strength of acids and bases (1 Viewer)

[shkspeare]

how can we tell if an acid or base is a strong one?

so far i have only known the strengths by general knowledge (hcl stong acid.. NaOH strong base...)

 

[CM_Tutor]

There are only six strong acids: nitric, sulfuric, hydrochloric, hydrobromic, hydroiodic, perchloric.

There are not many strong bases: soluble metal hydroxides (sodium hydroxide, barium hydroxide, ...) and conjugate bases of weak bases (like the amide ion, NH₂^-, for example) are the major categories.

All others are weak.

So, in short, you have to memorise it - Sorry

(Note - for the acids, hydrofluoric acid, HF, is weak.)

 

[shkspeare]

how can u have a conjugate base of a weak base?

i thought u can only get a conjugate of something that is the opposite...?

 

[Xayma]

Well you could have a conjugate that is ampiprotic so could function as a base in a solution, but I think its just a simple mistake.

 

[shkspeare]

ohh thx to both of u

 

[Xayma]

Technically it would be the conjugate acid, and I think it was just a typo, but the conjugate acid can still act as a base.

eg.
HCl_(aq)+NH₂^-_(aq)--->Cl^-_aq+NH_{3 (aq)}

HCl_(aq)+NH₃_(aq)--->Cl^-_aq+NH₄⁺¹_(aq)

 

[CM_Tutor]

Sorry, but it wasn't a typo. The amide ion is the conjugate base of the weak base ammonia. That is, it is formed when ammonia, NH₃, acts as an acid and donates a proton, forming its conjugate base, NH₂^-.

The resulting ion is a strong base because it was formed when a weak base was forced to act as an acid - ie. we took a species that would normally gain a proton and removed a proton - the result is a species that really wants to gain a proton, and will rip them off just about anything - the amide ion is a sufficiently strong base to remove protons from terminal alkynes.

 

[Xayma]

ahh ok, I didnt read it carefully enough I thought you meant the conjugate of NH₂^- which can act as an acid and a base.

 

[xiao1985]

oh yea btw... is strong acid 100% dissociation or near 100% dissociation???

 

[CM_Tutor]

Strong acids do undergo complete ionisation (note - acids ionise, they don't dissociate. The two terms are not interchangeable, despite the careless use of the terms by trial exams, teachers and some text books.)

 

[xiao1985]

Originally posted by CM_Tutor
Strong acids do undergo complete ionisation (note - acids ionise, they don't dissociate. The two terms are not interchangeable, despite the careless use of the terms by trial exams, teachers and some text books.)

yea sorry... :$ hsc chem too much...

 

[mushroom_head]

what's the difference between dissociation and ionisation??

 

[t-i-m-m-y]

Originally posted by mushroom_head
what's the difference between dissociation and ionisation??

i don't think there is any difference

 

[xiao1985]

Originally posted by mushroom_head
what's the difference between dissociation and ionisation??

i fink dissociation is when say HNO3 becums H+ and NO3 -...

whereas, in ionisation process, the ions actually ionises the water molecule:

HNO3 + H2O -> H3O+ + NO3-

 

[CM_Tutor]

There is definitely a difference between dissociation and ionisation - I know the terms are frequently used as if they are interchangeable, but they are not. It is better to learn to use terminology correctly, so here is an explanation of the difference between them:

An ionisation is a process in which new ions are formed - that is, ions that were not previously present. Both strong and weak acids undergo ionisation reactions, both when considered from an Arrhenius and from a Lowry-Bronsted point of view. All of the following are examples of ionisation:

HCl_(g) ---H₂O---> H⁺_(aq) + Cl^-_(aq)

HCl_(g) + H₂O_(l) ---> H₃O⁺_(aq) + Cl^-_(aq)

H₂SO_{4 (aq)} + H₂O_(l) ---> H₃O⁺_(aq) + HSO₄^-_(aq)

HSO₄^-_(aq) + H₂O_(l) <---> H₃O⁺_(aq) + SO₄^2-_(aq)

By contrast, a dissociation process involves ions that already exist separating from one another. Dissolution of ionic solids is an example of a dissolution process, such as in cases like:

NaCl_(s) ---H₂O---> Na⁺_(aq) + Cl^-_(aq)

NaOH_(s) ---H₂O---> Na⁺_(aq) + OH^-_(aq)

 

[t-i-m-m-y]

good stuff CM-tutor how do u know all this?

but for HSC i always thought they were the same

 

[CM_Tutor]

Originally posted by t-i-m-m-y
good stuff CM-tutor how do u know all this?

but for HSC i always thought they were the same

I know all this stuff because I'm a chemist, and I have the degree to prove it.

I agree that no one is going to penalise an HSC student for using these terms interchangeably, but it is better to learn the correct usage now, rather than having to unlearn and then relearn later. This is one of those areas were terminology is used very losely, by students, by teachers, and certainly by text books. That can cause problems down the track.

 

[xiao1985]

uhm... cm_tutor:

does that mean that by arhhenius's definition of acid, they dissociate rather than ionises??

and yea, thx soo much for clearing it up... =) so gald we can have a chemist to help our chemistry... =)

*remembers forgot to attend the free bbq for 1st chem student from the alchemist society

DOH!!!

 

[CM_Tutor]

Originally posted by xiao1985
does that mean that by arhhenius's definition of acid, they dissociate rather than ionises??

No. Arrhenius definition is still based on the formation of new ions from a net uncharged, covalently bonded molecule, so it's still an ionisation - in fact, the first example I wrote of ionisation uses the Arrhenius definition.

*remembers forgot to attend the free bbq for 1st chem student from the alchemist society

DOH!!!

Oh well, alchemists will certain hold more BBQs - they just won't de free to first years!

 

[t-i-m-m-y]

CM-tutor: oops i didn't see the "teacher" status but i see it now.hehehe