Question

\[\ce{∆_f U^Θ}\] of formation of \[\ce{CH4 (g)}\] at certain temperature is – 393 kJ mol^–1. The value of \[\ce{∆_f H^Θ}\] is ______.

Options

• zero

• \[\ce{< ∆_f U^Θ}\]

• \[\ce{> ∆_f U^Θ}\]

• equal to \[\ce{∆_f U^Θ}\]

Answer 1

\[\ce{∆_f U^Θ}\] of formation of \[\ce{CH4 (g)}\] at certain temperature is – 393 kJ mol^–1. The value of \[\ce{∆_f H^Θ}\] is \[\ce{< ∆_f U^Θ}\].

Explanation:

The balanced equation for the combustion of methane is

\[\ce{CH4 (g) + 2O2 (g) -> CO2 (g) + 2H2O (l)}\]

Therefore, `∆n_g = 1 - 3 = - 2`

\[\ce{∆H_o = ∆U_o + ∆ng}\] RT

\[\ce{∆H_o = - 393 - 2}\] RT

∴ \[\ce{∆H_o < ∆U_o}\]