monoprotic, diprotic acids (1 Viewer)

[kobler]

I've been told H2SO4 is a weaker acid than HCl because it is diprotic, but why does this make it weaker?
is it because it has 2 ions to give away, thus intermolecular forces are weaker?
thanks!!!

 

[vds700]

As far as I know, HCL and H2SO4 are both strong acids and strong acids have no degrees of strongness, like weak acids, which can be moderately weak,, extremely weak etc. If an acid is strong, it is completely ionised. If not, then it is weak.

NB: it is the first step of H2SO4 ionising that is strong, ie ionising to form one H+ ion and HSO4-.

 

[minijumbuk]

Either the person who has told you was wrong, or you have forgotten what the person has told you.

H2SO4 is a strong acid, and so is HCl.
However, H2SO4 would be MORE acidic than HCl. You'll understand after this example:

H2SO4 --> 2H+ + SO4 2-
HCl --> H+ + Cl-

So, 1 mol H2SO4 would release 2 mol of H+, whereas 1 mol of HCl would release 1 mol of H+

This means that for every mole of H2SO4, there would be twice the number of moles of H+ in HCl.

 

[undalay]

I've been told H2SO4 is a weaker acid than HCl because it is diprotic, but why does this make it weaker?
is it because it has 2 ions to give away, thus intermolecular forces are weaker?
thanks!!!

You could interpret thats its weaker in this way:

H2S04 -> H+ + HSO4- (very strong)
HSO4- -> H+ + SO4(2-) (weak)

thus overall it is a weaker then HCl (which is always 100%)
Although i've been told that in the HSC we always consider the second step to be 100% as well, even though in reality it isn't.

 

[minijumbuk]

You could interpret thats its weaker in this way:

H2S04 -> H+ + HSO4- (very strong)
HSO4- -> H+ + SO4(2-) (weak)

thus overall it is a weaker then HCl (which is always 100%)
Although i've been told that in the HSC we always consider the second step to be 100% as well, even though in reality it isn't.

Wow, I always thought H2SO4 is stronger than HCl...
But let's say the first step is 100%, then 1 mol of H2SO4 is 1 mol of H+ and HSO4-
If the ionisation of HSO4- is weak, let's make up a figure of...5%? Then there would be H+ and HSO4 -. So in the second step, there would be 5/100 x 1 = 1/20 mol of H+

So, in the total procedure, wouldn't there be 1 mol + 1/20 mol = 1.05 moles, so having a higher [H+] than HCl with only 1 mol anyway?

 

[undalay]

the amount of [H+] merely reflects the acidity.
Not the strength.

For example taking your values of 100% and 5%

then the average ionisation % is (100%+5%)/2
=52.5%

but once again i've been told to consider both steps of H2SO4 as strong.

 

[minijumbuk]

Ohhh no wonder why =\

We never got taught that there is actually a weak ionisation at our school =\

Haha

 

[solomarc20]

diprotic acids are weaker than monoprotic acids because of the steps they take to completely ionise. Similarly, triprotic acids (H3PO4) are weaker still.

 

[minijumbuk]

Quote from wiki:
A diprotic acid is an acid such as H2SO4 (sulfuric acid) that happens to contain within its molecular structure two hydrogen atoms capable of dissociation (i.e. ionizable) in water. The complete dissociation of diprotic acids is of the same form as sulfuric acid:

H2SO4 → H+(aq) + HSO4−(aq) Ka = 1 × 103

HSO4− → H+(aq) + SO42−(aq) Ka = 1 × 10−2

The dissociation does not happen all at once due to the two stages of dissociation having different Ka values. The first dissociation will, in the case of sulfuric acid, occur completely, but the second one will not. Diprotic acids are of particular note in regards to titration experiments, where a pH versus titrant volume curve will clearly show two equivalence points for the acid. This occurs because the two ionization capable hydrogen atoms on the acid molecule do not leave the acid at the same time.

In bold texts:
If it's completely ionised, shouldn't Ka= infinity?

 

[izzah1]

Completely off topic but dude your deathnote sig is hilarious, <3 DeathNote for life.

 

[xiao1985]

Hmm, I would like to argue that H2SO4 is a strong acid, stronger than HCl (Not that comparative strength of strong acids are needed for the HSC, but regardless)

To quote both steps of ionisation of H2SO4 as the strength of the acidity of H2SO4 is not completely right IMO, since, by LB theory of acid and bases, acid merely donates one proton. This implies that the strength of H2SO4 should be based on the below ionisation:

H2SO4 + H2O ---> H3O + HSO4 -

This ionisation step, IIRC, is very strong (stronger than the ionisation of HCl).
Now that been said, the strength of HSO4- is alot weaker than HCl (as a lot of you have mentioned already).

edit: #minijunk: on the topic of complete ionisation, it's a over simplification. No reaction in the universe is complete. Just at times, the equilibrium constant is so huge (such as for first step of sulfuric acid ionisation), that for all practical purposes, it can be treated as infinite (i.e. complete reaction)

 

[blake.stuart]

Ok, so you've all established now that the first step of H2SO4 is a strong ionisation just like HCl - in equality really. They are both strong acids therefore complete ionisation occurs FOR H2SO4 AND HCl.

However, HSO4- is a different story in that it has a negative charge and will want to GAIN a proton, not lose another one. Thus, it becomes more difficult for the HSO4- to ionise completely due to the electronegativity of the molecule so it doesn't.

In essence, both are strong acids but if you had to rank them, HCl would be stronger slighty because it does not need to fight the electro-attractive forces as much (it still does of course) and can therefore ionise more completely and READILY than H2SO4

 

[thongetsu]

Rofl at bump

 

[hscishard]

Rofl at bump

Lol. It's funny when they're clueless.

 

[proletariat]

Ok, so you've all established now that the first step of H2SO4 is a strong ionisation just like HCl - in equality really. They are both strong acids therefore complete ionisation occurs FOR H2SO4 AND HCl.

However, HSO4- is a different story in that it has a negative charge and will want to GAIN a proton, not lose another one. Thus, it becomes more difficult for the HSO4- to ionise completely due to the electronegativity of the molecule so it doesn't.

In essence, both are strong acids but if you had to rank them, HCl would be stronger slighty because it does not need to fight the electro-attractive forces as much (it still does of course) and can therefore ionise more completely and READILY than H2SO4

you're wrong.

H2SO4 + H2O -> H3O+ + HSO4-

HSO4- + H2O <-> H3O+ + SO4-

With the electronegativity crap, dude, the more electronegative the molecule is, the more "strong" the acid is. We dont go any further than "HCl, H2SO4, HNO3, etc etc are strong acids" but their strengths differ. more electronegative = more acidic.