How to determine if a molecule is a strong base or acid or weak acid or base...

[letaps]

Hey,

How do you determine if a molecule is a strong base or acid or weak acid or base...

Thanks

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[loveoforganic]

You can determine relative acidities pretty well based on predicting stability of the conjugate base, but as far as knowing what's a strong acid, you just have to memorize some.

 

[Seraph 84]

Yeah I always look at the stability of the conjugate base or the stability of the base itself.

If a question asks me "which of the following is the strongest acid" I qualitatively determine the most stable conjugate base.

Or if they ask "which of the following is the strongest base" I would look for the stability of that base; or look at the strength of the conjugate acids because the strongest base will have the weakest conjugate acid.

A few strong acids off the top of my head include

HCl
HNO3
HI (very strong acid because I- anion is very stable due to its large, diffuse electron cloud)
H2SO4

Some weak acids

HF (F- anion is very unstable because the electron cloud is very tight and not diffuse like Cl- or I-; this makes the F- anion a strong base)
HClO
H2CO3

Hope this helps

 

[letaps]

how do you determine the stability of the conjugate acid or base?

 

[loveoforganic]

Have you taken organic chemistry? Resonance, inductive effects...

 

[redcap]

care to ELABORATE.....cuz i am sure there are many people who do not know this very well...

 

[loveoforganic]

How long ago did you take organic chem? I have no idea how much I should say. Do you know what resonance is? What an inductive effect is?

 

[letaps]

i took it a while back...can you explain it briefly?

 

[loveoforganic]

It's a pretty big topic.

http://en.wikibooks.org/wiki/Organic_Chemistry/Foundational_concepts_of_organic_chemistry/Resonance

^Resonance.

Inductive effects are easier to understand, but less important

http://en.wikipedia.org/wiki/Inductive_effect

 

[sv3]

this is like a key topic and those are some fundamental questions. Read up on this people! It's key for acids and bases, organic chemistry, and likely alot more that is not as obvious.

now when you get stuck, come back!

 

[letaps]

i read up on resonance, and it said that the more resonance structures a molecule has, the more acid or more basic it is...is that how resonance relates to this?

I didn't understand how inductive effects works?...can someone explain this to me?

Also, how is CH3ONa a strong base?

Thanks

 

[ThirdEye]

When you pull off an H from an acid, it leaves a negative charge.

A molecule will be more stable if the negative charge of the conj. base is on an electronegative atom (The more elec. neg., the more stable). This is why HCL is so damn strong because CL- is incredibly stable. The way to think about this intuitiviely is to know what an acid does. An acid donates a proton. So the stronger the molecule will become after donating the proton, the more it wont hesitate to give up that proton, and the more resistant it will be to take that proton back. Cl- is stable, why would it want to take back it's proton?

Inductive effect is when you have elect. neg. atoms within the molecule to help pull the neg. charge toward them, thus spreading the charge out and making the molecule more stable.

Resonance is also a balancing (sharing) of the charge, thus it too is a factor in stability.