Thermodynamics And Thermochemistry Result Question 1
1. Enthalpy of sublimation of iodine is $24 $ $\mathrm{cal}$ $ \mathrm{g}^{-1}$ at $200^{\circ} \mathrm{C}$. If specific heat of $\mathrm{I}_2(s)$ and $\mathrm{I}_2$ (vap.) are $0.055$ and $0.031 $ $\mathrm{cal} $ $\mathrm{g}^{-1} \mathrm{~K}^{-1}$ respectively, then enthalpy of sublimation of iodine at $250^{\circ}$ $ \mathrm{C}$ in cal $\mathrm{g}^{-1}$ is
(2019 Main, 12 April I)
(a) $2.85$
(b) $5.7$
(c) $22.8$
(d) $11.4$
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Answer:
Correct Answer: 1. ( c )
Solution:
- Key Idea When $q$ is the amount of heat involved in a system then at constant pressure
$q=q_p \text { and } C_p \Delta T=\Delta H$
Given reaction :
$\mathrm{I}_2(\mathrm{~s}) \longrightarrow \mathrm{I}_2(\mathrm{~g})$
Specific heat of $\mathrm{I}_2(s)=0.055 $ $\mathrm{cal} $ $\mathrm{g}^{-1} \mathrm{~K}^{-1}$.
Specific heat of $\mathrm{I}_2$ (vap) $=0.031 $ $\mathrm{cal} $ $\mathrm{g}^{-1} \mathrm{~K}^{-1}$.
Enthalpy $\left(H_1\right)$ of sublimation of iodine $=24 $ $\mathrm{cal}$ $ \mathrm{g}^{-1}$
If $q$ is the amount of heat involved in a system then at constant pressure $q=q_p$ and
$\begin{aligned} \Delta H & =C_p \Delta T \\ H_2-H_1 & =C_p\left(T_2-T_1\right) \\ H_2 & =H_1+\Delta C_p\left(T_2-T_1\right) \\ H_2 & =24+(0.031-0.055)(250-200) \\ H_2 & =24+(-0.024)(50)=24-1.2=22.8 \mathrm{cal} / \mathrm{g} \end{aligned}$
Thus, the enthalpy of sublimation of iodine at $250^{\circ} \mathrm{C}$ is $22.8 $ $\mathrm{cal} / \mathrm{g}$.
BETA
