Dissociation (chemistry)

Dissociationinchemistryis a general process in which molecules (or ionic compounds such assalts,orcomplexes) separate or split into other things such as atoms, ions, orradicals,usually in a reversible manner. For instance, when anaciddissolves in water, acovalent bondbetween anelectronegativeatom and a hydrogen atom is broken byheterolytic fission,which gives a proton (H⁺) and a negativeion.Dissociation is the opposite ofassociationorrecombination.

Dissociation ofGomberg's dimer

Dissociation diagram ofphosphoric acid

Dissociation constant

For reversible dissociations in achemical equilibrium

{\ce {AB <=> A + B}}

thedissociation constantK_dis the ratio of dissociated to undissociated compound

K_{d}=\mathrm {\frac {[A][B]}{[AB]}}

where the brackets denote the equilibrium concentrations of the species.^[1]

Dissociation degree

The dissociation degree $\alpha$ is the fraction of original solute molecules that have dissociated. It is usually indicated by the Greek symbol α. More accurately, degree of dissociation refers to the amount of solute dissociated into ions or radicals per mole. In case of very strong acids and bases, degree of dissociation will be close to 1. Less powerful acids and bases will have lesser degree of dissociation. There is a simple relationship between this parameter and thevan 't Hoff factor $i$ .If the solute substance dissociates into $n$ ions, then

i=1+\alpha (n-1)

For instance, for the following dissociation

{\ce {KCl <=> K+ + Cl-}}

As $n=2$ ,we would have that $i=1+\alpha$ .

Salts

A video ofsodium chloridecrystals dissolving and dissociating in water

The dissociation of salts bysolvationin asolution,such aswater,means the separation of theanionsandcations.The salt can be recovered byevaporationof the solvent.

Anelectrolyterefers to a substance that contains free ions and can be used as anelectrically conductivemedium. Most of the solute does not dissociate in a weak electrolyte, whereas in a strong electrolyte a higher ratio of solute dissociates to form free ions.

A weak electrolyte is a substance whose solute exists in solution mostly in the form of molecules (which are said to be "undissociated" ), with only a small fraction in the form of ions. Simply because a substance does not readily dissolve does not make it a weak electrolyte.Acetic acid(CH₃COOH) andammonium(NH+4) are good examples. Acetic acid is extremely soluble in water, but most of the compound dissolves into molecules, rendering it a weak electrolyte. Weak bases and weak acids are generally weak electrolytes. In an aqueous solution there will be someCH₃COOHand someCH₃COO⁻andH⁺.

A strong electrolyte is a solute that exists in solution completely or nearly completely as ions. Again, the strength of an electrolyte is defined as the percentage of solute that is ions, rather than molecules. The higher the percentage, the stronger the electrolyte. Thus, even if a substance is not very soluble, but does dissociate completely into ions, the substance is defined as a strong electrolyte. Similar logic applies to a weak electrolyte. Strong acids and bases are good examples, such as HCl andH₂SO₄.These will all exist as ions in an aqueous medium.

Gases

The degree of dissociation ingasesis denoted by the symbol $α$ ,where $α$ refers to the percentage of gas molecules which dissociate. Various relationships between $K p$ and $α$ exist depending on thestoichiometryof the equation. The example ofdinitrogen tetroxide(N₂O₄) dissociating tonitrogen dioxide(NO₂) will be taken. ${\ce {N2O4 <=> 2NO2}}$

If the initial concentration of dinitrogen tetroxide is 1moleperlitre,this will decrease by $α$ at equilibrium giving, by stoichiometry, $α$ moles ofNO₂.Theequilibrium constant(in terms of pressure) is given by the equation $K_{p}={\frac {p{\bigl (}{\ce {NO2}}{\bigr )}^{2}}{p\,{\ce {N2O4}}}}$

where $p$ represents thepartial pressure.Hence, through the definition of partial pressure and using $p T$ to represent the total pressure and $x$ to represent themole fraction; $K_{p}={\frac {p_{T}^{2}{\bigl (}x\,{\ce {NO2}}{\bigr )}^{2}}{p_{T}\cdot x\,{\ce {N2O4}}}}={\frac {p_{T}{\bigl (}x\,{\ce {NO2}}{\bigr )}^{2}}{x\,{\ce {N2O4}}}}$

