strength vs proton donation (1 Viewer)

[Farmerism]

im abit confused about the relationship between the relative strength of an acid, and.. the number of protons that theyre able to donate..ie. whether the acid is monoprotic, diprotic or triprotic...

im confused because i thought that when there are more H ions, (or something) present within the acid molecule, this meant that it was more acidic, cept its not the case cuz.. it undergoes an equilibrium reaction, which classifies it as a weak acid (to me anyway..)

and im not sure about the degree of ionistation either, if the percentage is lower, then does this mean that its a weaker acid? [i was reading macmillian and to me it contradicts itself]

i also dont understand the statment:
" the more concentrated the weak acid, the lower its degree of ionisation"

because i thought that.. neh i dunno im confused..

help please

 

[insert-username]

A strong acid will dissociate completely in solution. It may be monoprotic (HCl), diprotic (H₂SO₄) or triprotic (H₃PO₄) - this has nothing to do with whether or not it is a strong acid. A weak acid does not dissociate completely in solution, and again, it could be monoprotic (HCOOH), diprotic (H₂CO₃) or triprotic (citric acid). This is where degree of ionisation comes in - the degree is how far the acid dissociates. The degree of ionisation for a strong acid is always 100% no matter how many protons it can donate. The degree of ionisation for weak acids is always lower than 100%.

i also dont understand the statment:
" the more concentrated the weak acid, the lower its degree of ionisation"

The term "concentrated" refers to how much acid is in the solvent. Weak acids only dissociate to a certain extent in solution. If you add more acid (i.e. more concentrated), it won't dissociate further, but you're increasing the amount of acid in the solution. Since the degree of dissociation is equal to H₃O+ concentration divided by total concentration, if you add more to the total concentration (the denominator), you decrease the degree of ionisation.

I hope that helps,


I_F

 

[Riviet]

There are only 3 strong acids that you need to know for the HSC course:

1. HCl - hydrochloric acid
2. H₂S0₄ - sulfuric acid
3. HNO₃ - nitric acid

Each of these strong acids will ionise completely (100%), although in reality, it's actually 99.9% as it is still an equilibrium with less than 0.1% of the reaction going the opposite way, which is a negligible amount. Any other acid that you come across in this course will be weak.

 

[Farmerism]

hey cool that clarifies things

thanks heaps!

 

[Farmerism]

actually, okay. i have another query conerning what insert-username said

A strong acid will dissociate completely in solution. It may be monoprotic (HCl), diprotic (H₂SO₄) or triprotic (H₃PO₄) - this has nothing to do with whether or not it is a strong acid.

but lets say that we were just talking about strong acids. and their proton donation. if a strong acid was to have the ability to donate one proton, then would this mean that it was weaker, compared to another strong acid which had two protons to donate?

also, when you work out the pH of:

0.01mol/L of HCl
pH=-log[0.01] =2

and 0.01mol/L of H2SO4
pH=-log[0.01 x 2] =1.70

the relationship shows that as more H is present within the molecule, the more acidic it will be...........hm

 

[tennille]

There is no such thing as a strong acid being weaker than another strong acid. If the acid is strong, it means it will completely dissociate in solution. If it dissociates 99.99%, it is a weak acid; it is not strong. It may be stronger than another acid, but technically, it is not strong. It's a bit confusing.

 

[Farmerism]

mkay i can understand what Tennille is saying.

but what about

There are only 3 strong acids that you need to know for the HSC course:
HCl - hydrochloric acid
H2S04 - sulfuric acid
HNO3 - nitric acid
Each of these strong acids will ionise completely (100%), although in reality, it's actually 99.9% as it is still an equilibrium with less than 0.1% of the reaction going the opposite way, which is a negligible amount. Any other acid that you come across in this course will be weak.

in conjuction with what i said earlier

0.01mol/L of HCl
pH=-log[0.01] =2

and 0.01mol/L of H2SO4
pH=-log[0.01 x 2] =1.70

dispite the differig pH levels, are they still of the same acidic strength? or.. are they just both concidered strong because the pH is < 4

 

[Riviet]

dispite the differig pH levels, are they still of the same acidic strength? or.. are they just both concidered strong because the pH is < 4

For your own better understanding of the topic, HCl is the strongest of the three, hence the lowest pH, but this is only for you to know in your head, and not to be written in an exam/assessment. Generally speaking, you should only refer to the 3 strong acids as strong, and don't state which is strongest. They are considered strong because they ionise 100% with of course that negligble amount reacting in reverse. Other acids may ionise from less than 1% up to 90ish%, but are still considered weak.