Question

The standard heats of formation in kcal mol⁻¹ of NO_2(g) and N₂O_4(g) are 8.0 and 2.0 respectively. The heat of dimerization of NO₂ in kcal is ____________.

\[\ce{2NO2_{(g)} ⇌ N2O4_{(g)}}\]

Options

• 10.0

• −6.0

• −12.0

• −14.0

Answer 1

The standard heats of formation in kcal mol⁻¹ of NO_2(g) and N₂O_4(g) are 8.0 and 2.0 respectively. The heat of dimerization of NO₂ in kcal is −14.0.

\[\ce{2NO2_{(g)} ⇌ N2O4_{(g)}}\]

Explanation:

Considering the forward reaction,

∆H⁰ = `∆_"f""H"_("(products)")^0 - ∆_"f""H"_("(reactants)")^0`

∆H⁰ = `∆_"f""H"^0 ("N"_2"O"_4) - 2∆_"f""H"^0 ("NO"_2)`

= 1 mol × (2 kcal mol⁻¹) − 2 mol × (8 kcal mol⁻¹)

= −14 kcal