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One of the back exercises (13(C)) from the book for practice provided at my school asked whether the following relation is true:

$\Delta_\mathrm{f}H^\circ(\ce{I2,g}) = \Delta_\mathrm{sub}H^\circ[\ce{I2,s}]$ at $\pu{25 °C}.$

I know how to solve the question, but the answer would depend on the meaning of $\Delta_\mathrm{sub}H^\circ[\ce{I2,s}].$ Which of the following reaction does the symbol $\Delta_\mathrm{sub}H^\circ[\ce{I2,s}]$ represent?

$$ \begin{align} \ce{I2(g) &-> I2(s)}\tag{a}\\ \ce{I2(s) &-> I2(g)}\tag{b} \end{align} $$

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  • $\begingroup$ So Chaitanya Garg's answer is correct. And the answer is (b). $\endgroup$
    – Maurice
    Feb 28, 2021 at 9:59
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    $\begingroup$ Sublimation by definition refers to phase transition from solid to gas (no textbook or reference required I would think, but see: en.wikipedia.org/wiki/Enthalpy_of_sublimation). $\endgroup$
    – Buck Thorn ♦
    Feb 28, 2021 at 10:28
  • $\begingroup$ @BuckThorn I'm more interested in why they used square brackets. Have you ever seen them used like this for the thermodynamic quantities? Or am I looking for the meaning where there is none? $\endgroup$
    – andselisk ♦
    Feb 28, 2021 at 10:31
  • $\begingroup$ @andselisk Rather odd to have different types of brackets on both sides of the OPs equation. Typo? $\endgroup$
    – Buck Thorn ♦
    Feb 28, 2021 at 10:36

1 Answer 1

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$\ ΔH_{sub}(I_2,s)$ can be interpreted as the change in enthalpy when one mole of solid $\ I_2$ converts to gaseous $\ I_2$ at a constant temperature. So the answer should be reaction (b).

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    $\begingroup$ Could you please provide with a reference text that says so. $\endgroup$
    – Aseem Mittal
    Feb 28, 2021 at 9:57
  • $\begingroup$ @Aseem Mittal, Pg No. 50 Atkins' Physical Chemistry by Peter Atkins and also Pg. No. 165 of NCERT textbook for class 11th. $\endgroup$
    – Chaitanya Garg
    Feb 28, 2021 at 13:11

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