Question

The standard heat of formation, in kcal/mol of Ba²⁺ is:

[Given: Standard heat of formation of \[\ce{SO^{2-}4}\] ion (aq) = −216 kcal/mol, standard heat of crystallisation of BaSO₄(s) = −4.5 kcal/mol, standard heat of formation of BaSO₄(s) = −349 kcal/mol]

Options

• −128.5

• −133.0

• +133.0

• +220.5

Answer 1

−128.5

Explanation:

\[\ce{Ba^{2+}_{ (aq)} + SO^{2-}_{4(aq)} -> BaSO4(s)}\]

ΔH_r = ∑ΔH_f(Products) − ∑ΔH_f(Reactants)

−4.5 = [ΔH_f(BaSO₄)] − [ΔH_f(Ba²⁺) + ΔH_f\[\ce{(SO^{2-}4)}\]]

−4.5 = [−349] − [ΔH_f(Ba²⁺) + (−216)]

−4.5 = −349 − ΔH_f(Ba²⁺) + 216

−4.5 = −133 − ΔH_f(Ba²⁺)

ΔH_f(Ba²⁺) = −133 + 4.5

ΔH_f(Ba²⁺) = −128.5 kcal mol⁻¹