rest of the compound we shall call it A

so Tada! ^_^ we have HA as our acid

when HA donates away its H+

this donation looks like…

HA -> H+   &   A-

whereas the first traveller we met on the way, HA is an acid…

when HA donates a H+ to become A-, we meet A-

which is a Conjugate Base

The 2nd traveller is the direct OPPOSITE of the 1st one that we met!

because a base is something that can
“take up” the H+ our acidic friend has donated!

so in other words, a rather interesting situation arises where

HA                      ->                       H+                      &                       A-
our acid      donates          hydrogen ion   and produces    a conjugate base!

Act 2 : The Tavern

Now that you have made friends with HA, both of you happily head down to the tavern to have a nice drink and a good chat!

At the tavern, you took a good look around…

and realised that there are actually many different kind of acids and conjugate bases around!

A sad-looking CH3COOH wants to buy you a drink

It tells you ” As an Acid, i feel so weak!  its all my conjugate base CH3COO- fault!”

“why is it so?” you ask CH3COOH

CH3COOH ponders for a while, before turning its head and beckoning you to look its way.

“Now look over that, thats CL-”  It says.

“You don’t see HCL around because HCL is such a strong acid that when it is in contact with water, it is TOTALLY converted to H+ and CL- !”  CH3COOH explains.

“But what is the difference between HCL and yourself? you guys are both acids right?” you question CH3COOH inquisitively.

“Big Difference” CH3COOH sighs. “because when i try to donate my H+ ion, i have to form my conjugate base CH3COO- ”

“However, it is so strong that most of the time, it gobbles up the H+ ions whenever i try to donate… so i am back to my original form again”

“On the other hand, CL- is such a weak conjugate base that it does not take  up the hydrogen ion HCL is donating!!!” CH3COOH grumbles.

The creaky door of the tavern swings wide open and in steps a forlorn looking CH3CH2COOH. Just when you are wondering why it is so unhappy…

“Man! Am i surprised to see you CH3CH2COOH! I am so happy to meet you here, cousin!” Exclaims CH3COOH loudly. Much to your own surprise.

CH3CH2COOH replies sarcastically.  “yeah, cuz i am even weaker than you right, pal?”

Just as the depressing mood CH3CH2COOH seem to be in is making you feel terribly awkward, CH3COOH decides to spare a thought for your feelings.

It says ” Take it easy alright, just hear me out”

“as you can see, both CH3CH2COOH and myself have alkyl groups as part of our structure.”

Now CH3COOH takes a deep breath.  “The thing is, alkyl groups exert an electron donating inductive effect which intensifies the negative charge on the COO- of our already negative Conjugate Bases”

“and unfortunately for CH3CH2COOH, his alkyl group is even bigger than mine, therefore the electron donating inductive effect is going to intensify the negative charge on its Conjugate ion more than mine” CH3COOH explains.

“Yet I would be very happy if i can have a slight structural modification to myself… which is to add a halogen to my alkyl side chain and perhaps be  CH2(CL)COOH”  it says dreamily.

“Having a halogen atom in my structure to exert an electron withdrawing inductive effect will instead disperse the negative charge on COO- of my conjugate ion, allowing me to donate more H+ and hence dissociate more easily in water!” CH3COOH concludes.

You could barely hear what CH3COOH was saying as a previously quiet group has erupted into a ferocious argument. The din was incredibly loud and the look on CH3COOH grew soured.

Finally CH3COOH slammed the table and said to you in as loud a voice he can muster “Those jokers, GRRR, they are our cousins CH3CH(CL)COOH and CH2(CL)CH2COOH and CH3CH(Br)COOH and CH2(F)CH2COOH. Almost always forever bickering with each other and creating SOOO MUCH trouble for all of us!!!!!!”

“They seem to have similar structures, why aren’t they treating each other as pals but as enemies instead?” You asked quizzically.

“Precisely because they are so similar, that is why they are so envious about the minor differences between each other!!!” CH3COOH explains.

“You don’t get it? Ah well let me go into the finer details” CH3COOH dawdles.

“Take CH3CH(CL)COOH and CH2(CL)CH2COOH for example! just because the CL halogen for CH2(CL)CH2COOH is further away from the crucial COOH group than in CH3CH(CL)COOH,  ceteris paribus, CH2(CL)CH2COOH is a weaker acid than the latter.”

“Just when you thought i would proceed on to discuss about the next pair of OLD MAIDDSSS, i will show you that the worst is not over for the poor loser CH2(CL)CH2COOH because… although the CL is as far away from the sooo important COOH group as F in CH2(F)CH2COOH, just because F is more electronegative than CL, CH2(CL)CH2COOH is weaker!!!  i can imagine how it feels to be such a complete loser in that group but oh dear, what a poor thinggg”

“And you won’t believe it, you totally won’t…. BUT even CH2(F)CH2COOH has an axe to grind with CH3CH(Br)COOH! You might be fooled into thinking that F, being much more electronegative than Br, is going to allow CH2(F)CH2COOH to be a stronger acid than CH3CH(Br)COOH……… BUT it is the direct OPPOSITE!!!. Apparently for us acids, whether the position of our good old electron-withdrawing Halogens is closer to the COOH takes much more importance than their electronegativity!”

“I shall not drag the story on about the really bad blood between the unlikely hero of the previous paragraph CH3CH(Br)COOH and our first winner CH3CH(CL)COOH… you know the story! But how much closer is it this time? Incredibly close margin to lose by, because the very important distance to the COOH is same, and now it is the less important electronegativity difference that makes the CUT!!!”

Act 3 : The Roll of Honour

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