Question

In the reaction CS2 ($\mathcal{l}$) + 3O2 (g) $\longrightarrow$CO2 (g) + 2SO2 (g) $\Delta$H = – 265 kcal

The enthalpies of formation of CO2 and SO2 are both negative and are in the ratio 4:3. The enthalpy of formation of CS2 is +26kcal/mol. Calculate the enthalpy of formation of SO2.

• A. – 90 kcal/mol
• B. – 52 kcal/mol
• C. – 78 kcal/mol
• D. – 71.7 kcal/mol

Answer 1

Answer: (D)

– 71.7 kcal/mol

Answer 2

CS2 ($\mathcal{l}$) + 3O2 (g) $\longrightarrow$ CO2 (g) + 2SO2 (g)

$\Delta$ H = –256 Kcal

Let $\Delta$Hf (CO2 , g) = –4 x and$\Delta$Hf (SO2 , g) = –3x

$\Delta$Hreaction = $\Delta$Hf (CO2 ,g) + 2 $\Delta$Hf (SO2 .g) – $\Delta$Hf (CS2, $\mathcal{l}$)

– 265 = –4 x – 6x –26

x = + 23.9

$\therefore\Delta$Hf (SO2,g) = 3x = –71.7 Kcal / mol.