Question

If,
i) \(\text{C}+2\text{S}\rightarrow \text{CS}_{2}\), \(\Delta H=117\ \text{kJ}\)
ii) \(\text{C}+\text{O}_{2}\rightarrow \text{CO}_{2}\), \(\Delta H=-393\ \text{kJ}\)
iii) \(\text{S}+\text{O}_{2}\rightarrow \text{SO}_{2}\), \(\Delta H=-297\ \text{kJ}\)
Find the heat of combustion of \(\text{CS}_{2}\).

• A. \(-1104\ \text{kJ mol}^{-1}\)
• B. \(1104\ \text{kJ mol}^{-1}\)
• C. \(807\ \text{kJ mol}^{-1}\)
• D. \(-807\ \text{kJ mol}^{-1}\)

Hint:

Write the combustion equation of carbon disulfide and combine the three given equations to match it.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept
Hess's law: the enthalpy change depends only on the initial and final states, so enthalpies of combined equations add up.

Step 2: Target reaction
\(\text{CS}_{2}+3\text{O}_{2}\rightarrow \text{CO}_{2}+2\text{SO}_{2}\)

Step 3: Combine the equations
Target = (ii) + 2 \(\times\) (iii) \(-\) (i)
(i) reversed gives \(\text{CS}_{2}\rightarrow \text{C}+2\text{S}\), \(\Delta H=-117\ \text{kJ}\).

Step 4: Calculation
\(\Delta H=-393+2(-297)-117=-393-594-117=-1104\ \text{kJ mol}^{-1}\)

Final Answer:
The heat of combustion of CS2 is minus 1104 kJ per mole. \[ \boxed{\text{(A) }-1104\ \text{kJ mol}^{-1}} \]