The standard heat of formation, in kcal/mol, of Ba^2+ is: [Given: standard heat of formation of SO4^2- ion (aq) = -216 kcal/mol,…

Question

The standard heat of formation, in kcal/mol, of Ba^2+ is: [Given: standard heat of formation of SO4^2- 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]

1. A. -128.5 (Correct answer)
2. B. -133.0
3. C. +133.0
4. D. +220.5

Correct Answer

Option A — -128.5

Detailed Solution & Explanation

The correct answer is -128.5 kcal/mol.

Key Points

• Combine: (1) formation of SO$_4^{2-}$ ($-216$), (2) crystallisation Ba$^{2+}$(g) + SO$_4^{2-}$(g) $\to$ BaSO$_4$ ($-4.5$), (3) formation of BaSO$_4$(s) ($-349$).
• $\Delta H_f(\text{Ba}^{2+})=\Delta H_f(\text{BaSO}_4)-\Delta H_f(\text{SO}_4^{2-})-\Delta H_{cryst}$.
• $=-349-(-216)-(-4.5)=-349+216+4.5=-128.5$ kcal/mol.