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

Assertion $(A)$ : Boron is unable to form $BF _{6}^{3-}$

Reason $(R)$ : Size of $B$ is very small.

In the light of the above statements, choose the correct answer from the options given below.

$(i)\, 6C(s) + 3H_2(g) \to C_6H_6(l) ; \Delta H = +45.9\,kJ$

$(ii)\, H_2(g) +\frac {1}{2}O_2(g) \to H_2O(l) ; \Delta H = -285.9\,kJ$

$(iii) \,C(s) + O_2(g) \to CO_2(g) ; \Delta H = -393.5\,kJ$

.....$kJ$

(Given : $\Delta_{\mathrm{r}} \mathrm{H}_{298 \mathrm{~K}}^{\circ}=-54.07 \mathrm{~kJ} \mathrm{~mol}^{-1}, \Delta_{\mathrm{I}} \mathrm{S}_{298 \mathrm{~K}}^{\circ}=10 \mathrm{JK}^{-1} \mathrm{~mol}^{-1}$ and $\mathrm{R}=8.314 \mathrm{JK}^{-1} \mathrm{~mol}^{-1} ; 2.303 \times 8.314 \times 298=5705$ )