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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.

Related Questions:

What are "pyro" oxy acids?

What are "Ortho" or "Meta" oxyacids?

What is Meta Boric Acids?

What are the structural difference between oxides ( P4O6 and P4O10) of phosphorous?

What are common structural features of oxides (P4O6 and P4O5) of phosphorous?

What is "calgon" ? Give structure and its uses?

Structure of Oxy acids of Phosphorous:

What is structure of (HPO3) metaphosphoric acid?

What is metaphophoric (HPO3)?

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