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5. Thermodynamics and thermochemistry — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistry5. Thermodynamics and thermochemistry1 Mark
MCQ
The molar enthalpies of combustion of $C_2H_2(g),\,C$ (graphite) and $H_2\,(g)$ are $-1300, -394$ and $-286\,kJ\,mol^{-1},$ respectively. The standard enthalpy of formation of $C_2H_2\,(g)$ is.......$kJ\,mol^{-1}$
  • A
    $-226$
  • B
    $-626$
  • ✓
    $226$
  • D
    $626$
✓

Answer

Correct option: C.
$226$
c
(i) $C + O _{2} \rightarrow CO _{2} ; \Delta H =-393.5$

(ii) $H _{2}+\frac{1}{2} O _{2} \rightarrow H _{2} O ; \Delta H =-282.6$

(iii) $C _{2} H _{2}+\frac{5}{2} O _{2} \rightarrow 2 CO _{2}+ H _{2} O ; \Delta H =-1300 KJ$

Reaction: $2 \times( i )+( ii )-($ iii $)$ will result into:

$2 C + H _{2} \rightarrow C _{2} H _{2}$

$\Delta H =2 \times(-393.5)+(-282.6)+1300=226.8$

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