Enthalpy of sublimation of iodine is 24\,cal g-1 at 200C. If specific heat of I2(s) and I2 (vap) are 0.055 and 0.031\,cal g-1K-1 respectively, then enthalpy of sublimation of iodine at 250C in cal g-1 is:

JEE Main · 2019 · Shift-IhardTHERMO-125

Enthalpy of sublimation of iodine is 24\,cal g-1 at 200C. If specific heat of I2(s) and I2 (vap) are 0.055 and…

Thermodynamics · Class 11 · JEE Main Previous Year Question

Question

Enthalpy of sublimation of iodine is $24\,\text{cal g}^{-1}$ at $200^\circ\text{C}$ . If specific heat of $\text{I}_2(s)$ and $\text{I}_2$ (vap) are 0.055 and $0.031\,\text{cal g}^{-1}\text{K}^{-1}$ respectively, then enthalpy of sublimation of iodine at $250^\circ\text{C}$ in $\text{cal g}^{-1}$ is:

Options

a
11.4
b
2.85
c
22.8
✓
d
5.7

Correct Answerc

22.8

Detailed Solution

🧠 Kirchhoff's law: $\Delta H_{T_2} = \Delta H_{T_1} + \Delta C_P(T_2 - T_1)$ The enthalpy of sublimation changes with temperature at a rate equal to the difference in heat capacities between the vapor and solid phases.

🗺️ Calculation $\Delta C_P = C_P(\text{vapor}) - C_P(\text{solid}) = 0.031 - 0.055 = -0.024\,\text{cal g}^{-1}\text{K}^{-1}$

$\Delta H_{250} = 24 + (-0.024)(250 - 200) = 24 - 1.2 = 22.8\,\text{cal g}^{-1}$

$\boxed{\text{Answer: (c) 22.8 cal g}^{-1}}$

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