1I dont understand amphoteric substances. If we consider water as a standard medium, then does HSO4- dissociate in water to form H+ or OH-??
(1) Is CH3COOH amphiprotic? It has a conjugate base CH3COO- as well as a conjugate acid CH3COOH2+, so ideally shouldnt it be amphiprotic?
(2) Is the strength of an acid/base and degree of ionisation the same?
(3) Does pH decrease with increase in temperature for bases also?
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8 Answers
1357The questions are good
(1) Normally we consider only aqueous titration. In aqueous titration CH3COOH always acts as an acid. So it should not be considered as amphiprotic for aqueous titration.
But for non aqueous titration it can act as an acid as well as base. Their we can say that it is amphiprotic
1357(2)strength is a qualitative term and degree of ionisation is quantitative term.
degree of ionisation is a way of expressing the strength of acid/base
1357for (3) are you confirmed that pH of acid decrease with increase in temperature
1Thanks a lot manish!!
So what i have understood is that from (1) the acidic or basic nature of a substance is always relative to the medium. Pls. correct if wrong!! But in that case we generally consider substances like water, HSO4-etc. as amphiprotic. Are these considerations also made relative to aqueous titrations??
For(3) thats what i thought - pH decreases with increase in temperature - why is it wrong??
1357Yeah acidic or basic nature of a substance is always relative to the medium. Normally whatever acid or base we consider, it is relative to water i.e. they are stronger acid or base than water.
If we change the medium
e.g. consider solution of acetic acid in HClO4. acetic acid behaves as base in this medium as HClO4 is highly acidic.
Generally we consider these substances relative to aqueous titrations only.
1357For (3) what I think pH depends of ionisation constant.
and it varies with enthalpy change as
d(lnK)/dT=ΔH/RT2 (van't hoff eqn.)
So we can't find whether K increases or decreases unless we know whether ionization is endothermic or exothermic
I'm not sure of this but I think in this way.
see if you are agree with me
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1Amphoteric substances behave as acids or bases.
HSO4- dissociates to form H+, i think!