JEE Advanced · 2010 · 11 Apr 2010 · Paper 1 · Chemical Thermodynamics

The species which by definition has ZERO standard molar enthalpy of formation at 298 K is

(A) $Br_{2}$ (g)
(B) $Cl_{2}$ (g)✓ Correct
(C) $H_{2} O (g)$
(D) $CH_{4}$ (g)

Answer: (B)

Solution

By definition, the standard enthalpy of formation (ΔH°f) of an element in its most stable form at 298 K and 1 atm is zero. At 298 K, chlorine exists as Cl₂(g), which is its standard state. Bromine's standard state is Br₂(l), not Br₂(g). H₂O(g) and CH₄(g) are compounds, not elements in their standard states, so their ΔH°f values are non-zero.

JEE Advanced · 2010 · 11 Apr 2010 · Paper 1 · Single correct option

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