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  4. The values of heat of formation of SO2 and SO3 are - 298.2 kJ and - 98.2 kJ. The heat of formation of the reaction, SO2 +(1/2)O2 arrow SO3 will be

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Q. The values of heat of formation of $SO_2$ and $SO_3$ are $- 298.2 \, kJ$ and $- 98.2\, kJ$. The heat of formation of the reaction, $ SO_{2} +\frac{1}{2}O_{2} \rightarrow SO_3$ will be

JIPMERJIPMER 2014Thermodynamics

Solution:

Given,
(i) $S + O _2 \longrightarrow SO _2 ; \Delta H =-298 \cdot 2 kJ$
(ii) $S ^{+} \frac{3}{2} O _2 \longrightarrow SO _3 ; \Delta H =-98 \cdot 2 kJ$
$SO _2+\frac{1}{2} O _2 \longrightarrow SO _3 ; \Delta H =$ ?
Subtracting eqn. (i) from eqn. (ii), we get,
$\Rightarrow SO _2+\frac{1}{2} O _2 \longrightarrow SO _3$
$\Delta H =-98 \cdot 2+298 \cdot 2=+200 kJ$