IIT-JEE 2010 · Paper 1 · Chemistry Q7

ChemistrySingle correct+3 / -1Question 7 of 84

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

\ce{Br2(g)}

\ce{Cl2(g)}

\ce{H2O(g)}

\ce{CH4(g)}

Answer and worked solution

Official answer: B

Standard enthalpy of formation is zero for an element in its standard (reference) state at 298 K.

$\ce{Cl2(g)}$ : chlorine's standard state is the gas. Zero.
$\ce{Br2(g)}$ : bromine's standard state is the liquid, so the gas has $\Delta H_f^\circ>0$ .
$\ce{H2O(g)}$ and $\ce{CH4(g)}$ are compounds.

(B)

Topics:Hess's law and enthalpies of reaction

IIT-JEE 2010 · Paper 1, Chemistry question 7