Oxyacids:
The acids contains oxygen and must have one -OH group directly attach with central atom and also
oxidation state of central atom is (+) positive. Most of covalent (acidic)
oxide react with water to form oxy-acid that yield hydronium (H3O+)ions
in solution. Mostly non metal of P-block formed oxy-acids except some
metals.
Strength
of Oxy acid:
The strenght of
oxy acid define by degree of dissociation (pka) in water or ability to furnise
H+ ions in water.
In general the
relative strength of oxy acid can pridicted on the basis of electronegativity and oxidation state of central non metal atom.
(1) For example perchloric (HClO4)
is stronger acid than sulphuric acid (H2SO4), which
should be a stronger acid than phosphoric acid (H3PO4)
because the electronegativity of the central atom increses as (Cl>S>P).
Acidic srength of acid as :
(HClO4)>(H2SO4)>(H3PO4)
(2)
For given non metal acid the strength of oxy acid increases as the oxidation
state of central atom increases for example sulphuric acid (OS=+6) (H2SO4)
is more acidic then sulphurous acid (OS=+4)(H2SO3).
Similarly nitric acis (HNO3)(OS=+5) is more acidic than Nitrous
acid(HNO2)(OS=+3).
Acidic srength of acid as :
(1)
(H2SO4)>(H2SO3)
(2)
(HNO3)> (HNO2)
(3)
(HClO4)>(HClO3)>(HClO2)>(HClO)
Special Note: Oxy acid of phosphorous do not follow the concept of oxidation state. Because acidic strength of phosphorous acid depends upon degree of dissociation (Ka or Pka value): the order acidic nature follow as H3PO2 > H2PO3 > H3PO4
Levelling effect and relative strength of acids:
When strong acids like HClO4, HCl, HBr, HI, HNO3
etc are dissolved in water they are equally ionised (100%) it means they are
equally strong in water this is called levelling effect of water.
Similarly for strong bases also , NaOH, KOH Ca(OH)2 etc are in
water behave the same. hence relative strength of strong acids or base can not
be compare in water .
(1) Water does no show levelling effect
for weak acids or base because they are ionised upto different extent in water.
(2) All the acids are stronger than H3O+
ion, consider as strong acids and weak than H3O+ ion are consider as
weak acids.
(3) All the bases stronger than OH-
are consider as strong base and weaker than OH-, then they are
consider as weak base.
Definition: If more than one acids or bases are showing same acidic or basic
strength in same solvent it is called labelling effect and the solvent is
called labelling solvent for example:
(1) HClO4 and HI equally dessociation in aqueous
medium (99.99%) it means they have equal strength.
(2) HCl, HNO3
and H2SO4 are equally strong in water because
their strength are “levelled” to solvent species H3O+. only by
putting them into a more acidic solvent do they weak acids. With determinate Pka values which differetiate their strengths. Thus in glacial
ethanoic acid (acetic acid ) as solvent, the order of acidic strength is as: H2SO4 >
HCl > HNO3 .
(3) Similarly HF and HCl are equally dessociation in NaOH (100
%).
If their strength differ in same solvent ,it is called differential effect and the solvent is called differential solvent.
(4) For example HClO4 dessociate 99.99 % in acetic
acid while HI dissociate 99.8% in acetic acid, it means acidic strength of HClO4 is
more than HI in acetic acid.
(5) Many of inorganic oxoacid are strong acid (with more
negative Pka value) inaqueous solution. But, as we have seen, use of solvent
with a lower proton affinity than water like acetic acid, makes it possible to
differentiate between the strength of these acids and measure Pka value.
H2CO3 < H3PO4
< H2SO4 < HClO4 (Increasing acidic
sterngth) these acidic strength can
also explain if the folmulae of the these oxy acids are written as base
or OH formate or like that OC(OH)2 < OP(OH)3<
O2S(OH)2< O3Cl(OH) then it is clear that the acidic strength incrases as the number
of oxygen atoms not involve in O-H bonding increases.
The reason behind levelling and diffential
effect is ability of solvent to donate or accept protons .
For acids, acidic solvents are differential and basic solvent are levelling.
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