Question

# Given, $$C(s)+{O}_{2}(g)\rightarrow {CO}_{2}(g); \Delta H=-395\ kJ$$$$S(s)+{O}_{2}(g)\rightarrow {SO}_{2}(g);\Delta H=-295\ kJ$$$$C{S}_{2}(l)+3{O}_{2}(g)\rightarrow {CO}_{2}(g)+2{SO}_{2}(g);\Delta H=-1110\ kJ$$The heat of formation of $$C{S}_{2}(l)$$ is:

A
250 kJ
B
62.5 kJ
C
31.25 kJ
D
125 kJ

Solution

## The correct option is A $$125\ kJ$$Since, the formation reaction of $$CS_2$$ is,$$C(s) + 2S(s) \rightarrow CS_2(l)$$Since, the formation reaction of $$CS_2$$ can also be derived using the given reaction as,$$1^{st} \text{reaction} +2\times 2^{nd} \text{ reaction} - 3^{rd} \text{ reaction}$$Hence, the enthalpy of the reaction, $$[1^{st} \text{reaction} +2\times 2^{nd} \text{ reaction} - 3^{rd} \text{ reaction}]$$ $$=\Delta H_{1^{st} \text{reaction}} +2\times \Delta H_{2^{nd} \text{ reaction}} -\Delta H_{3^{rd} \text{ reaction}}$$ $$=\text{heat of formation of CS}_2(l)$$Hence, $$\Delta H_{f(CS_2)} = -395 + 2\times (-295) - (-1110) = 125kJ$$Hence, the answer is option $$D$$.Chemistry

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