The total number of moles at equilibrium is $(1 - α) + 2 α$ ,which is equivalent to $1 + α$ .Thus, substituting the mole fractions with actual values in term of $α$ and simplifying; $K_{p}={\frac {p_{T}(4\alpha ^{2})}{(1+\alpha )(1-\alpha )}}={\frac {p_{T}(4\alpha ^{2})}{1-\alpha ^{2}}}$

This equation is in accordance withLe Chatelier's principle. $K p$ will remain constant with temperature. The addition of pressure to the system will increase the value of $p T$ ,so $α$ must decrease to keep $K p$ constant. In fact, increasing the pressure of the equilibrium favours a shift to the left favouring the formation of dinitrogen tetroxide (as on this side of the equilibrium there is less pressure since pressure is proportional to number of moles) hence decreasing the extent of dissociation $α$ .

Acids in aqueous solution

The reaction of an acid in water solvent is often described as a dissociation

{\ce {HA <=> H+ + A-}}

where HA is a proton acid such as acetic acid, CH₃COOH. The double arrow means that this is an equilibrium process, with dissociation and recombination occurring at the same time. This implies that theacid dissociation constant

K_{\ce {a}}={\ce {\frac {[H^{+}][A^{-}]}{[HA]}}}

However a more explicit description is provided by theBrønsted–Lowry acid–base theory,which specifies that theprotonH+ does not exist as such in solution but is insteadacceptedby (bonded to) a water molecule to form thehydroniumion H₃O⁺.

The reaction can therefore be written as

{\ce {HA + H2O <=> H3O+ + A-}}

and better described as anionizationor formation of ions (for the case when HA has no net charge). The equilibrium constant is then

K_{\ce {a}}={\ce {\frac {[H_{3}O^{+}][A^{-}]}{[HA]}}}

where ${\ce {[H_2O]}}$ is not included because in dilute solution the solvent is essentially a pure liquid with athermodynamic activityof one.^[2]^: 668

K_ais variously named adissociation constant,^[3]anacid ionization constant,^[2]^: 668anacidity constant^[1]or anionization constant.^[2]^: 708It serves as an indicator of the acid strength: stronger acids have a higherK_avalue (and a lower pK_avalue).

Fragmentation

Fragmentationof a molecule can take place by a process ofheterolysisorhomolysis.

Receptors

Receptorsareproteinsthat bind smallligands.The dissociation constantK_dis used as indicator of theaffinityof the ligand to the receptor. The higher the affinity of the ligand for the receptor the lower theK_dvalue (and the higher the pK_dvalue).

References

^^a ^bAtkins P. and de Paula J.Physical Chemistry(8th ed. W.H.Freeman 2006) p.763ISBN 978-0-7167-8759-4
^^a ^b ^cPetrucci, Ralph H.; Harwood, William S.; Herring, F. Geoffrey (2002).General chemistry: principles and modern applications(8th ed.). Upper Saddle River, N.J: Prentice Hall.ISBN 978-0-13-014329-7.LCCN 2001032331.OCLC 46872308.
^Laidler K.J.Physical Chemistry with Biological Applications(Benjamin/Cummings) 1978, p.307ISBN 978-0-8053-5680-9

[Atkins-1] Atkins P. and de Paula J.Physical Chemistry(8th ed. W.H.Freeman 2006) p.763ISBN 978-0-7167-8759-4

[:0-2] Petrucci, Ralph H.; Harwood, William S.; Herring, F. Geoffrey (2002).General chemistry: principles and modern applications(8th ed.). Upper Saddle River, N.J: Prentice Hall.ISBN 978-0-13-014329-7.LCCN 2001032331.OCLC 46872308.

[3] Laidler K.J.Physical Chemistry with Biological Applications(Benjamin/Cummings) 1978, p.307ISBN 978-0-8053-5680-9

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