Question

Given, the enthalpy of formation of `MgCl_((s))` is `-125 kJ mol^(-1)` and the entahpy of formation of `MgCl_(2(s))` is`-642kJ mol^(-1)`. Predict whether `MgCl` will undergo disproportionnation or not? If yes, calculate the enthalpy of disproportion.

Answer 1

Given, (a). `Mg_((s))+(1)/(2)Cl_(2(g))rarrMgCl_((s)) ,DeltaH_(1)^(Θ)=-125 kJ mol^(-1)`
(b). `Mg_((s))+Cl_(2(g))rarrMgCl_(2(s)),DeltaH_(2)^(Θ)=-642 kJ mol^(-1)`
(C ). Disproportionation of `MgCl` is
`overset((1+))(2MgCl_((s)))rarroverset((2+))(MgCl_(2(s)))+overset((0))(Mg),DeltaH_(3)^(Θ)=?`
`:.DeltaH_(3)^(Θ)=DeltaH_(2)^(Θ)-2DeltaH_(1)^(Θ)`
`=[-642-2(-125)]kJ mol^(-1)`
`=-392 kJ mol^(-1)`
Since the enthalpy of dispropotionation is exothermic and much higher compared to the formation of `MgCl_((s))`, therefore, `MgCl_((s))` will redily undergo disproportionation reaction.
Enthalpy of disproportionation of `MgCl_((s))=392 kJ mol^(-1)`.
Statement: Alkani nearth metals do not exhibit an oxidation state of `+3` in their compounds or `MX_(3)` type of compounds are not known.
Explanation: For the formation of `M^(3+)` ion, the third electron has to be removed from an inert gas core, which is highly stable.
`underset(["Noble gas"]ns^(2))(M_((g)))overset(IE_(1))underset(-e^(Theta))rarr underset(["Noble gas"]ns^(1))(M^(o+))overset(IE_(2))rarr underset(["Noble gas"]ns^(0))(M^(2+))`
Third third ionisation energy of therse elements is too high to allow the formation of `M^(3+)`. Hence, `MX_(3)` type of compounds are not known.