Reactions are carried out at $298 K$ with all gases ideal ( $R = 8.314 J mo l^{- 1} K^{- 1}$ ). Select the correct statement(s).

AFor $C (graphite) + O X_{2} (g) CO X_{2} (g)$ , $Δ H = Δ U$
BFor $H X_{2} (g) + I X_{2} (g) 2 HI (g)$ , $Δ H > Δ U$
CFor $CaCO X_{3} (s) CaO (s) + CO X_{2} (g)$ , $Δ H - Δ U = + 2.48 kJ mo l^{- 1}$
DFor $N X_{2} (g) + 3 H X_{2} (g) 2 NH X_{3} (g)$ , $Δ H < Δ U$

Answer: A, C, D

Δ H - Δ U = Δ n_{g} RT

counting gases only.

N X_{2} + 3 H X_{2} 2 NH X_{3}

Δ n_{g} = - 2

, so

Δ H < Δ U

CaCO X_{3}

decomposition:

Δ n_{g} = + 1

, so the difference is

8.314 \times 298 = 2.48 kJ mo l^{- 1}

H X_{2} + I X_{2} 2 HI

has

Δ n_{g} = 0

, so the claim

Δ H > Δ U

for it is false; graphite combustion has

Δ n_{g} = 0

, so

Δ H = Δ U

there is true.