Re: How can we determine between strong/weak acids and bases based on chemical formul
Simple answer:
HCl ionises to a greater extent than HF.
HCl completely ionises to form a hydronium ion and chlorine ion:
HCl + H2O -> H3O+ + Cl-
whereas, HF only partially ionises to form a hydronium ion and a fluorine ion:
HF + H2O <-> H3O+ + F-
(note the reversible nature of the 2nd equation, indicating that the reaction can take place in both directions - hence only partial ionisation)
As ionisation is directly linked to strength of an acid - HCl is stronger than HF. That's because, HCl will react with water to form more hydronium ions then HF will and the amount/concentration of hydronium ions produced is proportional to the strength of the acid.
OP clearly stated that he knows what strong and weak acids are and yet you've simply restated their definitions. Clearly, that's not what the OP wanted.
And some corrections:
The first bolded bit would make sense to most people when they read it, but it's nevertheless not scientifically accurate. You need to say "AT EQUAL CONCENTRATIONS".
Make your terminologies more accurate, especially if you are tutoring chem.
'Proportional' has a precise mathematical meaning = linear proportionality. Where you said conc of H3O+ ions produced is proportional to the strength of the acid, it's unclear what the 'strength' actually means mathematically with what you've stated above. Hence you cannot say 'proportional' without defining it.
In answer to OP's question: The reason (unless you're specialising in quantum physics/chemistry and so can give a better/more rigorous one!) is that F- is not very stable by itself and HF bond is strong. F- has a very electronegative nucleus and so has small ionic radii and holds onto its electrons very tightly - it is not easily polarised --> very localised electron density --> high energy. (in general, the more easily electrons can be spread over a larger distance, the lower the energy (or more stable).)
but Cl- is stable (holds onto its electrons much less tightly than F-).
but you don't need to know that... for HSC chem at least.
