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Thursday, January 14, 2021

What are the oxy-acids?

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.

 Related Questions:

What are "pyro" oxy acids?

What are "Ortho" or "Meta" oxyacids?


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